GB2252833A

GB2252833A - Spectrum analysers

Info

Publication number: GB2252833A
Application number: GB9103244A
Authority: GB
Inventors: Roderick Leonard Walla Stevens
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1991-02-15
Filing date: 1991-02-15
Publication date: 1992-08-19
Anticipated expiration: 2011-02-15
Also published as: GB2252833B; GB9103244D0

Abstract

A spectrum analyser comprises a first mixer (14A) fed with signals for analysis (11, 12) (the wanted signal), together with reference signals (14) in a different frequency band to the wanted signal, and with a synthesised local oscillator signal (15), thereby (16) to produce an intermediate frequency (IF) wanted signal channel (17, 19, 23) and an IF reference signal channel (16, 20, 26), means being provided for digitising (19, 20) and combining (27, 24) these two channels whereby to provide an output signal (28) in which unwanted sidebands of the synthesised local oscillator signal are reduced by cancellation. As described, a local oscillator model generator (27) produces a local oscillator digital signal which includes components in anti-phase to unwanted sideband signals present in the signal from the phase-lock loop (15), so that the unwanted sidebands are cancelled in the digital mixer (24). <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO SPECTRUM ANALYSERS This invention relates to spectrum analysers.

It is a common requirement in spectral analysis systems to identify the presence of wanted low power signals situated very close in frequency to unwanted high power signals. This situation can arise either when designing a covert communication system (i.e.

where a signal is deliberately located close to a very strong cover signal in order to remain undetected) or when operating in a hostile jamming environment. This problem of maintaining frequency selectivity is exacerbated when frequency hopping techniques are employed due to the frequency agility required of the receiver components, and in particular the "downconversion" local oscillator (L.O). This is because noise sidebands on the L.O. are transferred to the spectrum to be analysed during a "downconversion" process and often limit the selectivity obtainable using standard frequency analysis methods such as the use as a filter bank or the use of a fast fourier transform technique (F.F.T.) for example.

A traditional approach to this problem has been to improve the spectral purity of the L.O. using one of the following three techniques, which comprise; a) selecting one of a multitude of free running single frequency oscillators. This approach is only really suitable where limited frequency agility is required, or where large amounts of space and power are available for the receiver, or; b) using direct synthesis techniques to produce the frequency by combining the outputs of a multitude of free running single frequency oscillators. This approach uses less hardware space, but is more complex than the option (a) above and such synthesisers tend to be expensive to design or to purchase, or; (c) using a phase-lock loop (PLL.) to provide frequency agility and carefully choosing the loop filter components to minimise residual noise level.This is the normally preferred solution and a typical high performance fast switching P.L.L. can achieve - 50dB noise sidebands 50 kHz away from the centre band frequency.

It is also possible to use a direct digital synthesiser (D.D.S) as the frequency agile local oscillator but the present state of D.D.S technology does not permit the practical use of low power high spectral purity wide bandwidth systems.

According to the present invention a spectrum analyser comprises a first mixer fed with signals for analysis hereinafter called wanted signal, and with reference signals in a different frequency band to the wanted signal, and with a synthesised local oscillator signal, thereby to produce an intermediate frequency (IF) wanted signal channel and an IF reference signal channel, means being provided for digitising and combining these two channels thereby to provide an output signal in which unwanted sidebands of the synthesised local oscillator signal are reduced by cancellation, whereby detection of the presence of a signal within the band of signals for analysis i.e. the wanted signal in facilitated.

According to one aspect of the present invention a spectrum analyser comprises a reference signal generator operative to generate a signal in a first frequency band, signal combiner means to which a wanted signal to be analysed is fed for combination with the said reference signal thereby to produce a combined signal, the wanted signal being chosen to fall within a second frequency band which is outside the limits of the said first frequency band, a first mixer fed with the combined signal, a synthesiser which feeds the said first mixer with a first local oscillator signal thereby to produce in a wanted signal channel a wanted first IF signal and in a reference signal channel a reference first IF signal, a wanted signal channel mixer fed with the wanted first IF signal, a first stabilised oscillator arranged to feed the wanted signal channel mixer thereby to produce a wanted second IF signal in the wanted signal channel, a reference signal channel mixer fed with the reference first IF signal, a second stabilised oscillator arranged to feed the reference signal channel mixer thereby to produce a reference second IF signal in the reference signal channel, the frequency of the first and second stabilised oscillators being chosen such that the second IF signals in the reference signal channel and the wanted signal channel respectively occupy the same band, a sample clock oscillator, analogue to digital converter means to which the second IF signals in the reference signal channel and the wanted signal channel respectively are applied to be sampled in dependence upon the sample clock oscillator thereby to produce digital signals corresponding to the wanted and reference signals respectively, a local oscillator model signal generator to which the digital reference signal is fed and a digital mixer fed with the wanted digital signal and with a signal from the local oscillator model signal generator thereby to produce a digital output signal in which sidebands of the synthesiser are reduced by cancellation, whereby detection of a signal within the band for analysis, i.e. the wanted signal is facilitated.

The synthesiser in a spectrum analyser according to the present invention may comprise at least one phase-lock loop.

The digital output signal may be fed to a fast fourier transform network or alternatively to a filter bank comprising a plurality of parallel fed filters whereby detection of a signal within the band for analysis, i.e. the wanted signal is facilitated.

According to one embodiment of the invention the analogue to digital converter means comprises a signal combiner which is fed with signals from the wanted signal channel and from the reference signal channel at the second intermediate frequency, an output signal from the signal combiner means being fed to an analogue to digital converter which is clocked under control of the sample clock oscillator to produce a pair of output signals one of which is fed to the digital mixer and the other of which is fed to the local oscillator model generator, the said model generator being arranged to produce a signal which is fed back to the digital mixer thereby to produce the said digital output signal.

According to an alternative embodiment of the invention the analogue to digital converter means comprises a pair of analogue to digital converters fed with the second IF frequency from the reference signal channel and the wanted signal channel respectively, the analogue to digital converters being clocked with a signal derived from the sample clock oscillator whereby an output signal from the reference signal channel is fed to the local oscillator model generator to produce an output signal for a digital mixer, a digital version of the signal at second IF from the analogue digital converter in the wanted signal channel being fed as a further input to the digital mixer thereby to produce the said digital output signal.

In systems according to the present invention the synthesiser would normally be switched to select a frequency band of interest and the use of a filter bank or F.F.T technique to detect the presence of a signal within a band of interest would be facilitated due to the absence of interfering synthesiser sideband signals.

Some embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a generally schematic block diagram of a known spectrum analyser.

Figure 2 is a generally schematic block diagram of a spectrum analyser according to one embodiment of the invention.

Figure 3 is a generally schematic block diagram of a spectrum analyser according to another embodiment of the invention and Figure 4 is a generally schematic block diagram of a spectrum analyser according to a further embodiment of the invention.

Referring now to Figure 1 a known spectrum analyser for analysing the signal received by an aerial 1 comprises a bandpass filter 2 via which input signals are fed to a mixer 3. The mixer 3 is also fed from a phase-lock loop 4 which forms a part of a local oscillator synthesiser. Intermediate frequency output signals from the mixer 3 are fed via a second bandpass filter 5 to an analogue to digital converter 6. The analogue to digital converter 6 is clocked by a clock signal generator 7 to produce a digital output signal which is fed to a data storage unit 8. Output signals from the data storage unit 8 are fed to a digital signal processing unit 9 which embodies a fast fourier transfer network 10 which serves to analyse in a relatively narrow frequency band the signals received.

In order to cover a wide frequency band, for the purpose of detecting the presence of a signal, the phase-lock loop 4 is normally stepped through a plurality of frequency bands and output signals appertaining to each band are fed to the fast fourier transform network 10 for analysis whereby the detection of signals in one of a plurality of channels within each band is facilitated.

One of the problems with this known system is that unwanted sidebands produced by the phase-lock loop 4 result in spurious signals being fed to the fast fourier transform network 10 whereby the detection of low level wanted signals is rendered difficult if not impossible.

One embodiment of the invention will now be described with reference to Figure 2 wherein input signals received by an aerial 11 are fed via a bandpass filter 12 to a signal combiner 13. The signal combiner 13 is fed from a high spectral purity combed frequency generator 14 to provide a plurality of equally spaced out of band reference signals. Signals from the combiner 13 are fed to a first mixer 14A which is also fed from a phase-lock loop 15 which provides a local oscillator signal. Output signals from the mixer 14A are fed to a second combiner 16 which feeds a bandpass filter 17 in an analysis signal channel which is referred to herein a wanted signal channel and a bandpass filter 18 in a reference signal channel.Thus the bandpass filter 17 defines the second intermediate frequency limits of a wanted signal channel and the bandpass filter 18 defines the second intermediate frequency limits of the reference signal channel. The filters 17 and 18 are arranged to feed A to D converters 19 and 20 respectively which are clocked under control of a signal generator 21 which feeds the A to D converters 19 and 20 via a signal combiner 22. Digital signals from the A to D converter 19 are fed via a digital store 23 to a digital mixer 24 which forms part of a digital signal processing unit 25.

Digital signals from the A to D converter 20 on the other hand are fed to a digital store 26 which is arranged to feed a local oscillator model generator 27 which serves to produce a local oscillator signal in digital form which includes the unwanted sidebands present in the signal produced by the phase-lock loop 15 but in anti-phase therewith whereby an output signal from the digital mixer 24 on line 28 is produced, in which cancellation of the sidebands is effected. The signal on the line 28 is fed to a fast fourier transform network 29 which serves to analyse signals in a channel chosen in accordance with the frequency setting of the phase-lock loop 15.

It will be appreciated that in the embodiment of the invention just before described, rather than seeking to improve the spectral purity of the local oscillator, its deleterious effects are minimised on the subsequent spectral analysis by cancelling the undesirable sideband features thereof, whereby overall selectivity of the analysis system is enhanced.

In order to facilitate a better understanding of the invention a further embodiment will now be described with reference to Figure 3 wherein a particular set of frequency conditions will be considered.

Referring now to Figure 3 an input signal is received by an aerial 30 and fed to a bandpass filter 31 having a 30MHz to 90 MHz passband.

Output signals from the bandpass filter 31 are fed to a signal combiner 32 together with signals from a comb frequency generator 33 which are fed to the combiner via a bandpass filter 34. The comb generator produces spectral lines at 4 MHz spacing in a band which extends from 161 MHz to 217 MHz. Thus the passband of the bandpass filter 34 is chosen to be 161 MHz to 217MHz. Output signals from the combiner 32 are fed to a first mixer 35 which is fed also from a phase-lock loop 36 which forms part of a synthesiser.

The phase-lock loop 36 produces signals in the 4MHz to 60 MHz band in 4 MHz steps, whereby an input signal in the 30MHz to 90 MHz band as determined by the bandpass filter 31 can be examined in comparatively narrow bands which are 4MHz wide. Output signals from the mixer 35 are fed to a signal combiner 37. The signal combiner 37 feeds a bandpass filter 38 which defines a 90MHz to 94MHz first intermediate frequency band for wanted signals to be analysed. The combiner 37 also feeds a bandpass filter 39 which defines a 220MHz to 225MHz band for reference signals. The bandpass filter 38 is arranged to feed a mixer 40 which is fed from a crystal oscillator 41.The crystal oscillator 41 provides an 84.5MHz local oscillator signal whereby a second intermediate frequency signal is provided for a bandpass filter 42 which is arranged to pass second intermediate frequencies in the 5.5MHz to 9.5MHz range.

Similarly the bandpass filter 39 is arranged to feed a mixer 43 which is fed with a local oscillator signal from a 214MHz crystal oscillator 44. Output signals from the mixer 43 at a second intermediate frequency are fed to a bandpass filter 45 which serves to provide second intermediate frequency reference signals in a 6MHz to 8MHz range. Second intermediate frequency wanted signals are fed to an A to D converter 46 and second intermediate frequency reference signals are fed to an A to D converter 47. The A to D converters 46 and 47 are fed from a 10MHz crystal clock oscillator 48 via a signal combiner 49. Thus the A to D converters 46 and 47 operate in a sampling mode under control of the crystal clock oscillator 48.

An output signal from the A to D converter 46 is fed to a digital mixer 50 and an output signal from the A to D converter 47 is fed to a local oscillator model generator 51 which provides a local oscillator model including unwanted sidebands which correspond to those produced by the phase-lock loop 36 but which are in antiphase.

Thus the unwanted sidebands are cancelled in the digital mixer 50 and a much purer signal is fed to a fast fourier transform unit 52 for analysis of a signal band chosen in accordance with the setting of a phase-locked loop 36.

In accordance with an alternative embodiment of the invention as shown in Figure 4 wherein parts corresponding to Figure 3 use the same numerical designations, signals from the bandpass filters 42 and 45 are fed to a signal combiner 53 which feeds a single analogue to digital converter 54 which is sampled under control of the 10MHz crystal clock oscillator 48. Output signals from the A to D converter 54 are fed to the digital mixer 50 and the local oscillator generator 51 respectively whereby unwanted sideband cancellation is effected and a spectrally pure signal is provided for the fast fourier transform network 52.It will be appreciated that the arrangement just before described with reference to Figure 4 is basically similar to the arrangement described in Figure 3 except that a digital filter technique is used to separate the reference signal from the wanted signal for analysis but in other respects the technique is identical. It will be appreciated that by using a sideband cancellation system as just before described with reference to Figure 2, Figure 3 and Figure 4 prior to spectral analysis, the application thereafter of standard fast fourier transform techniques are facilitated whereby a far higher selectivity is achievable than would otherwise be possible with uncorrected data.

Various modifications may be made to the arrangements just before described without departing from the scope of the invention and for example various other signal analysis techniques may be utilised such as filter banks instead of standard fast fourier transform techniques.

Claims

IMPROVEMENTS IN OR RELATING TO SPECTRUM ANALYSERS 1. A spectrum analyser comprising a first mixer fed with signals for analysis hereinafter called the wanted signal, and with reference signals in a different frequency band to the wanted signal, and with a synthesised local oscillator signal, thereby to produce an intermediate frequency (IF) wanted signal channel and an IF reference signal channel, means being provided for digitising and combining these two channels thereby to provide an output signal in which unwanted sidebands of the synthesised local oscillator signal are reduced by cancellation, whereby detection of the presence of a signal within the band of signals for analysis i.e. the wanted signal is facilitated.
2. A spectrum analyser as claimed in claim 1 comprising a reference signal generator operative to generate a signal in a first frequency band, signal combiner means to which a wanted signal to be analysed is fed for combination with the said reference signal thereby to produce a combined signal, the wanted signal being chosen to fall within a second frequency band which is outside the limits of the said first frequency band, a first mixer fed with the combined signal, a synthesiser which feeds the said first mixer with a first local oscillator signal thereby to produce in a wanted signal channel a wanted first IF signal and in a reference signal channel a reference first IF signal, a wanted signal channel mixer fed with the wanted first IF signal, a first stabilised oscillator arranged to feed the wanted signal channel mixer thereby to produce a wanted second IF signal in the wanted signal channel, a reference signal channel mixer fed with the reference first IF signal, a second stabilised oscillator arranged to feed the reference signal channel mixer thereby to produce a reference second IF signal in the reference signal channel, the frequency of the first and second stabilised oscillators being chosen such that the second IF signals in the reference signal channel and the wanted signal channel respectively occupy the same band, a sample clock oscillator, analogue to digital converter means to which the second IF signals in the reference signal channel and the wanted signal channel respectively are applied to be sampled in dependence upon the sample clock oscillator thereby to produce digital signals corresponding to the wanted and reference signals respectively, a local oscillator model signal generator to which the digital reference signal is fed and a digital mixer fed with the wanted digital signal and with a signal from the local oscillator model signal generator thereby to produce a digital output signal in which sidebands of the synthesiser are reduced by cancellation, whereby detection of a signal within the band for analysis, i.e. the wanted signal is facilitated.
3. A spectrum analyser as claimed in claim 2 wherein the synthesiser includes a phase locked loop.
4. A spectrum analyser as claimed in claim 3 wherein the digital output signal is fed to a fast fourier transform network or alternatively to a filter bank comprising a plurality of parallel fed filters whereby detection of a signal within the band for analysis, i.e.

the wanted signal is facilitated.
5. A spectrum analyser as claimed in claim 3 wherein the analogue to digital converter means comprises a signal combiner which is fed with signals from the wanted signal channel and from the reference signal channel at the second intermediate frequency, an output signal from the signal combiner means being fed to an analogue to digital converter which is clocked under control of the sample clock oscillator to produce a pair of output signals one of which is fed to the digital mixer and the other of which is fed to the local oscillator model generator, the said model generator being arranged to produce a signal which is fed back to the digital mixer thereby to produce the said digital output signal.
6. The spectrum analyser as claimed in claim 3 wherein the analogue to digital converter means comprises a pair of analogue to digital converters fed with the second IF frequency from the reference signal channel and the wanted signal channel respectively, the analogue to digital converters being clocked with a signal derived from the sample -clock oscillator whereby an output signal from the reference signal channel is fed to the local oscillator model generator to produce an output signal for a digital mixer, a digital version of the signal at second IF from the analogue digital converter in the wanted signal channel being fed as a further input to the digital mixer thereby to produce the said digital output signal.
7. A spectrum analyser as claimed in claim 1 and substantially as hereinbefore described with reference to Figure 2, Figure 3 or Figure 4 of the accompanying drawings.