SE539238C2 - Method and devices for measuring phase noise and constructing a phase noise representation for a set of electromagnetic signals - Google Patents

Abstract

ABSTRACT 5 There is provided methods and devices for measuring the phase noise of a set ofelectromagnetic signals, the set comprising at least two electromagnetic signals, inorder to construct a representation of the phase noise where the correlatedcontribution and uncorrelated contribution to the phase noise is determined. There isalso provided a device for constructing a phase noise representation comprising the 10 correlated contribution and uncorrelated contribution as well as a computer program and computer program product having the same purpose. FIG. 1

Description

Method and devices for measuring phase noise and constructing a phase noise representation for a set of electromagnetic signals Technical field The proposed technology relates to methods and a corresponding devices formeasuring phase noise and constructing a representation of the phase noise of a setof electromagnetic signals. The invention relates generally to the measurement andCharacterization of phase noise of two or more signals in order to construct a phasenoise representation that explicitly comprises the correlated and uncorrelated phasenoise contributions from the different signals. The proposed technology alsocomprises a computer program constructing a representation of the phase noise of aset of electromagnetic signals as well as a computer program product comprising the computer program.

Background of the invention Phase noise of electromagnetic signals - electrical, radio frequency, or microwavesignals - is the random fluctuation around the nominal carrier frequency. The phasenoise is random variable that is a function of time. Being a random variable, phasenoise is described by its statistical properties. Phase noise can be described in timedomain, in which case the autocorrelation function can be determined. ln frequencydomain phase noise is described by the phase noise spectrum. The phase noisespectrum of a signal is the power spectrum of the signal, normalized by the meanpower of the signal and shown vs offset frequency from the nominal carrier frequency.The time or frequency domain representations are equivalent and one representation can be transformed into the other.

Phase noise is also relevant for characterizing signal generators and oscillators usedin several emerging applications, e.g. in concurrent multiple band amplifiers intelecommunications. Multiple signals of different centerfrequency are amplified by oneamplifier. Phase noise affects the performance of compensation techniques in thedigital domain, and knowledge about the phase noise may therefore be of value.1 Another example is in satellite communication where signals of different centerfrequency go through one satellite transponder. The phase noise also affects theperformance of the digital processing algorithms of the transmitters and receivers,hence a solid knowledge about phase noise is highly beneficial for countering orreducing the negative impact of phase noise.

There are many methods for measuring phase noise [Wendler2010, Jong2003,Roth2015]. Phase noise can be measured by most spectrum analysers. Even thoughthese methods are well-developed in today's instrumentation, the present invention isnot related to any of them in particular. Furthermore, the disclosed invention usesphase noise measurements that can be obtained using any of these well-developedphase noise measurements. ln these measurements, the power spectral density(PSD), normalized by the signafs power is then measured. The method requires thatthe spectrum analysers phase noise is much smaller than the tested signal[Go|dberg2000]. A reference source with low or known phase noise can be used forphase noise measurement. The reference is phase locked to the same frequency andthe test signal and reference are mixed. The power spectrum can then be measured[Goldberg2000], [Rohde2013a]. In the cross- correlation technique a signal is split intotwo; one of the signals is delayed and the two signals and the cross correlation of thetwo signals is taken [Goldberg2000], [Rohde2013a] [Rohde2013b]. The cross-PSDof two signals can be determined if the two signals can be sampled and the fast Fouriertransform (FFT) of each signal calculated [Fest1983].

Additive phase noise is the phase noise that is added to a signal that passes througha component. Additive phase noise is also referred to as residual phase noise.Component that may add phase noise are amplifiers, frequency dividers, frequencymultipliers, and mixers [Baker2012, Sariaslani2013, Koji2013].

Signal generators use oscillators that are designed to generate a signal of a specifiedfrequency. One' important property of an oscillator is the phase noise. There arenumerous designs for achieving oscillators with low phase noise or phase noise at aspecified level. Some generators are designed to give output signals at different frequencies. [Minassian2011]. A signal generator that is phase coherent and 2 generates a continuous phase signal that includes fast switched multiple differentfrequency burst is reported in [Fawley2014]. The company Holzworth instrumentationInc. Boulder Colorado offers a radio frequency signal generator for multiple phasecoherent output signals of arbitrary output frequency. lt is not specified to what degreeor in what sense the signals are phase coherent.

Summary There is a general object of the proposed technology invention to provide amechanism that enable improved or more detailed phase noise measurements.Improved phase noise measurements will provide developer of transmitters and alikewith better possibilities to counter and/or reduce the negative effects that emanatesfrom phase noise. This and other objects are met by embodiments of the proposed technology.

According to a first aspect there is provided a method for constructing a representationof the phase noise for a set of electromagnetic signals, EM-signals, where the setcomprises at least two EM-signals. The method comprises the step of measuring aphase noise spectrum for each mixing product of two different EM-signals in the setof EM-signals and a phase noise spectrum of each of the two different EM-signals.The method also comprises the step of determining, based on the measured phasenoise spectrum of the mixing products and based on the measured phase noisespectrum of each of the two different EM-signals, a correlated phase noise contributionto the phase noise spectrum and an uncorrelated phase noise contribution to thephase noise spectrum for each EM-signal in the set of EM-signals. The method alsocomprises the step of constructing a representation of the phase noise for the set ofEM-signals comprising the determined correlated and uncorrelated contribution to the phase noise for each EM-signal.

According to a second aspect there is provide a phase noise measuring device. Thephase noise measuring device comprising two separate signal inputs, a first input anda second input , the inputs being adapted to receive signals from a signal source. The device also comprises a signal mixer that is connected to the first input and to the3 second input. The device further comprises a unit for phase noise measurements. Theunit for phase noise measurements is connected to the first input, to the second inputand to the signal mixer. The device also comprises a processing unit for processingthe values of the phase noise measurements. The processing unit is connected to theunit for phase noise measuring. Each of the first and second inputs of the devicecomprises means for directing signals to the unit for phase noise measurements eitherdirectly over a first and second channel, respectively, or indirectly over a channelcomprising the signal mixer. The unit for phase noise measuring is adapted tomeasure the phase noise spectrum, Lun, of a first signal when receiving the first signalover the first channel from the first input. The unit for phase noise measuring is alsoadapted to measure the phase noise spectrum, Lwz, of a second signal when receivingthe second signal over the second channel from the second input. The unit for phasenoise measuring is further adapted to measure the phase noise spectra of the mixingproducts between the first and second signal when receiving the signal over a channelfrom the signal mixer. The unit for phase noise measuring is also adapted tocommunicate the outcome of the measurements to the processing unit to enable the processing unit to construct a representation of the phase noise.

According to a third aspect there is provided a processing unit connectable to a phasenoise measuring apparatus. The processing unit is configured to construct arepresentation of the phase noise for a set of electromagnetic signals, EM-signals,where the set comprises at least two EM-signals. The processing unit is configured toread values obtained from the phase noise measuring apparatus, the values beingrelated to measurements of a phase noise spectrum for each mixing product of twodifferent EM-signals in the set of EM-signals and a phase noise spectrum of each ofthe two different EM-signals. The processing unit is also configured to determine,based on the measured phase noise spectrum of the mixing products and based onthe measured phase noise spectrum of each of the two different EM-signals, acorrelated phase noise contribution to the phase noise spectrum and an uncorrelatedphase noise contribution to the phase noise spectrum for each EM-signal in the set ofEM-signals. The processing unit is also configured to construct a representation of thephase noise for the set of EM-signals that comprises the determined correlated and uncorrelated contribution to the phase noise for each EM-signal. 4 According to a fourth aspect there is provided a computer program comprisinginstructions, which when executed by at least one processor, cause the at least oneprocessor to: 5 -read values of the measured phase noise spectrum for each mixing product of two different EM-signals in the set of EM-signals and read values of the measured phasenoise spectrum of each of the two different EM-signals; -determine, based on differences between the measured phase noise spectrum of themixing products and based on the measured phase noise spectrum of each of saidtwo different EM-signals, a correlated phase noise contribution to the phase noisespectrum and an uncorrelated phase noise contribution to the phase noise spectrumfor each EM-signal in the set of EM-signals; -construct a representation of the phase noise for the set of EM-signals comprisingthe determined correlated and uncorrelated contribution to the phase noise for eachEM-signal.

According to a fifth aspect there is provided a computer program product comprisingthe computer program of the fourth aspect.

Embodiments of the proposed technology enables/makes it possible to obtain a morefinely grained information about the phase noise for a set of electromagnetic signals.This will in turn enable the development of better counter measures to reduce the negative effects of phase noise.

Other advantages will be appreciated when reading the detailed description.

Description of drawingsFig.1 provides, in a flow diagram, a method according to the proposed technology.

Fig.2 provides, in a flow diagram, an exemplary embodiment of the method accordingto the proposed technology. The flow diagram illustrating how a correlated anduncorrelated contribution to the phase noise is determined.

Fig. 3a shows an exemplary embodiment of a system that can be used for measuringthe phase noise spectra of the second order mixing products at frequencies of fi-fz and f1+f2, respectively.

Fig. 3b shows an exemplary embodiment of system that can be used for measuringphase noise spectra of the third order mixing products at frequencies of 2f1-f2 and2f1+f2, respectively. The phase noise spectra around 2f2-f1 and 2f2+f1 are measuredwith one mixer at the output signal of signal source 2.

Fig. 4 provides an illustrating outline of a system for measuring signals that can used to determine phase noise spectra from sample signals Fig. 5. ls a diagram showing the power spectra - i.e. the amplitude vs frequency - oftwo signals y1 and y2 of nominal frequency f1 and f2, respectively. Also shown is thepower spectrum of the mixed signal y1> Fig. 6. Provides a number of diagrams illustrating the phase noise spectra for anormalized amplitude vs. an offset frequency for two signals, y1 and yz. Also shown are phase noise spectra of the mixed signals, y1> Fig. 7 illustrates a particular embodiment of the proposed method in flow diagram form.lt is illustrated how a relative contribution of correlated and uncorrelated phase noise is obtained from measured phase noise spectrum.

Fig 8a provides a diagram illustrating a particular representation of the phase noisespectra of signals at f1 and f2, respectively.

Fig. 8b provides a diagram illustrating a particular representation of the phase noisespectra of the mixing products measured at frequencies f1-f2 and f1+f2, respectively.

Fig. 9a is a diagram illustrating a plot of the metrics p1 and pz vs offset frequency. Alsoshown is the noise limit of the measurement. At the offset frequency of 1 Hz, p1=1/p2and both pi and p: are above the noise limit. The multiplicative function k is determinedat these offset frequencies.

Fig. 9b is a diagram illustrating a representation of the phase noise comprising thecorrelated phase noise contribution, that is, the contribution that is common to the 6 signals at f1 and fz, and the uncorrelated part of the phase noise of the signal at f1 andfz, respectively.

Fig. 10 is a diagram illustrating a representation of the phase noise in the form of theratio of the coherent phase noise to the total phase noise of signals at frequency fi and fz, respectively.

Fig. 11 is a block diagram illustrating an embodiment of a phase noise measuringdevice according to the proposed technology.

Fig.12 is a block diagram illustrating an exemplary embodiment of a phase noisemeasuring device according to the proposed technology. lt is illustrated how switchesare used to separate the measurement of the phase noise spectrum of two different signals, and of mixed signal related to the different signals.

F ig.13 is a block diagram illustrating another embodiment of a device according to theproposed technology. lt is illustrated how splitters are used to separate themeasurement of the phase noise spectrum of two different signals, and of mixed signal related to the different signals.

Fig. 14 is a block diagram illustrating an embodiment of the proposed technology. lt isshown how a phase noise measuring device may be connected to a processing unit and an optional display.

Fig. 15 is a block diagram illustrating an embodiment of the processing unit accordingto the proposed technology. The processing unit comprises at least one processor andcorresponding memory. The processing unit may be lncorporated as a part in a phase noise measuring device according to the proposed technology.

Fig. 16 is a block diagram providing an alternative embodiment of a processing unit,here the processing unit also comprises a communication circuit which will enable theprocessing unit to obtain data or information relating to performed measurements andcommunicate the results of the processing performed on the obtained measurementdata.

Fig. 17 is a block diagram illustrating how a processing unit can utilize a computerprogram and computer program product according to the proposed technology in order to process measurement data.7 Detailed description of the invention ln new technologies in telecommunication and satellite communication transmittersand receivers (transceivers) are used in which multiple input and multiple output(MIMO) signals are transmitted and received simultaneously. Some of thecomponents are shared by the multiple signals. The signals can have the same ordifferent nominal center frequency. ln many transceivers today, digital signalprocessing methods are used to compensate for hardware impalrments, such asnonlinear effects, additive noise, memory effects, and cross talk between differentchannels. The correlation properties of the phase noise of the different signals mayaffect how well hardware impairments may be compensated for. ln today's techniquesit is assumed that the phase noise between different channels is either perfectlycorrelated or totally uncorrelated. However, it would be beneficial to be able to quantifythe degree of correlation of the signals in different channels. Some examples are: - When designing algorithms for digital pre distortion of MIMO transmitters forcompensating nonlinear effects, the phase noise correlation properties should be taken into account - When designing equalization algorithms for receivers for multiple signals the phase noise correlation properties should be taken into account.

- When designing so called phase noise trackers that are used in receivers to compensate for phase noise.- ln the development of oscillators or signal generators for MIMO applications.

When testing oscillators and signal generators that are to be used in MIMOtransceivers for phase noise.

Another example is in satellite communication where signals of different centerfrequency go through one satellite transponder. The phase noise also affects theperformance of the digital processing algorithms of the transmitters and receivers,hence a solid knowledge about a phase noise representation is highly beneficial for countering or reducing the negative impact of phase noise. 8 The proposed method and the proposed devices may therefore be seen as methodsthat enables the above mentioned designs and enables a reduction of the negative effects of phase noise. ln the present invention two or more electromagnetic signals are analyzed. The signalscan have the same or different nominal frequency. The signals can be the output of signal generators or oscillators with multiple output channels. ln the invention the phase noise of the signals are separated in a correlated part thatis shared by the signals and an uncorrelated part that is different for the differentsignals. The correlated and uncorrelated parts are random variables. The correlated part is multiplied with a constant that is not a random variable.

The invention presents a method for measuring the correlated and uncorrelated partsand multiplicative parameters of the phase noise of a number of signals. ln the invention phase noise of single signals should be measured by one of manyexisting techniques. Such measurements are standard in spectrum analyzers or signal analyzers. Alternatively, instrument that samples the signals coherently could be used.

The phase noise spectra of the signals are measured. The signals are also mixed andthe phase noise spectra are measured around the mixing frequencies (i.e. the sumand difference frequencies). The measured phase noise spectra are processed insuch a way that the correlated and uncorrelated parts of the signals and themultiplicative parameter, are determined. Different measures for partial correlated phase noise of the signals are included in the invention.

Before providing a detailed description of various embodiments it may be beneficial toprovide some details about the terminology used as well as the notation used. To this end we begin by providing some explanatory wordings regarding certain used terms.

With phase noise spectra is generally intended the one-sided power spectra of asignal. The signal may be normalized by the mean power and given as a function of an offset frequency that is relative to the nominal frequency.

With mixing product is generally intended a signal component that is the result of themultiplication - or mixing - of one or more signals. Fig.3a and Fig.3b as well as Figs. 5 and 6 provides illustrations of mixing products.

With mixing frequency is generally intended the frequencies that are present in the inthe product of multiplication, or equivalently mixing, of signals. lf two signals, one withthe nominal frequency m1 and the other of nominal frequency wz, are mixed there willbe frequencies at m1- wz and w1+w2 in the product signal. lf the first signal is mixedwith itself and then mixed with the second signal, there will be frequency componentsat Zool-wa and 2 w1+w2 in the product signal. Note that in the present applicationfrequencies is denote with f as well as uu. The only difference is that the frequency represented with f is related to frequency represented with w by means of: f= 2Trw.

With correlated phase noise is generally intended the part of the phase noise of twosignals that are statistically the same. Correlated phase noise can only be identified ifthere are at least two signals. With uncorrelated phase noise is generally intended thepart of the phase noise of a signal that is statistically independent from the phase noiseof another signal. Also uncorrelated phase noise can only be defined if there are at least two signals.

With representation of phase noise is generally intended that the phase noise of asignal can be expressed as composed of different components. These differentcomponents are themselves phase noise and each of them can be described by aphase noise spectrum or any other way of describing phase noise. Hence aconstruction of a representation of phase noise that comprises different componentsrefers to some way of providing an output of the phase noise, in the form of a phase noise spectrum or some other suitable description, such as a time domain description. ln this application the phase noise of N different electromagnetic signals is considered where N is equal to or larger than 2. The signals will generally be denoted with y1(t) , y2(í) and yN (f), and will, in the time domain, be written as: y1(t): A1 COSQÜII T 991yz (f) = A2 <><>S(w2f + wo) yN (t) = AN cos(cuNt + çøN (t))whereco, = Zfrf, with fi being the nominal frequency of signal 1 And A1 denotes the amplitude of the signal. The variables (01 (02 (f) , (DN (i) denote the random variables that describes the phase noise of each signal. .

The phases are random variables and their relations can be described by statisticalmeasures. We write the phases of the signals in a particular representation as vi (f) = (ß. (f) + n., (f),v20) = kz (f) * w. (f) + (at. (f), CÛNU)=kN(f)*(0C(f)+Q1N,u(f)»i.e. the phases ç0,(t),..., çøN(t) are composed of an uncorrelated part ¶0L,,(I),....,¶0N,,,(í), and a correlated part (060). The correlated phase noise of the signals çøc(t) is convolved by deterministic multiplicative functions IQU) ,...,kN(t) where * denotesconvolution. Here the phase noise w, (t) is used as a reference for the correlated part, i.e. for the functions 160). lt is arbitrary which of the signalsçq(t),..., çøN(t) in a set ofsignals that is used as a reference. The frequencies are arbitrary: f1 could be higher,lower, or the same frequency as fz, etc. ln a frequency domain representation the phases of the signals are instead given by:<0l(w)=@(w)+n,.,(w), (92 : kz X (90 "l" (924, (m), (ÛN z krv X WC + CÛNJ) (WL1 'l where ço1(a2) is the Fourier transform of çøl (t), etc.

The phase noise spectra of the various signals are given by lçø1(w)l2, |ç0^,(w)|2. Thephase noise spectra of the uncorrelated parts of the signals are in turn given by lçou,(w)|2 ,...,lçøN_u(w)|2 and finally the phase noise spectra of the correlated part of the signals is given by |çpc(cu)lz. |k2l2 çøc(w)lz, ...,|kNl2|@c(a>)|2.

The proposed technology aims to provide a method wherein a novel representation ofthe phase noise for a set of EM-signals is obtained from particular measurements ofthe phase noise. As such the method may also be seen as a phase noise measuríngmethod where the correlated part of the phase noise is determined as well as theuncorrelated part of the phase noise. The inventors have realized that the phase noisefor a set of EM-signals can be measured and provided in a representation where thecorrelated part of the phase noise is separated from the uncorrelated part of the phasenoise. Here the term correlated is used with the meaning that it is a random variablethat is the same for the two signals. The uncorrelated parts are statisticallyindependent of each other. Signals that are statistically identical can also be referredto as coherent; coherent signals are therefore correlated. ln the same way non- coherent signals are uncorrelated.

According to the proposed technology there is thus provided a method for constructinga representation of the phase noise for a set of electromagnetic signals, EM-signals,where the set comprises at least two EM-signals. The method comprises the step S1of measuríng a phase noise spectrum for each mixing product of two different EM-signals in the set of EM-signals and a phase noise spectrum of each of the twodifferent EM-signals. The method also comprises the step S2 of determining, basedon the measured phase noise spectrum of the mixing products and based on themeasured phase noise spectrum of each of the two different EM-signals, a correlatedphase noise contribution to the phase noise spectrum and an uncorrelated phasenoise contribution to the phase noise spectrum for each EM-signal in the set of EM-signals. The method also comprises the step S3 of constructing a representation ofthe phase noise for the set of EM-signals comprising the determined correlated and12 uncorrelated contribution to the phase noise for each EM-signal. The method isschematically illustrated in the flow diagram of Fig.1.

The method as stated above may, as has been stated, equally well be viewed as amethod for measuring the phase noise of a set of EM-signals, the set comprising atleast two signals, where the output of the method provides phase noise in arepresentation or form where the correlated and uncorrelated contributions has beenextracted. There is in other words provided a method where measurements of thephase noise spectrum for two different EM-signals are performed in a step S1. Themeasurements are performed for the individual EM-signals as well as for the mixingproducts of the two different EM-signals. The output of the measurements, i.e. thephase spectrum values obtained for the individual EM-signals as well as for the mixingproducts of the two different EM-signals, provide the input to a step S2 that isperformed in order to determine both the correlated phase noise contribution to thephase noise spectrum as well as the uncorrelated phase noise contribution to thephase noise spectrum. Having obtained the value of the correlated and uncorrelatedcontribution, the method proceeds and constructs, in a step S3, a representation ofthe phase noise pertaining to the different EM-signals that explicitly comprise thecorrelated and uncorrelated contributions phase noise contributions to the phasenoise. That is, the phase noise of the signals is, according to the proposed method,separated into a correlated part that is shared by the signals and into an uncorrelatedpart that is different for the different signals. The fact that the method provides arepresentation of the phase noise that explicitly highlights the correlated part anduncorrelated part of the phase noise make the method a suitable first step fordesigning counter measures to either reduce the phase noise or to compensate for the phase noise when transmitting signals. lt should be noted that the measurements of the phase noise of each signal in the setof signal or of the phase noise at the mixing products of the mixed signal may be doneusing any of a number of existing techniques for performing phase noisemeasurements. Such measurements are standard in spectrum analyzers or signalanalyzers. Still another alternative for performing the measurement is to use instruments that samples signals coherently. 13 A possible embodiment of the proposed method, provides a method wherein the stepS1 of measuring the phase noise spectrum for each of the two different EM-signalscomprises to measure the phase noise value at an at least one offset frequency in thevicinity of a nominal frequency m1 of a first EM-signal and the phase noise spectrumvalue at least one offset frequency in the vicinity of a nominal frequency wz of a second EM- signal.

Put in slightly different words, according to the proposed embodiment, phase noisemeasurements should be performed on offset frequencies having values close to thenominal frequency values of the different EM-signals. Measurements should beperformed on at least one offset frequency, preferably on a number of offsetfrequencies having values close to the nominal frequency of the different ElVl-signals.The offset frequencies selected for measurements may differ slightly between the twoEM-signals, but it is preferred if they are the same within some error margin. Theoutput of this step is that a number of phase noise spectrum values are obtained, onefor each chosen offset frequency. These obtained values provide part of the input toa subsequent step of determining the correlated and uncorrelated contribution to thephase noise. The other input is the corresponding measurements of the mixingproduct of the two EM-signals. These measurements are, according to anotherpossible embodiment of the proposed method, provided by step S1 of measuring thephase noise spectrum for each mixing product of two different EM-signals wherein thestep S1 comprises to measure the phase noise spectrum value of the mixing productsfor at least one offset frequency in the vicinity of a first mixing frequency nw1+mw2 andat least one offset frequency in the vicinity of the second mixing frequency nwl-mwz,m1 is the nominal frequency of the first EM-signal and wz is the nominal frequency ofthe second EM-signal.

The offset frequencies selected for measurements may differ slightly between thedifferent mixing frequencies nw1+mw2 and nwr-mwz, but it is preferred if they are thesame within some error margin. lt may also preferred if the offset frequencies arechosen so as to more or less coincide with the offset frequencies selected formeasuring the phase noise of the individual EM-signals around their nominal values 14 wi and wz. The output of this step is that a number of phase noise spectrum valuesare obtained for offset frequencies having values around the chosen mixingfrequencies, one for each chosen offset frequency. These values provide, togetherwith the corresponding values of the individual EM-signals around their nominalfrequency wi and wz, the input for determining the correlated and uncorrelatedcontribution to the phase noise. According to a particular version of the abovedescribed embodiment the values of n and m may be chosen to be, n=1 and m=1.These optional values yields measurements that are particularly simple to perform.

According to a particular embodiment there is provided a method wherein the step S2of determining a correlated phase noise contribution to the phase noise spectrum andan uncorrelated phase noise contribution to the phase noise spectrum for each EM-signal comprises the steps of: - computing S21, for a combination of two different EM-signals in the set of EM-signals, a difference between a phase noise spectrum value as measured at an offsetfrequency in the vicinity of the first mixing frequency and a phase noise spectrum valueas measured at an offset frequency in the vicinity of the second mixing frequency;-creating S22, for the combination of two different EM-signals, a first metric and asecond metric based on the computed difference; -obtaining S23, for the combination of two different EIVI-signals, a multiplicativeparameter k based on the created first and second metric, -extracting S24, for the combination of two different EM-signals, the correlatedcontribution and the uncorrelated contribution of the phase noise based on anexpression relating the measured phase noise spectrum value of each of the twodifferent EM-signals, the computed difference and the obtained multiplicativeparameter k. This particular embodiment is schematically illustrated in the flowdiagram of Fig.2.

This particular embodiment provides for a method where the correlated phase noisecontribution to the phase noise spectrum and an uncorrelated phase noise contributionto the phase noise spectrum is determined from the obtained measurement values bymeans of a sequence of steps. Various embodiments of these steps will be provided in what follows.

According to a particular embodiment of the above there is provided a method,wherein the step S21 of computing the difference comprises to compute a differencebetween the measured value of phase noise spectrum at an offset frequency in thevicinity of the first mixing frequency, nwfimwz, and the measured value of the phasenoise spectrum at an offset frequency in the vicinity of the second mixing frequencyvalue nwi-mwz, where n and m are numbers, preferably integers and wi and wz are the nominal frequencies of the two different EM-signals. lt should be noted that the difference may be computed for each single distinct offsetfrequency, hence if, for example, four offset frequencies are used, there will be fourdifferent computed differences.

By way of example, the proposed technology provides another embodiment of amethod wherein the step S22 of creating first and second metrics based on thecomputed difference comprises to create a first metric, defined as a ratio between thecomputed difference and the value of the phase noise spectrum as measured at atleast one offset frequency in the vicinity of the nominal frequency value wi of a firstEM-signal, and a second metric defined as a ratio between the computed differenceand the value of the phase noise spectrum as measured at at least one offset frequency in the vicinity of the nominal frequency value wz of a second EM-signal.

According to another possible version of the method there is provided an embodiment,wherein the step S23 of obtaining a multiplicative parameter k for the combination oftwo EM-signals in the set of El\/l-signals comprises to compare the value of the createdfirst metric with the value of the inverse of the created second metric and set theparameter k to the value 1 if the comparison yields that the difference between thevalue of the first metric and the value of the inverse of the second metric fulfills a predetermined criterion.

The predetermined criterion may, for example, be that a difference between the firstmetric and the inverse of the second metric is above a certain number. lf the difference between the first metric and the inverse of the second metric is above a certain 16 number, i.e. an error margin, than it is an indication that the phase noise of the twodifferent signals are uncorrelated. An additional and optional criteria for setting k = 1 is that the values of the first and second metric is above the noise floor level.

According to another version of the proposed method, the step S23 of obtaining amultiplicative parameter k for the combination of two different EM-signals in the set ofEM-signals comprises to compare the value of the created first metric with the valueof the inverse of the created second metric and set the parameter k to the value of thefirst metric if the comparison yields that the difference between the value of the firstmetric and the value of the inverse of the second metric fulfills a predetermined criterion.

The predetermined criterion may, for example, be that a difference between the firstmetric and the inverse of the second metric is below a certain number. lf the differencebetween the first metric and the inverse of the second metric is below a certainnumber, i.e. an error margin, than it is an indication that the phase noise of the twodifferent signals are correlated. An additional and optional criteria for setting k to thevalue of the first metric, or equivalently, to the value of the second metric is that the values of the first and second metric should be above the noise floor level.

A particular embodiment of the proposed method comprises a method, wherein thestep S24 of extracting the correlated contribution and the uncorrelated contribution ofthe phase noise is based on an expression relating the measured phase noisespectrum values as measured at an at least one offset frequency in the vicinity of thenominal frequencies m1 and m2 of the two different EM-signals, respectively, thedetermined difference between the phase noise values as measured at offsetfrequencies in the vicinity of the mixing frequency nw1-mw2, n and m being integers,or at offset frequencies in the vicinity of the mixing frequency nwl-mwz, to a linearcombination of obtained value of k and the correlated part and uncorrelated part of the phase noise for the two distinct El\/l-signals.

A particular version of the embodiment described above provides a method whereinthe expression is given by the following matrix equation: 17 2 Lwl 1 1 0 wc 2 Lab = lklz o 1 low2 LMZ l1-kl 1 1 løzauz where Lwl corresponds to the measured phase noise spectrum at the nominalfrequency value of the first ElVl-signal, Lwz corresponds to the measured phase noisespectrum of the nominal frequency value of the second ElVl-signal Lwkwzcorresponds to the measured value of the phase noise spectrum at the mixingfrequency wi-wz and where çoc denotes the correlated contribution of the phase noisebetween the first and second signal, om denotes the uncorrelated contribution of thephase noise from the first EM-signal and om denotes the uncorrelated contribution of the phase noise from the second EM-signal.

The proposed technology also provides a method according wherein the method stepsare repeated for each of a plurality of offset frequencies in the vicinity of the mixingfrequencies. This may be done in order to generate a more detailed representation ofthe phase noise for two different EM-signals, where the phase noise comprises thedetermined correlated as well as the uncorrelated contribution to the phase noise foreach EM-signal.

That is, the earlier described steps of the methods are performed for each of a pluralityof chosen offset frequencies in order to generate a representation of the phase noisespectrum for two different EM-signals that comprises the determined correlated anduncorrelated contribution to the phase noise for each EM-signal. So in the illustrativeand exemplary case where four offset frequencies are used, the method step areperformed for each of these four frequencies separately.

The method may be applied to obtain a representation of the phase noise for a largeset of EM-signals. To this end there is provided an embodiment wherein the steps ofthe method are repeated for every combination of two different EM-signals in the setof EM-signals. By repeating the method for every combination of two distinct signals in the set of ElVl-signals it will be possible to obtain a representation of the phase noise 18 spectrum for all EM-signals in the set of EM-signals that comprises the determined correlated and uncorrelated contribution to the phase noise.

That is, the earlier described method steps are performed for every permutation of twosignals in the set of EM-signals in order to construct a representation of the phasenoise that is representative for all signals in the set signals. The method steps should,in the illustrative and exemplary case with three signals, y1,y2 and ya, be performedseparately for the following combinations: y1y2,y1ya and yzya in order to obtain arepresentation of the phase noise that is representative for the three signals.

Having described various embodiments of the proposed method, in what follows wewill provide more detailed illustrative examples of the earlier described embodiments.These examples are intended to be illustrative to facilitate the understanding of theinvention and they should not be considered to limit the scope of the invention. ln this example we will consider the case where the set of signals comprises twodistinct signals, a first signal and a second signal. The first signal has a nominalfrequency un, and the second signal has a nominal frequency m2. The mixing productdefines, in this particular example, corresponding mixing frequencies w1+ wz and wi-wz. We will further consider the case with one offset frequency A, that is, a singlemeasurement is performed on an offset frequency A in the vicinity of the nominalfrequency un, the nominal frequency wz and the mixing frequencies w1+ wz and w1- M2. ln the formulation any signal can be used as a reference signal. The other signals areanalyzed relative to the reference signal. ln the first step S1 of the method the phasenoise spectra of the respective signals and their mixing products are measured. Apossible set up for measuring the phase noise spectra of mixing products are shownin Fig. 3a and Fig.3b. The latter drawing illustrating how higher order mixing productscan be obtained.

The phase noise spectra of the two signals are measured using any of the methodsknown for measuring the phase noise of one signal. The phase noise spectra is 19 denoted as Lml and Lmz for signal 1 and 2, respectively. The signals are then mixed to create mixing product at the frequencies f1+ fz and f1.f2 (or equivalently un and wz,with w=2trf). multiplication of two signals. Preferably a mixer should be used that has additive phase Mixing is, as has been stated earlier, equivalent to an analogue noise that is lower than the phase noise of the signals to be measured. The set up forcreating and measuring the phase noise of the mixing products is shown in Fig. 1. We denote the measured phase noise spectra at f1+ fz and f1-f2, L and L Wai, Wu, 1respectively. Fig. 5 and Fig. 6 illustrate the signals that are used in frequency domain.ln Fig. 5 are the power spectra of two signals at nominal frequency f1 and fz illustrated.The amplitude is shown vs the frequency. The spectra are broadened around thenominal frequencies fl and fz. Also shown is the power spectrum of the mixed signaly1 x yz. This signal has frequency components at two frequencies, f1-f2 and f1+f2,respectively. ln Fig. 6 it is illustrated how the signals in Fig. 5 are represented as phasenoise spectra. ln these examples are the amplitudes normalized and the amplitude isgiven vs the offset frequency. For the mixed signals two phase noise spectra are measured, one at the mixing frequency f1-f2 and one at the mixing frequency f1+f2.

Having obtained the values of the measured phase noise spectra of the two EM-signals around their nominal frequencies and the values of the measured phase noisespectra around the different mixing frequencies by means of a first step S1, the methodproceeds and determines, based on the measured phase noise spectrum of the mixingproducts and based on the measured phase noise spectrum of each of the twodifferent EM-signals, a correlated phase noise contribution to the phase noise spectrum and an uncorrelated phase noise contribution to the phase noise spectrum.

The correlated phase noise contribution to the phase noise spectrum and theuncorrelated phase noise contribution to the phase noise spectrum are in thisparticular example determined based on differences between the measured phasenoise spectrum of the mixing products on the measured phase noise spectrum of eachof said two different EM-signals. To this end, in a step S21 of the method is the difference, Lwrmz - Læfiwz, of the phase noise spectrum around the different mixing frequencies is computed. The difference contains only the correlated phase noise. ln this particular example, with a single offset frequency, this amount to a single computation yielding a single value.

Fig. 7 provides a flow diagram that illustrates an example of the process for obtaininga phase noise representation according to the proposed technology.

Fig. 8a illustrates the spectra for Lwl and Lmz while Fig. 8b shows the spectra for Lmrwz and L According to the particular example of the proposed method, the difference Larm: - Lwæz , should be divided by a factor that is proportional to the corresponding phase noise spectrum value around the different nominal frequencies. The proportionalityfactor is preferably set equal to four. This creates a first metric p1 and a second metricpz, explicitly given by: L - L p __ Ûffwz Û)|"Û)21 _ 74Lwlå) 'FHI __ LQ) “fUp ___ l 2 l 22 .__4Lw2 Fig. 9a shows the functions pl and p2 as obtained from the phase noise spectra in Figs 8a and 8b. The metrics may also be obtained from the phase noise spectra ofhigher order mixing products. For example the numerator could be the differencebetween the spectra at mixing frequencies 2f1+f2 and 2f1-f2. lt could also be thedifference between the phase noise spectra at mixing frequencies 2f2+f1 and 2f2-f1. Orin general at mixing frequencies mf1+nf2 and/or mfi-nfz. To ensure a positivity of themixing frequency the expression can be read as abs (mf1+nf2) or abs (mf1-nf2) whereabs denotes the absolute value of the expression.

The created metrics obey the following equalities: 2 k w.2 2 ° (Se)wc + øLu P1 where k is a muitiplicative parameter, lçnulafllz and lça2,u(a>)|2 denotes the phase noise spectra of the uncorrelated parts of the different EM-signals and where the correlatedpart of the signals is given by lçoc(w)lz. From these expressions it is possible to obtain, in a step S23, the correlated and uncorrelated parts of the phase noise in two differentways. The first way demands that the signal source is well known, whereby themuitiplicative parameter may be known a priori. Typically k=1 or k=f2/f1. ln the casewhere k is known a priori it is possible to extract the correlated and uncorrelatedcontributions to the phase noise from expressions relating the correlated and uncorrelated contributions with the parameter k and the measured phase noise spectra. A particular example of such an expression is given by the matrix equation:2Lq 1 1 0 (DC1; - få o 1 2æz _ I T 2 *(01312Lat-w. ll-kl 1 1 lat., as described earlier.

According to the second way frequencies where p, w 1/p2 are identified. At these frequencies the phase noise is correlated between the two different signals. It ispossible to obtain the muitiplicative function k by looking at these frequencies. Hence, if there is no frequency where the approximate equality p, wl/pz holds, the phase noise of the two signals is deemed to be mostly uncorrelated. k=1 is used in the proceeding process. A criterion may be formulated that p, =1/p2 with a specified error, e.g. to within 10%. Hence k can be determined as follows: -Compare the value of the created first metric pl with the value of the inverse of thecreated second metric pz, 1/p2. - lf the comparison yields that the difference between the value of the first metric andthe value of the inverse of the second metric fulfills a predetermined criterion then theparameter k should be assigned the value 1.

The criterion might, as a non-limiting example, be that the difference between thevalues should be more than 10 %. Other values are possible. 22 -lf the comparison on the other hand yields that the difference between the value of the first metric and the value of the inverse of the second metric fulfills anotherpredetermined criterion, than the value of k should be set to either the value of the firstmetric, or to the value of the inverse of the second metric. The criterion might, as a5 non-Iimiting example, be that the difference between the values is less than 10 %.This particular way to obtain the multiplicative parameter k utilizes that the metricsprovide a measure of the amount of correlated phase noise between the different EM- signals, that is, a measure of the correlation between the first and second EM-signal.

That is the multiplicative constant k can be obtained from identifying pl =k and 10 pz =1/k.

Having obtained a multiplicative parameter k based on the created first and secondmetric in a step S23. lt is possible to extract or estimate, in a step S24, the correlatedcontribution and the uncorrelated contribution of the phase noise based on an 15 expression relating the measured phase noise spectrum value of each of the twodifferent EM-signals, the computed difference and the obtained multiplicativeparameter k.

According to a particular embodiment of the proposed method it is possible to extract20 the correlated contribution and the uncorrelated contribution of the phase noise fromthe following system of Iinear equations. For each n =0,i1,+ 12,... and m=1, 11, i2,...(except for the case where n=0 and m=0) a Iinear equation is formed.2 (ßVkic) ' L =ln+mkl2løc 2 +m2yçøm nwfimæz 2 2+ n løLu Here çøc(c0)|2, lçßlgfiáfllz and lçøzfljaàlz denotes the correlated and uncorrelated 25 contributions to the phase noise spectra . The phase noise spectra |¶pc(a,)|2, q,l,”(w)|2 and |¿,,2_”(w)|2 are all positive quantities. The system of equations can be solved using standard solvers.

The system of Iinear equations given above can be written on matrix form as: 2 (PcL = M int, lçfllu 2 2 Where L is a column vector with Lmvfimwz . M is matrix in which each row is given by: hn-rm/:IZ m2 m2_|. 5 For the case of n=1 and m=1 in the first row, n=0 m=1 in the second row and n=1 and m=~1 in the third row one get a particularly simple and preferred system of equations: Lwl 1 1 0 pc 22Lmz = 2 0 1 lfflhu (wM-x)2Lfüt-wz _ 1 1 ÄçÛLu 10 By solving the equation (***) or the simplified version (****) the correlated anduncorrelated contributions to the phase noise can be extracted.Having extracted the correlated and uncorrelated contributions to the phase noise it ispossible to construct, in a step S3, a representation of the phase noise for the set ofEM-signals where the representation comprises the determined correlated and 15 uncorrelated contribution to the phase noise for each EM-signal.

That is, a phase noise representation in the frequency domain may be written in terms of the uncorrelated parts of the signals lçphumfllz and|çp2,u(w)l2, and the correlated part of the signals çøc(w)l2.

A representation of the phase noise spectra |cpc(w)[2, lçø1,u(w)l2 and lçøujmlz as determined from the data in Figs. 8a and 8b and Fig 9a, are shown in Fig. 9b. lt is also possible to compute the ratio of correlated phase noise and the measured 25 phase noise spectra. These ratios provides measures of the part of the phase noise24 of the respective signals that are uncorrelated and correlated, respectively. Theseratios are between zero and one. An example of a representation provided by suchratios are shown in Fig. 10.

As an alternative to the earlier described it should be noted that it is possible that thesignals are measured and digitized individually, see Fig. 4 for an illustration. The signals should preferably be coherently sampled. The digitized signals are denotedy1(n) and y2(n). The mixing product y](n)> Fourier transform is calculated of the signals y,(n), y2(n) and yl(n)> spectra may then be calculated from the discrete Fourier transform. The powerspectral density may be normalized by means of the signals power. The phase noisespectra are determined around the frequencies f1,f2, f1 -fz and f1 + fz.

When the signals are directly digitized, see Fig. 4, it is possible also the calculate thediscrete Fourier transform of the signals 1 and 2. These signals are then amplitude normalized and shifted to the offset frequency. These new signals are denoted aredenoted Yln and Yzn. From these we can calculate the phase noise spectra L... = Yann* Lwz "_" Yznyzn* where * denotes the complex conjugation.

The sample rate in the measurement of the signals would preferably be two times thefrequency f1 + fz in order to avoid aliazing. lt is however possible to use undersamplingand get the sample signals with the frequency components at aliased frequencies.

Having described various examples of the method for determining, or equivalently,constructing a representation of the phase noise that comprises correlated anduncorrelated contributions, in what follows, we will describe various devices andimplementations of the proposed technology. The devices and implementationsprovides the same advantages as the methods described.

The proposed technology also provides a device that is adapted to perform themeasurements used to construct a phase noise representation comprising thecorrelated and uncorrelated contributions to the phase noise. Or, equivalently, adevice adapted to perform measurements to determine the correlated and uncorrelated contributions to the phase noise.

Fig 11 is a block diagram illustrating such a phase noise measuring device 100. Thephase noise measuring device comprising two separate signal inputs, a first input 110and a second input 120, the inputs 110, 120 being adapted to receive signals from asignal source 105, 106. The signal source may be a single signal source, e.g. signalsource 105, that is dedicated to both outputs but the outputs may also be fed by signalsfrom different signal sources 105, 106. The device 100 also comprises a signal mixer200 that is connected to the first input 110 and to the second input 120. The devicefurther comprises a unit 130 for phase noise measurements. The unit 130 is connectedto the first input 110, to the second input 120 and to the signal mixer 200. The devicealso comprises a processing unit 150 for processing the values of the phase noisemeasurements. The processing unit 150 is connected to the unit 130 for phase noisemeasuring. Each of the first 110 and second 120 inputs of the device 100 comprisesmeans 145 for directing signals to the unit 130 for phase noise measurements eitherdirectly over a first and second channel, respectively, or indirectly over a channelcomprising the signal mixer 200. The unit 130 for phase noise measuring is adaptedto measure the phase noise spectrum, Lun, of a first signal when receiving the firstsignal over the first channel from the first input 110. The unit 130 for phase noisemeasuring is also adapted to measure the phase noise spectrum, Lwz, of a secondsignal when receiving the second signal over the second channel from the secondinput 120. The unit 130 for phase noise measuring is further adapted to measure thephase noise spectra of the mixing products between the first and second signal whenreceiving the signal over a channel from the signal mixer 200. The unit 130 for phasenoise measuring is also adapted to communicate the outcome of the measurementsto the processing unit 150 to enable the processing unit 150 to construct a representation of the phase noise.

According to a particular embodiment, there is provided a phase noise measuringdevice 100, wherein the means 145 for directing signals to the unit 130 for phase noisemeasurements comprises a signal splitter 145a for splitting a signal received from asignal source 105, 106 so that part of the signal is directed toward the unit 130 forphase noise measuring and another part of the signal is directed toward the signalmixer 200. This particular embodiment is illustrated schematically in Fig.13. The signalsplitter 145a on the first input 110 is adapted to provide part of the signal along pathAo towards the unit 130, and part of the signal along the path As towards the signalmixer 200. The signal splitter 145b on the second input 120 is instead adapted toprovide part of the signal along path Bo towards the unit 130, and part of the signalalong the path Bs towards the signal mixer 200.

A signal from signal source 105 is therefore split into two parts by a splitter, or adirectional coupler, 145a. The signal of output A0 goes directly to the unit 130 formeasuring phase noise of this signal, Lun. A signal from signal source 106 may alsobe split into two parts by the splitter 145b. The signal of output Bo goes directly to theunit 130 for measuring phase noise of this signal, Lwz. The signals of output A3 andBa go to the mixer 200 and the mixed signal goes from the mixer 200 to the unit 130for measuring phase noise. ln this particular case, where the nominal frequencies ofthe signals are wi and wz, the unit 130 for measuring phase noise will measure mixed products around wi- wz and m1 + wz.

Another possible embodiment of the proposed device provides an alternative signaldirecting means, wherein the means are given by a switches arranged on the inputs105, 106.

According to this embodiment there is provided a phase noise measuring device 100,wherein the means 145 for directing signals comprises a first switch A dedicated tothe first input 105 and a second switch B dedicated to the second input 106. The firstswitch A is adapted to switch between at least two different states, a first state A1where a signal is directly transferred to the phase noise measuring unit 130 and astate A2 where a signal is transferred to the phase noise measuring unit 130 over the signal mixer 200. The second switch B is also adapted to switch between at least two 27 different states, a state B1 where a signal is transferred directly to the phase noisemeasuring unit 130 and a state Bz where a signal is transferred to the phase noisemeasuring unit 130 over the signal mixer 200. The unit 130 for phase noise measuringcomprises, in this particular embodiment, at least three detection channels. A firstdetection channel dedicated to receive signals transferred directly from the first input,a second detection channel dedicated to receive signals transferred directly from thesecond input and a third detection channel dedicated to receive signals transferreddirectly from the signal mixer. The unit 130 for phase noise measuring is in thisembodiment adapted to measure the phase noise spectrum, Lun, of a first signal whenthe first switch A is in a first state A1 and the signal is received in the first detectionchannel. The unit 130 for phase noise measuring is in this embodiment also adaptedto measure the phase noise spectrum of a second signal, Lwz, when the second switchB is in state B1 and the signal is received over the second detection channel. The unit130 for phase noise measuring is in this embodiment also adapted to measure thephase noise spectra of mixing products of a first and second signal when the firstswitch A is in state A2, the second switch B is in state Bz and the signal is receivedover the third detection channel.

Still another embodiment of the proposed device is illustrated in FIG 12. This figureillustrates a phase noise measuring device 100, wherein the device comprises a thirdswitch C, that is arranged between said unit 130 for phase noise measurements andsaid first switch A and second switch B. See FIG 3a. The third switch C is adapted tooperate in at least three different states, a state C1 where the unit 130 for phase noisemeasurements is connected to the first switch A, a state Cz where the unit 130 forphase noise measurements is connected to the second switch B and A state Cs wherethe unit 130 for phase noise measurements is connected to the signal mixer 200. Theunit 130 for phase noise measuring is in this embodiment adapted to measure thephase noise spectrum, Lw1, of a first signal when the first switch A is in a first positionA1 and the third switch C is in position C1. The unit 130 for phase noise measuring isin this embodiment also adapted to measure the phase noise spectrum of a secondsignal, L wz, when the second switch B is in position Bi and the third switch C is inposition Ca. The unit 130 for phase noise measuring is in this embodiment furtheradapted to measure the phase noise spectra of mixing products of the first and second 28 signal when the first switch A is in position A2, the second switch B is in position Bzand the third switch C is in position Cz. ln this particular case, where the nominal frequencies of the signals are m1 and wz,the unit 130 for measuring phase noise will measure mixed products around wi- wz and w1+w2.

The phase noise measuring device 100, as described in the embodiments providedabove comprises a processing unit 150. ln what follows we will describe variousembodiments of such a processing unit 150. The embodiments to be described mayhowever also be viewed as providing a processing unit that is configured to performthe earlier described and proposed method for constructing a representation of thephase noise for a set of electromagnetic signals, EM-signals, comprising at least twoEM-signals. As such it is optional whether the phase noise measuring device providingthe relevant phase noise spectrum values is the phase noise measuring device asdescribed above or some other device capable of providing the relevant measurementdata. Fig.14 illustrates how a phase noise measuring device 100 may be connectedto an external processing unit 10, 150, that comprises a memory 124 and at least oneprocessor 122 as well as a communication circuit 126. lt is also shown how theprocessing unit may be connected to an optional display unit 160 adapted to provide a visual representation of the determined phase noise. lt is therefore provided a processing unit 10, 150 connectable to a phase noisemeasuring apparatus. The processing unit 10, 150 is configured to construct arepresentation of the phase noise for a set of electromagnetic signals, EM-signals,where the set comprises at least two EM-signals. The processing unit 10, 150 isconfigured to read values obtained from the phase noise measuring apparatus 100,the values being related to measurements of a phase noise spectrum for each mixingproduct of two different EM-signals in the set of EM-signals and a phase noisespectrum of each of the two different EM-signals. The processing unit 10, 150 is alsoconfigured to determine, based on the measured phase noise spectrum of the mixingproducts and based on the measured phase noise spectrum of each of the two different EM-signals, a correlated phase noise contribution to the phase noise 29 spectrum and an uncorrelated phase noise contribution to the phase noise spectrumfor each EM-signal in the set of EM-signals. The processing unit 10, 150 is alsoconfigured to construct a representation of the phase noise for the set of EM-signalsthat comprises the determined correlated and uncorrelated contribution to the phase noise for each EM-signal.

The processing unit may be seen as a unit that is configured to perform the earlierdescribed method in order to process measurement data of phase noise to provide arepresentation of the phase noise where the correlated phase noise has beenseparated from the uncorrelated phase noise.

According to a particular example there is provided an embodiment of a processingunit 10, 150 where the processing unit is configured to read values of the phase noisespectrum for each of the two different EM-signals as measured at a nominal frequencym1 of a first EM-signal and the phase noise spectrum value at a nominal frequency wzof a second EM- signal.

According to another example of an embodiment there is provided a processing unit10, 150 where the processing unit is configured to read values of the phase noisespectrum for each mixing product of two different EM-signals as measured at an atleast one offset frequency in the vicinity of a first mixing frequency nw1+mw2 and atleast one offset frequency in the vicinity of the second mixing frequency nun-mwz,where n and m are integers, m1 is the nominal frequency of the first EM- signal and wzis the nominal frequency of the second EM-signal.

An optional version of the processing unit 10, 150 relates to the case where n=1 andm=1.

A particular embodiment provides a processing unit 10, 150 that is configured todetermine a correlated phase noise contribution to the phase noise spectrum and anuncorrelated phase noise contribution to the phase noise spectrum for each EM-signal. The processing unit 10, 150 is configured to compute, for a combination of twodifferent EM-signals in the set of EM-signals, a difference between a phase noise spectrum value as measured at an offset frequency in the vicinity of the first mixingfrequency and a phase noise spectrum value as measured at an offset frequency inthe vicinity of the second mixing frequency. The processing unit 10, 150 is alsoconfigured to create, for the combination of two different EM-signals, a first metric anda second metric based on the computed difference. The processing unit 10, 150 isalso configured to obtain, for the combination of two different EM-signals, amultiplicative parameter k based on the created first and second metric. Theprocessing unit 10, 150 is further configured to extract, for the combination of twodifferent EM-signals, the correlated contribution and the uncorrelated contribution ofthe phase noise based on an expression relating the measured phase noise spectrumvalue of each of the two different EM-signals, the computed difference and the obtained multiplicative parameter k.

According to a particular embodiment of the proposed processing unit 10, 150 thereis provided a processing unit 10, 150 that is configured to compute a differencebetween the measured value of phase noise at an offset frequency around the firstmixing frequency, nm1+mm2, and the measured value of the phase noise at an offsetfrequency around the second mixing frequency value nml-mmz, where n and m are integers and m1 and m2 are the nominal frequencies of the two different EM-signals.

Yet another embodiment provides a processing unit 10, 150 where the processing unit10, 150 is configured to create first and second metrics based on the computeddifference. The processing unit 10, 150 is therefore configured to create a first metric,defined as a ratio between the computed difference and the measured value of thephase noise spectrum at the nominal frequency value m1 of a first EM-signal, Theprocessing unit 10, 150 is also configured to create a second metric defined as a ratiobetween the computed difference and the measured value of the phase noisespectrum at the nominal frequency value m2 of a second EM-signal.

Still another embodiment provides a processing unit 10, 150 where the processingunit 10, 150 is configured to obtain a multiplicative parameter k for the combination oftwo EM-signals in the set of EM-signals. The processing unit 10, 150 is therefore configured to compare the value of the created first metric with the value of the inverse 31 of the created second metric and configured to set the parameter k to the value 1 ifthe comparison yields that the difference between the value of the first metric and the value of the inverse of the second metric fulfills a predetermined criterion. 5 The processing unit 10, 150 is in an exemplary embodiment configured to obtain amultiplicative parameter k for the combination of two different EM-signals in the set ofEM-signals by being configured to compare the value of the created first metric withthe value of the inverse of the created second metric and by being configured to setthe parameter k to the value of the first metric if the comparison yields that the 10 difference between the value of the first metric and the value of the inverse of the second metric fulfills a predetermined criterion.

A possible version ofthe proposed processing unit 10, 150 relates to a processing unit10, 150 that is configured to extract the correlated contribution and the uncorrelated 15 contribution of the phase noise based on an expression relating the measured phasenoise spectrum values at the nominal frequencies w1 and wz of the two different EM-signals, the determined difference between the phase noise values as measured atoffset frequencies around the mixing frequency nwi-mwz, n and m being integers, orat offset frequencies around the mixing frequency nwi-mwz, to a linear combination of 20 obtained value of k and the correlated part and uncorrelated part of the phase noisefor the two distinct EM-signals.

An optional embodiment of the proposed processing unit 10, 150 relates to aprocessing unit that is configured to extract the correlated contribution and the25 uncorrelated contribution of the phase noise based on an expression thas given by the following matrix equation: 2 Lwl 1 1 o n,Lwz = |kl2 0 1 [gylf2Lwrwz l1-kl 1 1 Iwzj where LM corresponds to the measured phase noise spectrum at the nominal 30 frequency value of the first El\/l~signal, Lwz corresponds to the measured phase noise32 spectrum of the nominal frequency value of the second EM-signal, Lwkwzcorresponds to the measured value of the phase noise spectrum at the mixingfrequency w1-w2 and where cpc denotes the correlated contribution of the phase noisebetween the first and second signal, çollu denotes the uncorrelated contribution of thephase noise from the first EM-signal and cpzlu denotes the uncorrelated contribution of the phase noise from the second EM-signal.

Yet another embodiment provides a processing unit 10, 150 that is configured toconstruct a representation of the phase noise by being configured to repeat theconstruction for each offset frequency in a plurality of offset frequencies around themixing frequencies in order to generate a representation of the phase noise spectrumfor two different EM~signals that comprises the determined correlated anduncorrelated contribution to the phase noise for each EM-signal.

That is, the processing unit is configured to perform the earlier described operationsfor each of a plurality of chosen offset frequencies in order to generate arepresentation of the phase noise spectrum for two different EM-signals thatcomprises the determined correlated and uncorrelated contribution to the phase noisefor each EM-signal. So in the illustrative and exemplary case where four offsetfrequencies are used, the processing unit is configured to perform its operation foreach of these four frequencies separately.

A particular embodiment of the processing unit 10, 150 provides a processing unit 10,150 that is configured to construct a representation of the phase noise for everycombination of two different EM-signals in the set of EM-signals in order to obtain arepresentation of the phase noise spectrum for all EM-signals in said set of EM-signalsthat comprises the determined correlated and uncorrelated contribution to the phasenoise.

That is, the processing unit is configured to perform the described operations for everypermutation of two signals in the set of EM-signals in order to construct arepresentation ofthe phase noise that is representative for all signals in the set signals. ln the illustrative and exemplary case with three signals, y1, yz and ys, is the processing 33 unit configured to repeat the operations for the following combinations: y1y2,y1ys andyzys in order to obtain a representation of the phase noise that is representative for thethree signals.

According to an exemplary embodiment there is provided a processing unit 10, 150according to any of the earlier described embodiments, wherein the processing unit10, 150 comprises a processor and a memory, the memory comprising instructionsexecutable by the processor, whereby the processor is operative to construct arepresentation of the phase noise for a set of electromagnetic signals, EM-signals,where said set comprises at least two EM-signals. A particular embodiment of theprocessing unit 10, 150 is illustrated schematically in Fig.15. This schematic blockdiagram illustrates an example of a processing unit 10 comprising a processor 122and an associated memory 124. This embodiment of the processing unit 10, 150illustrates a processing unit that is connectable to the phase noise measuring apparatus 100.

Figure 14 is a schematic block diagram illustrating an example of a processing unit10, 150 connected to a phase noise measuring device 100 and connected to anoptional display unit 160. lt will be appreciated that the methods and devices described herein can be combinedand re-arranged in a variety of ways.

For example, embodiments may be implemented in hardware, or in software forexecution by suitable processing circuitry, or a combination thereof.

The steps, functions, procedures, modules and/or blocks described herein may beimplemented in hardware using any conventional technology, such as discrete circuitor integrated circuit technology, including both general-purpose electronic circuitry and application-specific circuitry. 34 Particular examples include one or more suitably configured digital signal processorsand other known electronic circuits, e.g. discrete logic gates interconnected to performa specialized function, or Application Specific Integrated Circuits, ASlCs.

Alternatively, at least some of the steps and/or blocks described herein may beimplemented in software such as a computer program for execution by suitable processing circuitry such as one or more processors or processing units.

Examples of processing circuitry includes, but is not limited to, one or moremicroprocessors, one or more Digital Signal Processors, DSPs, one or more CentralProcessing Units, CPUs, video acceleration hardware, and/or any suitableprogrammable logic circuitry such as one or more Field Programmable Gate Arrays,FPGAs, or one or more Programmable Logic Controllers, PLCs. lt should also be understood that it may be possible to re-use the general processingcapabilities of any conventional device or unit in which the proposed technology isimplemented. lt may also be possible to re-use existing software, e.g. byreprogramming of the existing software or by adding new software components.

Optionally, the described devices and units may also include a communication circuit toenable the device to communicate with external measurement equipment. Thecommunication circuit may include functions for wired and/or wireless communicationwith the measurement equipment or measurement apparatus. ln a particular example,the communication circuit may be based on radio circuitry for communication withexternal equipment including transmitting and/or receiving information. Thecommunication circuit may be interconnected to the processor and/or memory. ln particular examples, at least some of the steps, functions, procedures, and/or blocksdescribed herein are implemented in a computer program, which is loaded into thememory for execution by processing circuitry including one or more processors. Theprocessor(s) and memory are interconnected to each other to enable normal softwareexecution. An optional input/output device may also be interconnected to the processor(s) and/or the memory to enable input and/or output of relevant data suchas input parameter(s) and/or resulting output parameter(s).

The term processor' should here be interpreted in a general sense as any system ordevice capable of executing program code or computer program instructions toperform a particular processing, determining or computing task.

The processing circuitry including one or more processors is thus configured to perform,when executing the computer program, well~defined processing tasks such as thosedescribed herein.

The processing circuitry does not have to be dedicated to only execute the above-described steps, functions, procedure and/or blocks, but may also execute other tasks.

The proposed technology also provides a computer program 135 for constructing aphase noise representation. There is in other words provided a computer program 135comprising instructions, which when executed by at least one processor, cause the atleast one processor to: -read values of the measured phase noise spectrum for each mixing product of twodifferent EM-signals in said set of EM-signals and read values of the measured phasenoise spectrum of each of said two different EM-signals; -determine, based on differences between the measured phase noise spectrum of themixing products and based on the measured phase noise spectrum of each of saidtwo different EM-signals, a correlated phase noise contribution to the phase noisespectrum and an uncorrelated phase noise contribution to the phase noise spectrumfor each EM-signal in said set of EM-signals; -construct a representation of the phase noise for said set of EM-signals comprisingthe determined correlated and uncorrelated contribution to the phase noise for eachEM-signal .

The proposed technology also provides a computer-program product 235 comprisinga computer-readable medium having stored thereon a computer program 135according to the above.

By way of example, the software or computer program may be realized as a computerprogram product, which is normally carried or stored on a computer-readable medium,in particular a non-volatile medium. The computer-readable medium may include one ormore removable or non-removable memory devices including, but not limited to a Read-Only Memory, ROM, a Random Access Memory, RAM, a Compact Disc, CD, a DigitalVersatile Disc, DVD, a Blu-ray disc, a Universal Serial Bus, USB, memory, a Hard DiskDrive, HDD, storage device, a flash memory, a magnetic tape, or any other conventionalmemory device. The computer program may thus be loaded into the operating memoryof a computer or equivalent processing device for execution by the processing circuitrythereof.

The proposed technology also provides a carrier comprising the computer program,wherein the carrier is one of an electronic signal, an optical signal, an electromagneticsignal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or acomputer-readable storage medium.

References 5 [Goldberg2000] B.-G. Goldberg, ”Phase noise theory and measurements: A shortreview,” Microwave J., vol. 43, no. 1, pp. 6-9, Jan. 2000.[Rohde2013a] U.L. Rohde, A.K. Poddar, and A.M. Apte, ”Phase noise measurementsand system comparisons,“ Microwave J., vol. 56, no. 4, pp. 22-46 Apr. 2013.[Rohde2013b] U.L. Rohde, A.K. Poddar, and A.M. Apte, ”Getting its measure” IEEE 10 Microwave Mag., vol. 14, no. 6, pp. 73-86. Sep. 2013.

[Poddar2013] A. K. Poddar, U.L. Rohde, and A. M. Apte, “How Low Can They Go?”IEEE Microwave Mag., vol 14, no. 6, pp. 50-72, Sep. 2013.

[Fest1983] D. Fest, J. Groslambert, and. J.-J. Gagnepain, “Individual Characterizationof an Oscillator by Means of Cross-Correlation or Cross-Variance Methodf' IEEE 15 Trans. lnstrum. Meas., vol. 32, no. 3, pp. 447-450 Sep. 1983.

[Minassian2011] S. Minassian and E. Levi, “Low phase noise RF signal generatingsystem and method for calibrating phase noise measurement system using same"patent WO 2011/088278 A2 [Fawley2014] R.J. Fawley, H. Masoum, A. Soarbro, and A.D. Williams, “Phase 20 disciplined, direct digital synthesizer based, coherent signal generatorf' patent US 2014/0240004 A1 [Baker2012] J. Baker, E. Levi, “Phase noise measurement system and method,”patent US8248297 B1 [Wendler2010] W. Wendler, A. Roth, and H. Eckert, “Method and device for measuring 25 phase noise," patent US 2010/0048143 A1 [Sariaslani2013] D. Sariaslan and J.P Dunsmore, “SYSTEM FOR MEASURINGRESIDUAL PHASE NOISE” US2013197848 (A1) [Koji2013] RADA KOJI, YOKOYAMA MlTSURU, “PHASE NOISE EXTRACTIONAPPARATUS AND TECHNIQUE," US2013322570. 30 [Jong2003] MAR WING JONG, “Phase noise measurement module and method for a spectrum analyzerj' US2003080724 (A1) .

[R0th2015] ROTH ALEXANDER, BECHTELER THOMAS, “METHOD AND A DEVICE FOR MEASURING THE AMPLITUDE NOISE AND/OR PHASE NOISE OFA SIGNAL,” US2015016616 (A1)

Claims (32)

1. A method for constructing a representation of the phase noise for a set ofelectromagnetic signals, EM-signals, said set comprising at least two EM-signals, wherein the method comprises the steps of: - measuring (S1) a phase noise spectrum for each mixing product of twodifferent EM-signals in said set of EM-signals and a phase noise spectrumof each of said two different EM-signals; - determining (S2), based on the measured phase noise spectrum of themixing products and based on the measured phase noise spectrum of eachof said two different EM-signals, a correlated phase noise contribution to thephase noise spectrum and an uncorrelated phase noise contribution to thephase noise spectrum for each EM-signal in said set of EM-signals; - constructing (S3) a representation of the phase noise for said set of EM-signals comprising the determined correlated and uncorrelated contribution to the phase noise for each EM-signal.

2. The method according to claim 1, wherein the step (S1) of measuring the phasenoise spectrum for each of said two different EM-signals comprises to measurethe phase noise value at an at least one offset frequency in the vicinity of anominal frequency m1 of a first EM-signal and the phase noise spectrum valueat an at least one offset frequency in the vicinity of a nominal frequency wz of asecond EM- signal.

3. The method according to claim 1 or 2, wherein the step of measuring the phasenoise spectrum for each mixing product of two different EM-signals comprisesto measure the phase noise spectrum value of the mixing products for at leastone offset frequency in the vicinity of a first mixing frequency nw1+mw2 and atleast one offset frequency in the vicinity of the second mixing frequency nw1-mw2, where n and m are integers, m1 is the nominal frequency of said first EM- signal and w2 is the nominal frequency of said second EM-signal.

4. The method according to claim 3, wherein n=1 and m=1.

5. The method according to any of the claims 'l- 4, wherein the step (S2) ofdetermining a correlated phase noise contribution to the phase noise spectrumand an uncorrelated phase noise contribution to the phase noise spectrum foreach EM-signal comprises the steps of: - computing (S21), for a combination of two different EM-signals in said set ofEM-signals, a difference between a phase noise spectrum value asmeasured at an offset frequency in the vicinity of the first mixing frequencyand a phase noise spectrum value as measured at an offset frequency inthe vicinity of the second mixing frequency; - creating (S22), for the combination of two different EM-signals, a first metricand a second metric based on the computed difference; - obtaining (S23), for the combination of two different EM-signals, amultiplicative parameter k based on the created first and second metric, - extracting (S24), for the combination of two different EM-signals, thecorrelated contribution and the uncorrelated contribution of the phase noisebased on an expression relating the measured phase noise spectrum valueof each of the two different EM-signals, the computed difference and the obtained multiplicative parameter k.

6. The method according to claim 5, wherein the step (S21) of computing thedifference comprises to compute a difference between the measured value ofphase noise spectrum at an offset frequency around the first mixing frequency,nw1+mw2, and the measured value of the phase noise spectrum at an offsetfrequency around the second mixing frequency value nw1-mw2, where n andm are integers and m1 and m2 are the nominal frequencies of the two distinctEM-signals.

7. The method according to claim 6, wherein the step (S22) of creating first andsecond metrics based on the computed difference comprises to create a firstmetric, defined as a ratio between the computed difference and the value of the phase noise spectrum as measured at an at least one offset frequency in the vicinity of the nomina| frequency value wi of a first EM-signal, and a secondmetric defined as a ratio between the computed difference and the value of thephase noise spectrum as measured at an at least one offset frequency in the vicinity of the nomina| frequency value w2 of a second EM-signal.

8. The method according to claim 7, wherein the step (S23) of obtaining a multiplicative parameter k for the combination of two EM-signals in said set ofEM-signals comprises to compare the value of the created first metric with thevalue of the inverse of the created second metric and set the parameter k to thevalue 1 if said comparison yields that the difference between the value of thefirst metric and the value of the inverse of the second metric fulfills a predetermined criterion.

9. The method according to claim 7, wherein the step (S23) of obtaining a multiplicative parameter k for the combination of two different EM-signals in saidset of EM-signals comprises to compare the value of the created first metric withthe value of the inverse of the created second metric and set the parameter k tothe value of the first metric if said comparison yields that the difference betweenthe value of the first metric and the value of the inverse of the second metric fulfills a predetermined criterion.

10.The method according to any of the claims 5 to 9, wherein the step (S24) ofextracting the correlated contribution and the uncorrelated contribution of thephase noise is based on an expression relating the measured phase noisespectrum values as measured at an at least one offset frequency in the vicinityof the nomina| frequencies m1 and 002 of the two different EM-signals,respectively, the determined difference between the phase noise values asmeasured at offset frequencies in the vicinity of the mixing frequency nw1-mw2,n and m being integers, or at offset frequencies in the vicinity of the mixingfrequency nw1-mw2, to a linear combination of obtained value of k and thecorrelated part and uncorrelated part of the phase noise for the two distinct EM-signals.

11. The method according to claim 10, wherein said expression is given by thefollowing matrix equation: 2 Lä) 1 1 o fpc 2 Lä) = lk|2 o 1 law2 Lai-w; |1_k| 1 1 kpzßz where LM corresponds to the measured phase noise spectrum at the nominalfrequency value of the first EM-signal, Lwz corresponds to the measured phasenoise spectrum of the nominal frequency value of the second EM-signal,Lamm corresponds to the measured value of the phase noise spectrum at themixing frequency w1-w2 and where çoc denotes the correlated contribution ofthe phase noise between the first and second signal, om denotes theuncorrelated contribution of the phase noise from the first EM-signal and (pm denotes the uncorrelated contribution of the phase noise from the second EM-signal.

12. The method according to any of the claims 1-11, whereinthe steps are repeatedfor a plurality of offset frequencies around said mixing frequencies in order togenerate a representation of the phase noise spectrum for two different EM-signals that comprises the determined correlated and uncorrelated contributionto the phase noise for each EM-signal.

13. The method according to any of the claims 1-12, wherein the steps arerepeated for every combination of two different EM-signals in said set of EM-signals in order to obtain a representation of the phase noise spectrum for allEM-signals in said set of EM-signals that comprises the determined correlatedand uncorrelated contribution to the phase noise.

14. A phase noise measuring device (100), said device comprising:- two separate signal inputs, a first input (110) and a second input (120), saidinputs (110, 120) being adapted to receive signals from a signal source (105,106); a signal mixer (200) connected to said first (110) and second (120) inputs;a unit (130) for phase noise measurements, said unit (130) being connectedto said first input (110), to said second input (120) and to said signal mixer(200); a processing unit (150) for processing the values of the phase noisemeasurements, wherein said processing unit (150) is connected to said unit (130) for phase noise measuring; wherein each of said first input (110) and said second input (120) comprises means (145) for directing signals to said unit (130) for phase noise measurements either directly over a first and second channel, respectively, or indirectly over a channel comprising said signal mixer (200); and wherein said unit (130) for phase noise measuring is adapted to: measure the phase noise spectrum, Loui, of a first signal when receiving saidfirst signal over the first channel from said first input (110); and measure the phase noise spectrum, Lwz, of a second signal when receivingsaid second signal over the second channel from said second input (120);and measure the phase noise spectra of the mixing products between the firstand second signal when receiving the signal over a channel from the signalmixer (200); and communicate the outcome of the measurements to the processing unit (150)to enable said processing unit (150) to construct a representation of the phase noise.

15. A phase noise measuring device (100), according to claim 14, wherein said means (145) for directing signals to said unit (130) for phase noise measurements comprises a signal splitter (145a) for splitting a signal received from a signal source (105, 106) so that part of the signal is directed toward the unit (130) for phase noise measuring and another part of the signal is directed toward the signal mixer (200).

16.A phase noise measuring device (100) according to claim 14, wherein said means (145) for directing signals comprises a first switch (A) dedicated to the first input (105) and a second switch (B) dedicated to the second input (106);whereinsaid first switch (A) is adapted to switch between at least two different states, afirst state (A1) where a signal is directly transferred to the phase noisemeasuring unit (130) and a state (A2) where a signal is transferred to the phasenoise measuring unit (130) over the signal mixer (200), and said second switch(B) is adapted to switch between at least two different states, a state (B1) wherea signal is transferred directly to the phase noise measuring unit (130) and astate (Bz) where a signal is transferred to the phase noise measuring unit (130)over the signal mixer (200); and wherein the unit (130) for phase noisemeasuring comprises at least three detection channels, a first detection channeldedicated to receive signals transferred directly from said first input, a seconddetection channel dedicated to receive signals transferred directly from saidsecond input, and a third detection channel dedicated to receive signalstransferred directly from said signal mixer; wherein said unit (130) for phase noise measuring is adapted to: - measure the phase noise spectrum, Lw1, of a first signal when the firstswitch (A) is in a first state (A1) and the signal is received in said firstdetection channel; and - measure the phase noise spectrum of a second signal, Lw2, when thesecond switch (B) is in state (B1) and the signal is received over the seconddetection channel; and - measure the phase noise spectra of mixing products of a first and secondsignal when the first switch (A) is in state (A2), the second switch (B) is in state (Bz) and the signal is received over the third detection channel.

17.A phase noise measuring device according to claim 16, wherein said devicecomprises a third switch (C), arranged between said unit (130) for phase noisemeasurements and said first switch (A) and second switch (B), said third switchbeing adapted to operate in at least three different states, a state (C1) where theunit (130) for phase noise measurements is connected to the first switch (A), astate (Cz) where the unit (130) for phase noise measurements is connected tothe second switch (B) and A state (Cs) where the unit (130) for phase noise measurements is connected to the signal mixer (200); wherein said unit (130) for phase noise measuring is adapted to: measure the phase noise spectrum, Lw1, of a first signal when the firstswitch (A) is in a first position (A1) and the third switch (C) is in position (Cr),and measure the phase noise spectrum of a second signal, Lw2, when thesecond switch (B) is in position (B1) and the third switch (C) is in position(Cs), and measure the phase noise spectra of mixing products of the first and secondsignal when the first switch (A) is in position (A2), the second switch (B) is inposition (Bz) and the third switch (C) is in position (Cz).

18. A processing unit (10, 150) connectable to a phase noise measuring apparatus, said processing unit (10, 150) being configured to construct a representation of the phase noise for a set of electromagnetic signals, EM-signals, said set comprising at least two EM-signals, wherein: the processing unit (10, 150) is configured to read values obtained from thephase noise measuring apparatus (100), said values being related tomeasurements of a phase noise spectrum for each mixing product of twodifferent EM-signals in said set of EM-signals and a phase noise spectrumof each of said two different EM-signals; and the processing unit (10, 150) is configured to determine, based on themeasured phase noise spectrum of the mixing products and based on themeasured phase noise spectrum of each of said two different EM-signals, acorrelated phase noise contribution to the phase noise spectrum and anuncorrelated phase noise contribution to the phase noise spectrum for eachEM-signal in said set of EM-signals; the processing unit (10, 150) is configured to construct a representation ofthe phase noise for said set of EM-signals comprising the determinedcorrelated and uncorrelated contribution to the phase noise for each EM-signal.

19.The processing unit (10, 150) according to claim 18, wherein the device is configured to read values of the phase noise spectrum for each of said two different EM-signals as measured at a nominal frequency m1 of a first EM-signal and the phase noise spectrum value at a nominal frequency wz of a second EM-signal.

20.The processing unit (10, 150) according to claim 18 or 19, wherein the deviceis configured to read values of the phase noise spectrum for each mixingproduct of two different EM-signals as measured at an at least one offsetfrequency in the vicinity of a first mixing frequency nw1+mwz and at least oneoffset frequency in the vicinity of the second mixing frequency nwi-mwz, wheren and m are integers, wi is the nominal frequency of said first EM- signal and wz is the nominal frequency of said second EM-signal.

21.The processing unit (10, 150) according to claim 20, wherein n=1 and m=1.

22.The processing unit (10, 150) according to any of the claims 18- 21, wherein thedevice is configured to determine a correlated phase noise contribution to thephase noise spectrum and an uncorrelated phase noise contribution to the phasenoise spectrum for each EM-signal, wherein: - the processing unit (10, 150) is configured to compute, for a combination oftwo different EM-signals in said set of EM-signals, a difference between aphase noise spectrum value as measured at an offset frequency in thevicinity of the first mixing frequency and a phase noise spectrum value asmeasured at an offset frequency in the vicinity of the second mixingfrequency; and - the processing unit (10, 150) is configured to create, for the combination oftwo different EM-signals, a first metric and a second metric based on thecomputed difference; and - the processing unit (10, 150) is configured to obtain, for the combination oftwo different EM-signals, a multiplicative parameter k based on the createdfirst and second metric; and - the processing unit (10, 150) is configured to extract, for the combination oftwo different EM-signals, the correlated contribution and the uncorrelatedcontribution of the phase noise based on an expression relating the measured phase noise spectrum value of each of the two different EM- signals, the computed difference and the obtained multiplicative parameterk.

23.The processing unit (10, 150) according to claim 22, wherein the processing unit(10, 150) is configured to compute a difference between the measured value ofphase noise at an offset frequency around the first mixing frequency, nw1+mw2,and the measured value of the phase noise at an offset frequency around thesecond mixing frequency value nun-mwz, where n and m are integers and wi and wz are the nominal frequencies of the two distinct EM-signals.

24.The processing unit (10, 150) according to claim 23, wherein the processing unit(10, 150) is configured to create first and second metrics based on the computeddifference by being configured to create a first metric, defined as a ratio betweenthe computed difference and the measured value of the phase noise spectrumat the nominal frequency value om of a first EM-signal, and a second metricdefined as a ratio between the computed difference and the measured value ofthe phase noise spectrum at the nominal frequency value m2 of a second EM- signal.

25.The processing unit (10, 150) according to claim 24, wherein the processing unit(10, 150) is configured to obtain a multiplicative parameter k for the combinationof two EM-signals in said set of EM-signals by being configured to compare thevalue of the created first metric with the value of the inverse of the createdsecond metric and by being configured to set the parameter k to the value 1 ifsaid comparison yields that the difference between the value of the first metric and the value of the inverse of the second metric fulfills a predetermined criterion.

26.The processing unit (10, 150) according to claim 24, wherein the processing unit(10, 150) is configured to obtain a multiplicative parameter k for the combinationof two different EM-signals in said set of EM-signals by being configured tocompare the value of the created first metric with the value of the inverse of thecreated second metric and by being configured to set the parameter k to the value of the first metric if said comparison yields that the difference between the value of the first metric and the va|ue of the inverse of the second metric fulfillsa predetermined criterion.

27.The processing unit (10, 150) according to any of the claims 22 to 25, whereinthe processing unit (10, 150) is configured to extract the correlated contributionand the uncorrelated contribution of the phase noise based on an expressionrelating the measured phase noise spectrum values at the nominai frequenciesm1 and wz of the two different EM-signals, the determined difference betweenthe phase noise values as measured at offset frequencies around the mixingfrequency nwi-mwz, n and m being integers, or at offset frequencies around themixing frequency nwi-mwz, to a linear combination of obtained va|ue of k andthe correlated part and uncorrelated part of the phase noise for the two distinct EM-signals.

28. The processing unit (10, 150) according to claim 27, wherein said expression isgiven by the following matrix equation: 2 Lq 1 1 o (pc 2 Lab = lklz 0 1 law2 LWÛZ |1-kl 1 1 løzglf where LM corresponds to the measured phase noise spectrum at the nominaifrequency va|ue of the first EM-signal, Lwz corresponds to the measured phasenoise spectrum of the nominai frequency va|ue of the second EM-signal,Lwlwz corresponds to the measured va|ue of the phase noise spectrum at themixing frequency w1-w2 and where çac denotes the correlated contribution ofthe phase noise between the first and second signal, rpm, denotes theuncorrelated contribution of the phase noise from the first EM-signal and om denotes the uncorrelated contribution of the phase noise from the second EM-signal.

29.The processing unit (10, 150) according to any of the claims 18-28, wherein theprocessing unit (10, 150) is configured to construct a representation of the phasenoise by being configured to repeat the construction for each offset frequency ina piurality of offset frequencies around said mixing frequencies in order togenerate a representation of the phase noise spectrum for two different EM-signals that comprises the determined correlated and uncorrelated contributionto the phase noise for each EM-signial.

30. The processing unit (10, 150) according to any of the claims 18-29, wherein theprocessing unit (10, 150) is configured to construct a representation of the phasenoise for every combination of two different EM-signals in said set of EM-signalsin order to obtain a representation of the phase noise spectrum for all EM-signalsin said set of EM-signals that comprises the determined correlated and uncorrelated contribution to the phase noise.

31.The processing unit (10, 150) according to any of the claims 18-30, wherein theprocessing unit (10, 150) comprises; a processor and a memory, said memorycomprising instructions executable Iby the processor, whereby the processor isoperative to construct a representation of the phase noise for a set ofelectromagnetic signals, EM-signals, said set comprising at least two EM-signals.

32.The processing unit (10, 150) according to any of the claims 18-31, wherein theprocessing unit (10, 150) is connect:ab|e to a phase noise measuring apparatus(100) according to any of the claims 14-17.