WO2012002822A1

WO2012002822A1 - Vector network analyzer comprising synchronization device for simplified estimation of incident wave

Info

Publication number: WO2012002822A1
Application number: PCT/NO2011/000191
Authority: WO
Inventors: Karsten Husby; Bengt Holter; Jacob Kuhnle
Original assignee: Sinvent As
Priority date: 2010-07-02
Filing date: 2011-07-04
Publication date: 2012-01-05
Also published as: NO20100972A1; NO331964B1

Abstract

Vector network analyzer comprising synchronization device for simplified estimation of incident wave comprising signal source (1), cable (5), connecting line (2) and (6) measurement port (4) and response receiver (7). The vector network analyzer is arranged to send a signal from the signal source (1) by the connection line (6) and cable (5) to leave the measurement port (4), and where the reflected signals b from the measurement port (4) can be guided by cable (5) and connection line (2) to the response receiver (7) for sampling, and where the response receiver may carry out calibration measurements on known standards and measurements at DUT in sequence after one another. The synchronization device (20) transfers amplitude- and phase references between source (1) and response receiver (7). The synchronization device (20) is connected to the measurement port (4) by cable (5), and the incident wave a from the signal source (1) and reflected signal b from measurement port (4) is guided by the synchronization device (20) to the response receiver (7) for synchronized sampling in relation to each other.

Description

Vector network analyzer comprising synchronization device for simplified estimation of incident wave

The present invention relates to general equipment for measuring electrical parameters. In particular the invention relates to Vector Network Analyzers (VNA) that characterize electrical components by measuring the ratio between reflected wave and incident wave when the components are exposed to an exciting signal. Area of the invention

VNA measures the ratio between reflected wave and incident wave when exposing electrical components to a stimulus. This ratio is called the reflection factor. The reflection factor gives a complete characterization of the electrical parameters of a measurement object. VNA typically includes multiple reflectometers, one for each port of the VNA making it possible to do a complete characterization of a multi port measurement object. In addition VNA use calibration to remove all systematic errors.

Prior art

In the early seventies VNA equipment for complete removal of systematic errors became common. They were using a set of both known and partly unknown electrical parameters as described by Andrej Rumiantsev og Nick Ridler in "VNA Calibration" from IEEE Microwave Magazine June 2008.

A traditional 2-port VNA is a measurement equipment typically consisting of two reflectometers each containing two measurement receivers. It is also possible that these two reflectometers share some components in common. For example, they may share signal source and incident wave receiver. Such a VNA is often called a 3- sampler VNA. One of the samplers is called the reference receiver. One example is given in US patent 6920402 B2. Here one reference receiver is used and focus is put on multiport VNA including differential VNA by for instance utilizing response receivers at several ports when the number of ports is large and greater than 2.

Another example is given in US patent 0290880 A1. EP 0265073 A1 presents a six-port network for characterizing a microwave or radio frequency device which includes a signal source capable of generating three signals of the same frequency but relative phase differences, and combines each of the signals with a signal reflected or transmitted by the device. Two three-phase sources may be used in a dual six-port network analyzer device.

US 2003/0125894 A1 presents a method for extending dynamic range and a test system with extended dynamic range which compensate for compression effect on measured data caused by receiver channel of the test system being compressed. The measured data is magnitude and phase data. The method comprises

characterizing channels based on compression response and compensating to correct for the effect of compression. The test system comprises a receiver channel, and a computer program that implements the method.

DE 102005058433 A1 presents a VNA comprising n measurement ports to measure an object, where the VNA has one signal generator and one measurement receiver to receive the reference signal from the generator or the reflected or transmitted wave from the measurement object. Only one receiver is available. This receiver use un-synchronized sampling instants. In addition this solution comprises switches and couplers to make reference measurements in a reference channel.

US 7095294 B2 presents a directive bridge coupler for use in VNA systems.

US2008/0290880 A1 presents a VNA with a combined resistive bridge coupler for low frequencies and a hybrid coupler for higher frequencies.

Prior art solutions need to perform incident wave or reference signal measurement in a reference channel and often by the use of a dedicated incident wave or reference signal measurement receiver. This causes a disadvantage concerning complexity, amount of equipment, size and cost. Other prior art disadvantages are the use of directional couplers, bridge couplers, transmission line couplers and transformers to couple incident waves or reference signals and at the same time using an additional set of similar couplers to couple the wanted response or the reflected signal from the measurement port. These couplers are costly, complex and their bandwidth is limited.

Summary of the invention

VNA makes accurate measurements eliminating all systematic errors. This is possible by making measurements on known or partly known standards for calibration in addition to measurements on the DUT (device under test). An appreciable property of this invention is the simplification and reduction of number of components of the VNA without sacrificing VNA measurement principles and accuracy. Many of the components available in prior art VNA generates a reference signal or an incident wave signal. After this is generated it is often measured by the use of a reference receiver or an incident wave receiver. This reference signal a is then referred to the reflected signal b from the measurement port to estimate the un- calibrated reflection factor Γ: r=b/a assuming that a and b are measured while all amplitudes and phases are constant or known while the measurement takes place. By the use of this invention this operation is simplified using a synchronization device to find the reference signal a.

The synchronization device diverts both amplitude and phase of the reference signal and makes that information available for the response receiver (7). An essential property of the present invention is that the incident wave receiver, reference channel receiver or dedicated reference channels become superfluous and therefore are removed. This is possible if measured response at the response receiver (7) is referred to the reference a from the source (1) by the use of the synchronization device (20). As a consequence all incident wave receivers, reference receivers as well as reference channels may be removed including the associated couplers.

The incident wave a from the source (1) may be considered as a constant vector or a signal with constant phase and constant amplitude. After some time for example if the temperature inside the VNA changes both the incident wave and the reflected wave may anyway slowly change. The synchronization device then will give the opportunity to update the reference signal a to maintain the synchronicity between a and b. It is important that the relationship between a and b cannot change without an actual change in the DUT or in the calibration standard. VNA measurements like the 10- or 12-term SOLT calibration are relative measurements. This means that the actual value of the incident wave a is irrelevant as long as it is constant. This is the case for most types of VNA also included the one using self calibration techniques like for example TRL that is able to use 7-term calibration and calibration standards with partly unknown parameters. This invention therefore simplifies the VNA hardware no matter what kind of calibration used. The present invention will change the manufacturing of future VNA. The invention makes it possible to include a 2-port VNA in small handheld oscilloscopes that already have two receiver channels included. Users of such small handheld oscilloscopes will experience new kind of services previously only available at bigger stationary VNA. The new services will include very accurate impedance

measurements, filter measurement, amplifier measurement and time domain reflectometer measurements to mention the most important applications. Low cost oscilloscopes with build in VNA capabilities will offer improved measurement accuracy due to the elimination of systematic errors. The measurement results could be presented as S-parameters in the frequency domain which often is appreciated by RF engineers. However, the results may also be presented in time domain like an impulse response or like a step response. This invention also makes it possible to use only one response receiver connected to a switch even if the goal is a multiport VNA. However, to achieve great accuracy it might become necessary to isolate the switch impedance by extra isolators. This invention also makes it possible to offer an integrated VNA solution to the market based on standard PC equipment. Calibration and presentation are carried out in a PC while the VNA is placed at a data acquisition card. Also low cost sound cards augmented by the synchronization device (20) might be used to build a high quality VNA.

It is particularly useful and simple to use this invention to build a full 2-port VNA using SOLT, Quick-SOLT and robust-SOLT. Such an instrument with bandwidth up to some few GHz will have a broad usefulness regarding audio applications, radio applications, RF applications and line testing to mention a few. In addition a person skilled in the art may use this invention to measure at even higher frequencies, at more measurement ports and together with self calibrating methods like the TRL to mention on of the most popular methods.

One goal with the present invention is to provide a simplified VNA compared to prior art solutions.

Further goals with the given invention is to obtain solutions that are more compact, have less weight and are more easy to integrate into other systems. Further this solution will offer better functionality and higher capacity related to cost, size and complexity.

Additional goals are to improve solutions where this invention is included for example for increased cost-effectiveness. One additional goal is to simplify VNA for measurement on reciprocal measurement objects. In this case it is not necessary to synchronize to measure absolute phase or delay between the measurement ports. It is another benefit that the measurement accuracy may be increased by using isolators to isolate the potentially un-linear components sources and switches from the measurement ports. Such isolators will both stabilize the impedance and at the same time prevent reflected signals from the port (4) to form un-linear signal products inside switch and source. Among good isolators for this purpose are attenuators, circulators, directional couplers and isolation amplifiers to be mentioned.

Extra measurement accuracy will be gained by using filters to remove possible aliasing products. The invention also permits simplification in hardware by using the same source on more ports by the use of a switch.

One central aspect of this invention is a vector network analyzer comprising signal source, synchronization device, cable, measurement port and response receiver. The vector network analyzer lets a signal from the signal source mainly flow through the impedance, through the synchronization device, further by the cable and out to the measurement port, and then lets reflected signals from the measurement port be led to the response receiver. The synchronization device handles several signals. That is clock signals for the source and response receiver, and the signal coming from the impedance which contains the incident wave a. In addition the synchronization device handles the signal going to the response receiver that also includes the reflected wave b. Finally the synchronization device is connected to the measurement port. One aspect of the vector network analyzer is the synchronization device influencing phase or time by the use of a clock signal for sampling and a clock signal for signal generation. Another aspect is that phase and time are closely related and that broadband synchronization is possible by the use of clock signals containing more frequencies with more phases or time domain signals having a pulsed nature.

It is also an aspect that broadband synchronization includes frequencies down to DC (direct current) that are frequencies without oscillations. The use of broadband coupler (3) makes this feature possible.

Another aspect of the vector network analyzer is the synchronization device influencing incident wave a from the impedance and reflected wave b running to the response receiver.

The synchronization device is also connected to the measurement port.

Therefore the synchronization device controls all essential signals needed to make an estimate of Γ.

Yet another aspect of the vector network analyzer is that only changes in DUT or in possible calibration standards will give changes to Γ. Another aspect of the invention is that a filter is comprised between the source and the response receiver to remove frequencies that possibly may give rise to aliasing products.

Yet another aspect is a vector network analyzer comprising an isolator between the source and the synchronization device to stabilize the source signal. This isolator may be an attenuator, a circulator, a directional coupler or an isolating amplifier. Another essential aspect of this invention is a vector network analyzer that comprises more measurement ports. Such a network analyzer may have one or more sources that either do not send signals at the same time or that sends orthogonal signals or that sends on different frequencies in such a way that the signals can be separated by the response receivers.

Brief presentation of figures

Figure 1 presents example of error terms

Figure 2 illustrates one VNA according to the invention

Figure 3 presents a 2-port VNA

Figure 4 presents a 2-port VNA using 2 synchronized sources and 2 response receivers

Figure 5 presents a 2-port VNA using one source having 2 source ports

Figure 6 presents a 2-port VNA where both the source and response receiver have 2 ports

Figure 7 presents a VNA with preferred synchronization device (20) content

Figure 8 presents typical content of a response receiver having only one response receiver port (25)

Figure 9 presents a preferred response receiver with two response receiver ports for a 2-port VNA

Figure 10 presents a possible response receiver with 4 response receiver ports (25) for the use in a 4-port VNA

Figure 11 presents possible embodiments of broadband coupler (3) where the signal is referred to ground .

Figure 12 presents one example source (1) having 3 source signal ports (18)

Detailed description of the invention

Below different embodiments of the invention will be described in detail with reference to the figures. Before starting this description a more fundamental introduction will be given to help the understanding of the following. Any VNA measures the reflection factor Γ as the ratio between the reflected signal b and the incident wave a. r=b/a

All the electrical parameters from any multi port electrical circuitry can be measured if the reflection factors are measured at every measurement port when all ports sequentially are excited by an incident wave a. Without calibration this cannot be done without systematic errors. The systematic errors can be understood as constant deviations k both in amplitude and phase. Therefore in reality a VNA will measure:

where k₂ is measurement error on the reflected signal, k₃ is measurement error on the incident wave and ki is resulting measurement error on the reflection factor. Calibration standards and calibration algorithms are used to eliminate these errors.

The measured response b+k₂ in the response receiver (7) contains information about the measurement object being either a known calibration standard or an unknown circuitry. Measured signal in incident wave receiver a+k₃ will not contain information about the measurement object if this signal is isolated from the measurement port. If this isolation is carried out by a power splitter a reference signal is often measured. If however the isolation is done by a directional coupler the so-called incident wave is often measured. By a 100% effective isolation the measurement of the incident wave or the reference signal becomes a constant: k₄=a+k3

This constant will then become unaffected by any object connected to the

measurement port (4). If the phase of the receivers related to the source in addition is stabilized by the synchronization device, then all the constants k become static vectors independent of time. The result then becomes: r+k₁=(b+k₂)/k₄ which means that the reflection factor Γ with systematic errors ki becomes a function of the reflected signal b and the constants k2 and k₄.

In reality it is impossible to reach 100% isolation at every frequency component and particularly at high frequencies for measurement of k₄. However, since modern calibration algorithms remove all constant systematic errors the actual k4

measurement value becomes irrelevant for the result. Accordingly, the use of this invention makes it unnecessary to realize the ideal receiver to measure Ι<4· Instead any chosen value like k₄=1 can be used together with known calibration algorithms to remove systematic errors when thermal drift is not a problem.

It is important that measured response in the response receiver (7) is referred to phase and amplitude of the incident wave. A simple solution is to keep the amplitude of the incident wave as constant as possible e.g. by using thermally stabilized sources. In reality the measurements will slowly change due to thermal variations. Therefore the reference signal or the incident wave a will vary very slowly. To compensate for this an actual measurement of incident wave a is needed and not only a static estimate as mentioned above. In addition an actual isolator is needed preferably in the form of a switch that isolates the measurement port when a is to be measured. It is then a presumption that these measured values of a are used further on when calculating the un-calibrated Γ which finally is used to find the calibrated end result. How often new values of the incident wave a needs to be obtained is related to in what extend and how fast this reference is outdated. For example in a thermally stable environment and by the use of synchronized clock signals for signal generation and signal sampling the outdating will escalate so slowly that the incident value of k chosen during calibration perfectly well can be used when measuring the DUT at another point of time. Alternatively in other systems for example by the use of simple sound cards in an interrupt driven PC the synchronicity will only exist inside the duration of the last measured data buffer from measurement receiver (7) consisting of some thousands sample values. In these systems the a and b therefore needs to be measured in close proximity one after another making it possible to relate them to each other by the use of the synchronization device.

If technically possible it is often a benefit to establish a good reference to the phase of the incident wave. This can be done by using synchronized clock signals for signal generation and signal sampling in all reflectometers. Therefore all response receivers become synchronized with all sources either by distributing a trigger signal or another periodic signal that by the timeline marks the phase or starting point of the reference signal or the incident wave.

Standard error term methodology can be used by any person skilled in the art to exercise this invention. Never the less in Figure 1 partly new error terms are presented which more clearly express the characteristic features of this invention. Figure 1 shows an example of error terms E for port number n for a VNA together with constant source signal K_n and measured reflected wave B_n. It should be noted that error term E| and E_s operate in the opposite direction compared to normal standard error terms for 10- or 12-term SOLT calibration. Incident wave a_n

approaching from the source (1) will get modified by the incident wave error term E|. A lot of different methods are used to calibrate VNA and a lot of different methods can be used together with this invention. The common thing to do is to measure known and partly known calibration standards and thereafter solving a set of error terms E. Any person skilled in the art knows how to use these different methods. For example it is possible to use 10- or 12-term calibration or even 7-term calibration. Pay attention to Figure 1 and the 4 complex unknown parameters given that includes 3 error terms and one unknown source voltage K_n. For 10- or 12-term SOLT calibration the sources used in reality are different from each other and the

measurement done is always relative to both the source voltages. On the other hand for 2-port 7-term calibration or other multi-port calibration it is only possible to measure relative to one of the source voltages for example by normalizing source voltage Ki to 1 and solving the others as unknowns in addition to the error terms. Consequently 4n-1 unknown parameters must be solved to calibrate an n-port VNA. At least 7 measurements need to be done to solve 7 parameters. It is known that the Quick-SOLT method makes 4 measurements by the THRU standard and 3 measurements by the SHORT, OPEN and LOAD standards for example connected to only one of the measurement ports. By the use of self calibration methods like TRL more unknowns are introduced because the standards themselves have unknown parameters. In this case more measurements need to be done before the unknowns can be solved.

In Figure 2 a VNA is depicted. This one uses a response receiver (7) that measures the reflected signal from the port (4) by the use of a synchronization device (20) that also generates the clock signal (22) for the source (1) and the clock signal (23) for the response receiver (7). The synchronization device (20) also makes the incident wave a available for the un-calibrated reflection factor Γ to be estimated.

In Figure 3 a double set of components are used to generate a 2-port VNA. There is no synchronization between these sets operating by independent clock sources that may have different phase and frequency. Nevertheless this VNA may still measure absolute phase for reciprocal measurement objects connected to the measurement ports (4). This is particularly useful when for some reason wired connection cannot be used between the two measurement ports.

In Figure 4 a double set of components are used to generate a 2- port VNA. The clock signals (22) and (23) are shared between all sources (1) and all response receivers (7). In this case the signals from the two sources (1) become coherent. The two response receivers (7) also make coherent sampling with each other. All signal source ports (18) generates signals in such a way that the two response receivers (7) may separate signals approaching from each of the two sources. This is normally done by not letting the two sources send simultaneously but in series in time. It is however possible and some times preferable to let the two sources make

simultaneous transmissions on orthogonal codes or at different frequencies.

In Figure 5 a source (1) with2 signal source ports (18) is used for a 2-port VNA. The clock signal (23) is shared between all response receivers (7). The two response receivers (7) are making coherent signal sampling. All signal source ports (18) generate signals possible to separate by the two response receivers (7). This is normally done by not letting the two sources send simultaneously but in series in time. It is however possible and some times preferable to let the two sources make simultaneous transmissions on orthogonal codes or at different frequencies.

In Figure 6 as opposed to Figure 5 one response receiver (7) having 2 response receiver ports (25) is used. Therefore this 2-port VNA may include only one sampler (15) and one A/D-converter (16). Based on this figure it is straight forward to increase the number of measurement ports (4) by increasing the number of signal source ports (18) and the number of response receiver ports (25) together with possible more synchronization devices (20).

A 1-port VNA is shown in Figure 7. A preferred embodiment of the synchronization device (20) also appears. One common clock source (21) is used. This clock could be a free running oscillator or a phase locked loop. The clock signal for signal generation (22) and the clock signal for signal sampling distribute time or phase information from clock source (21) to source (1) and to response receiver (7). The information distributed may also include extra timestamps derived from the same clock source (21). The objective is to minimize clock drift between sender and receiver because this will give the best measurements. The synchronization device (20) preferably also includes a time guided isolator (1 1) that is able to isolate the measurement port (4) and thereby make it possible to estimate clock source drift between clock signal for signal generation (22) and clock signal for signal sampling (23) in such a way that incident wave a can be estimated in response receiver (7). By the use of properly synchronized clocks (22) and (23) and stable signal source ports (18) and response receiver ports (25) all incident waves on all ports become stable. However, in reality all components will slowly get changed and in addition the clock signals (22) and (23) do not need to be synchronized anyway. In that case an isolator (1 1 ) is necessary to be able to estimate the incident wave a. The isolator (11) may also be implemented as a frequency selective filter in such a way that phase or time reference of the incident wave a and partly also amplitude reference may be transferred in separate frequency bands. This solution occupies a frequency band that cannot be used for measurement. In addition this solution will not make it possible to measure the amplitude of a at the same frequencies used for

measurement at DUT. The benefit of a frequency selective isolator is that no guiding signal is needed.

Figure 8 presents a typical example of a response receiver (7) having only one response receiver port (25). The response receiver includes an anti aliasing filter (12) with defined input impedance in addition to a sampler (15) and an A/D- converter (16). A clock signal for signal sampling (23) is added. Sometimes the sampler is an integrated part of the A/D-converter. The digital signals are forwarded to a computer for processing, calibration and presentation of measurement results.

Figure 9 presents a typical solution for a 2-port with 2 response receiver ports (25). The benefit of this solution is presumption of stable input impedance to anti aliasing filter (12). The Figure also shows how sampling is done by a common clock signal for signal sampling (23).

In Figure 10 a response receiver is depicted. This receiver may be built to handle multiple ports by the use of only one sampler. Isolators (19) provide input impedance as stable as possible at every response receiver port (25) no matter the state of the switch (14). This is favorable with respect to keeping the error terms constant for the calibration. The Figure also shows how sampling is done by clock signal for signal sampling (23).

In Figure 1 1 four different examples of broadband coupler (3) for VNA is illustrated. Each coupler consists of 3 resistors that each could have a value of 0 ohm. Figure 13, a shows a bridge coupling while Figure 13,b shows a star coupling. In Figure 13,c two of the resistors are 0 ohm resistors and only one resistor is used. In example 13,d all resistors are 0 ohm. In balanced circuitry twice that number of resistors needs to be used. It is also possible to use many more resistors without changing the functionality. One of the three terminals connects to bring in a signal from a source (1), the other terminal transmits signal in the direction of a response receiver (7) while the third connects to receive signals going to or coming from a

measurement port (4). Figure 12 presents one example of source (1) containing 3 signal source ports (18). The source needs to present a stable impedance (8) as seen from the signal source ports (18). In this example D/A-converters (17) are used. It is also possible to use oscillators or phase locked loops. If these are turned on and off between active and passive mode some calibration methods like for example 7-term methods for 2-port VNA will require isolators (19) to hinder erroneous calibration. Other calibration methods like the 10- or 12-term SOLT are not sensitive to this kind of error because independent error terms are used for each active signal source port (18). Note however, it is also possible to let several ports be in active mode at the same time. All signal source ports (18) generates signals in such a way that all response receivers (7) may separate signals approaching from different ports. This is normally done by letting the signal source ports (18) not send simultaneously but following after one another in time. It is however possible and some times preferable to let them make simultaneous transmissions on orthogonal codes or at different frequencies.

Claims

1. Vector network analyzer comprising synchronization device for simplified estimation of incident wave, where the vector network analyzer comprises signal source (1) with signal source port (18), cable (5), connecting line (2) and (6), measurement port (4), and response receiver (7) with response receiver port (25), where the vector network analyzer is arranged to send a signal from the signal source (1) by the connection line (6) and cable (5) to leave the measurement port (5), and where reflected signals b from the measurement port (4) can be guided by cable (5) and connection line (2) to the response receiver (7) for sampling, and where the response receiver may perform calibration measurements on known standards and measurements at DUT in sequence after one another

characterized by

a synchronization device (20) transferring amplitude and phase references between the source (1) and the response receiver (7), and

where the synchronization device (20) is connected to the measurement port (4) by cable (5), and

where incident wave a from the signal source (1 ) and reflected signal b from measurement port (4) are guided by synchronization device (20) to the response receiver (7) for synchronized sampling in relation to one another.

2. Vector network analyzer according to claim 1 , comprising n measurement ports (4) where n>1 , at least one source (1) having a total of n signal source ports (18), at least one response receiver (7) having a total of n response receiver ports (25), and where n synchronization devices (20) by connection lines (6) and (2) and cable (5) are connected to all ports (4), (18) and (25), and where all sources (1 ) receive clock signals for signal generation (22) and all response receivers (7) receive clock signals for signal sampling (23) and where all clock signals (22) and (23) are synchronously generated by one of the n synchronization devices (20).

3. Vector network analyzer according to one of the above claims, where at least one of the synchronization devices (20) isolate itself from the measurement port (4) in time domain or frequency domain by the use of one isolator (11) when performing new measurements of incident wave a.

4. Vector network analyzer according to claim 3, where the isolator (11) is a switch for isolation in the time domain or a filter for isolation in the frequency domain.

5. Vector network analyzer according to one of the above claims, where at least one of the synchronization devices (20) comprises a broadband coupler (3).

6. Vector network analyzer according to claim 5, where the broadband coupler (3) comprises resistors connected in bridge- or star coupling where the resistors are chosen to minimize unwanted reflections and to optimize the signal propagation.

7. Vector network analyzer according to claim 6, where one or more of the resistors holds the value 0 ohm.

8. Vector network analyzer according to one of the above claims, further comprising a filter (12) arranged in the source (1) to remove frequencies that may give rise to aliasing products.

9. Vector network analyzer according to one of the above claims, comprising an isolator (19) arranged in the source (1) to stabilize the source signal.

10. Vector network analyzer according to claim 9, where the isolator (19) consists of an attenuator, a circulator, a directional coupler or an isolation amplifier.

11. Vector network analyzer according to one of the above claims, comprising two or more signal sources (1), where the signal sources (1) either do not transmit simultaneously or transmit with orthogonal signals or at different frequencies with signals separable by the response receivers (7).

12. Vector network analyzer according to one of the above claims, comprising balanced circuitry without ground reference in stead of circuitry with signal amplitudes referred to ground.

13. Vector network analyzer according to one of the above claims, comprising a response receiver (7) that comprises a sampler (15) and an A/D-converter (16) where the sampler (15) and the A/D-converter (16) are synchronized by assistance from the clock signal for signal sampling (23).