GB2248504A

GB2248504A - Measurement of the frequency of a signal, or of the frequencies contained in a composite signal.

Info

Publication number: GB2248504A
Application number: GB8407390A
Authority: GB
Inventors: Claude Aury; Jean-Luc Montardy
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1983-03-31
Filing date: 1984-03-22
Publication date: 1992-04-08
Anticipated expiration: 2004-03-22
Also published as: IT8467307A0; IT1235233B; FR2661506A1; DE3411654A1; FR2661506B1; DE3411654C2; NL8401025A; GB2248504B

Description

wIDE BAND PROCESS AND DEVICE FOR THE MEASUREMENT OF THE FREQUENCY OF A SIGNAL, ESPECIALLY FOR THE MEASUREMENT OF THE SEVERAL FREQUENCIES OF SIMULTANEOUS SIGNALS The presert invention rela+es to a wide band process and device for the measurement of the frequency of a signal and especially for the measurement of the several frequencies of simultaneous signals.

There is a number of types of device for measuring the frequency of a signal amongst which may be noted: - devices comprising a frequency discriminator fed by the signal itself and by the same signal after it has been delayed.

A device of this type is described, for example, in the article by N.E. GODDARD "Instantaneous Frequency Measuring Receivers" which appeared in IEEE Trans. 1972, MTT-20, pages 292-293.

Devices of this type cannot be used for the measurement of the several frequencies of simultaneous elementary signals.

- Devices producing a count of the number of signal periods per unit of time.

Devices of this type have the abovementioned disadvantage that they cannot be used in the case of simultaneous signals of different frequencies and, for technological reasons, in the case of a very high frequency signaL; - dispersive line devices in which, as the propagation time is dependent upon the signal frequency, the frequency of the latter may be obtained from the line input-output transition time. A device of this type makes it possible to measure the several frequencies of simul tantous signals but has a narrow band of operating frequericy.

The present invention makes it possible to overcome the above disadvantages and has as its objectives a process and a device for signal frequency measurement which make it possible to measure the several frequencies of simultaneous signals, especially for high frequency signals.

The measurement process and device according to the invention use the relative phase properties of two signals of the same frequency propagating in opposite directions in the same transmission unit.

According to the invention, the signal whose frequency f it is wished to determine is injected into a transmission unit in such a way that it gives rise to two signals propagating ir opposite directions from two initial points respectively. From this, a standing wave condition develops whose envelope has periodic variations as a function of the distance from a reference point. This envelope is detected end its frequency measured, this frequency being called spatial since its reciprocal, or spatial period, has a dimension equivalent to distance.

It can be shown that the frequency f of the injected signal 5 proportional to the spatial frequency Fs.

According to the invention, the process for the measurement of the frequency of a signal comprises the following successive stages: - injection of the signaL, called the incident signal, at the input of a transmission unit in such a way that it gives rise to a first and a second signal in the transmission unit, these signals being called input signals, whose frequency is identical and equal to that of the incident signal, and which propagate in opposite di rection from two-initial points respectively;; - generation of the envelope of the resulting wave which exists in the transmission unit, the envelope having periodic variations of spatial frequency as a function of the distance from a point of reference of the transmission unit for which the electrical paths traversed by the said input signals from the respective initial point of propagation are equal; - measurement of the spatial frequency of the envelope of the resultant wave; and - calculation of the frequency of the incident signal by multiplying the said envelope spatial frequency by the coefficient c/2 in which c is the speed of propagation of the waves in the transmission unit.

Other advantages and characteristics of the present invention will be apparent from reading the detailed description that follows with reference to the annexed figures which show: - Figure 1, a block diagram of the device according to the invention; - Figure 2, one embodiment of a first subassembly of the device of Figure 1; - Figure 3, one embodiment variant of the first subassembly of the device according to the invention; - Figure 4, another embodiment variant of the part of the device of the invention shown in Figure 2; and - Figure 5, an example of embodiment of a second subassembly of the device of Figure 1.

In the embomenTs shown My figures 1 to 4, reference 2 indicates a transmission unit fed by a signaL, called the incident signaL, of unknown frequency f. This incident signal gives rise to two signals of the same frequency f, propagating in the transmission unit 2, in opposite directions from points A and B respectively, these points also being called the initial points of propagation.

The means of generating these two signaLs, called input signals in the following text, from the i-ncident signal have not been shown.

Transmission unit 2 may consist of transmission lines (such as shown in Figures 2, 3, 4 for example) or circuits (not shown) in which the electrical path followed by the signal is shorter than the wavelength corresponding to the frequency f of the incident signal.

Figure 1 shows the block diagram of the device according to the invention: the resultant signal existing in the transmission unit 2 is applied to the input of a circuit 3 which determines its enveLope.

This envelope has periodic variations of the period, called the spatial period, T5 as a function of the distance s of each point of the transmission unit 2 fron a point of reference Mo.

A circuit 4 determines the spatial frequency Fs= 1/Ts of the envelope generated by circuit 3, and a circuit 5 caLcuLates the frequency f of the incident signal from the said spatial frequency Fs by using the formula (1):f = c.Fs/2, c being the speed of propagation of the waves in the transmission unit.

The operation of the device according to the invention shown by Figure 1 is described be low by referring to Figures 1 and 2. A first embodiment of transmission unit 2 and of envelope computation circuit 3 is shown by Figure 2.

The transmission unit 2 consists, for example, of a simple transmission line 200 at the two ends, A and B, of which are applied respectively the two input signals of same frequency f, as mentioned above.

The simultaneous presence of the two signals on the same line creates a standing wave condition.

As has already been stated, the present invention is based upon the use of the relative phase properties of two signals of the same frequency propagating in opposite directions in a transmission unit, and so the remaining processing is carried out on the resultant standing wave.

According to one non-limiting embodiment, the generated standing wave is sampled by N means of coupling 201, 202, ..., 20N. It is assumed that the couplings are weak in order not to interfere with the standing wave condition.

Circuit 3, which computes the envelope of the standing wave, may be composed in a non-limiting way of N detectors 301, 302, ..., 30N, corresponding respectively to the means of coupling 201, 202, ..., 20N, and each issuing a sample of the envelope of the standing wave. The number and positioning of the means of coupling 201, 202, ..., 20N, are defined by those skilled in the art, by using Shannon's theorem, in such a way as to reconstitute at the output of detectors 301, 302, ..., 30N, in the operating frequency band, the envelope of the standing wave as a function of the distance s from the point of reference Mo.

The detected samples issued by the N detectors 301, 302, ..., 30N, are applied to the input of a spacetime conversion device 31 which issues at its output 311 a periodic signal Sp which represents the sampled variations of the output voltages of the N detectors, that is to say the variations, as a function of time, of the envelope of the standing wave created in the transmission unit 2.

The detectors are preferably quadratic detectors. The conversion device may, according to one non-limiting exampLe which is not shown, be an N input commutator receiving the output signals of the detectors 301, 302, ..., 30N, respectively, and switching them in sequence onto the output 311 at a defined frequency controlled by an external pulsed signal of frequency fe.

Output 311 of the space-time conversion device 31 is applied to the input circuit 4 which computers the spatial frequency Fs of the sampled enveLope. This circuit 4 may, for exampLe, comprise a Fourier analyzer. Circuit 5 then calculates the frequency f = c.Fs/2 of the incident signal and of the input signals, c being the speed of propagation of the waves in the transmission unit.

Let Mo be a reference point of the transmission unit 2 for which the electrical paths traversed by the two input signals (same frequency, possible phase displacement 9) are equal.

The phase displacements experienced ty each of the two input signals between the initial respective propagation points A, B and the reference point Mo of the transmission line are equal and consequently their phase displacement relative to the reference point Mo is the same as their possible relative phase displacement 9 at the input A, B of the transmission Line 200.

At each point of the transmission line situated at a distance s from the reference point Mo the resultant voltage e(s) is expressed by the relationship (2): e(s)=E. cos (#/2 - #.5/c)cos#t (2) where - E is a voltage which is a function of the amplitude of the signals injected at each end of the line; P i is the phase difference of the two input signals of the transmission line; and - O 2 2 T.f is the pulse rate which corresponds to the unknown frequency f of the input signals.

It has been assumed that the means of coupling 201, 202, ..., 20N are designed in such a way as not to interfere with the standing wave which has been created.

The voltage E (s) detected along the line by quadratic detectors is proportional to E2. [1 + cos (# 2#.s)] The voltage obtained at the output of the spacetime converter 31 after filtering is then proportional to E2 . C1 + cos (2 F.t) for a convenient value of the start of time, F being the frequency of the resultant wave at the outFut of the space-time converter 31.

It can be shown that the resultant frequency F is linked to the spatial frequency Fs by the relationship (3): F = fe e s.Fs (3) in which: - fe is the envelope sampling frequency by the space-time conversion device 31 (frequency of pulsed signal driving the commutator according to the embodiment example referred to above); and - 1is is the interval between the means of coupling 201, 202, ..., 20N.

Circuit 4 determines the frequency F, by Fourier analysis for example, of the output signal Sp and calculates the spatial frequency Fs of the envelope using formula (3) above. From the spatial frequency Ps supplied by circuit 4, circuit 5 calculates the frequency f of the incident signal using formula (1) referred to earlier, namely f = c.Fs/2.

If the incident signal is a complex signal, that is to say composed of a plurality of elementary signals of different frequencies, it can be shown that the envelope of the standing wave is a carrier for data appertaining to each freqllency: the voltage detected along the transmission line 200 is the sum of the voltages that would be obtained for each elementary signal separately, applied at the input A and with a possible phase displacement ç at input B, the phase displacement being the same for all of the elementary signals.

In the case of a complex signal, circuit 4 makes it possible to isolate each frequency F corresponding to each elementary signal and to calculate each corresponding spatial frequency Fs.

According t another non-represented embodiment, the second initial propagation point B may be short circuited so that the second input signal is the first input signal propagating in the reverse direction, after reflection at the short circuit.

Figure 3 shows another embodiment of transmission unit 2 which, together with circuit 3 of Figure 2, can be used in the device according to the invention shown by Figure 1: The transmission unit 2 may also comprise two parallel transmission lines 210 and 220 terminated at one end by their characteristic impedances, Z1 and Z2 respectively, their other end being arranged facing the end of the other line which is terminated by the characteristic impedance.

The two input signals are applied to the open end A,B of the first and second transmission lines 210, 220 respectively.

Each means of coupling 201, 202, ..., 20N may compr;se a first and a second coupler sampling a part of the signal transmitted by the first and second Lines 210 and 220 respectively, and a circuit producing the summation of the sampled signaLs. In a simpler manner each means of coupling 201, 202, ..., 20N may be coupled at one and the same time to the first and second transmission lines.

Summation of the sampled signals on each line at the two points M1, M'1; ...; or MN, M'N situated opposite each other thus occurs. In such a construction the lines 210 and 220 operate in a progressive manner. This embodiment makes it possible to amplify the signals along the lines if there is need, whereas in the embodiment of Figure 2 amplification can only be provided at the inputs A and B of line 200.

Figure 4 shows another embodiment of circuits 2 and 3 of Figure 1.

Transmission unit 2 comprises a distributed constant transmission line 230, of the coaxial type for example, at the two ends A,B of which the two input signals are respectively applied, and whose linear inductances are indicated by L. The means of coupling for sampling the standing wave are composed of transistors shown in Figure 4 by their schematic equivalents 201, 202, ..., 20N. The stray input capacitance of the transistors appears in the calculation of linear impedance of the line and the input capacitance of each coupling transistor is shown by a single capacitance indicated by C.

Detectors 301, 302, .... 30N of subassembly 3 are each constituted by the placing of a rectifier diode D and a capacitance C' in parallel. A complete space-time converter 31, for example subassembly 3 as in Figure 2, issues at output 311 a signal Sp representing the envelope of the standing wave.

Figure 5 shows a detailed embodiment of the measurement circuit 4 where processing is performed in parallel. In this case the space-time conversion device 31 (Figures 2 and 4) is not necessary. The samples supplied by detectors 301, 302, ..., 30N of envelope computation means 3 are applied via weighting resistances 411, 41N; 421, ..., 42N; ...; 4M1, ..., 4MN to the positive or negative inputs of M adder circuits 41, ..., 4M in such a way that the output 401, ..., 40M of each of the M adder circuits 41, ..., 4M corresponds to one point of the discrete Fourier transform of the envelope. An assembly 40 of logic circuits makes it possible to determine the channel of maximum level amongst channels 401, ..., 40M and thereby to derive the corresponding spatial frequency F5. This spatial frequency F5 is coded and used by circuit 15 to calculate the frequency f = c.Fs/2 of the incident signal.

A device has thus been described which makes it possible to calculate, over a wide band of frequencies, the frequency of a signal as well as the several frequencies of simultaneous signals, especially at high frequency.

Claims

1. Process for the measurement of the frequency of a signal, which comprises successively: - a first stage for injection of the signal, called the incident signal of unknown frequency at the input of a transmission unit in such a way that it gives rise to a first and a second signal in the transmission unit, these signals being called input signals, whose frequency is identical and equal to that of the incident signal and which propagate in opposite directions from two initial points respectively; - a second stage for generating the envelope of the resulting wave which exists in the transmission unit, the envelope having periodic variations of spatial frequency (Fs) as a function of the distance from a point of reference of the transmission unit for which the electrical paths traversed by the said input signals from the respective initial point of propagation are equal;; - a third stage of measurement of the spatial frequency Fs of the envelope of the resultant wave; and - a fourth stage of calculation of the frequency of the incident signal by multiplying the said envelope spatial frequency by the coefficient c/

2 in which c is the speed of propagation of the waves in the transmission unit 2. Process of frequency measurement as claimed in claim 1, wherein the second stage comprises a sampling of the resultant wave in a plurality of N points along the transmission unit and a detection of each sampling.

3. Process of frequency measurement as claimed in claim 2, wherein the detection is followed by a spacetime conversion which makes it possible to generate a sampled curve which represents the variations in the envelope of the resultant wave as a function of time.

4. Process of frequency measurement as claimed in claim 3, wherein the third stage comprises the computation of frequency F of the sampled curve which corresponds to the incident signal of frequency f and the calculation of the spatial frequency of the corresponding envelope.

5. Process of frequency measurement as claimed in claim 4, wherein the frequency F of the sampled curve is determined by a Fourier analysis.

6. Process of frequency measurement as claimed in claim 2, wherein the third stage comprises the calculation of the discrete Fourier transform and the computation of the spatial frequency Fs corresponding to the maximum level of the said Fourier transform.

7. Process of frequency measurement as claimed in any of claims 2 to 6, wherein as the transmission unit comprises two transmission circuits at the input of which the two input signals are respectively applied, and in which the said input signals propagate in respective opposite directions, detection is performed on the plurality of samplings corresponding to the sum of two elementary samples 25 on the one and on the other transmission circuit respectively, at points situated opposite each other.

8. Process of frequency measurement as claimed in any of claims 2 to 7, wherein the detection is quadratic.

9. Device for measurement of the frequency of a signal, which comprises: - transmission unit (2) at the input of which is injected the signal, called the incident signal, of unknown frequency (f) and which comprises means for generating a first and a second signal, called input signals, of the same frequency (f) as the incident signal and propagating, in the said transmission unit, in opposite directions from a first and a second point (A*B) respectively, called the initial propagation points;; - means (3) of generating the envelope of the resultant wave which exists in the transmission unit, the envelope having periodic variations of spatial frequency (Fs) as a function of the distance from a reference point (Mo) of the transmission unit for which the electrical paths traversed by the said input signals from the respective initial propagation point are equal; - means (4) for measurement of the spatial frequency (Fs) of the envelope of the resultant wave; and - means (5) for calculating the frequency (f) of the incident signal equal to the product of the spatial frequency (Fs) of the envelope and the coefficient c/2 in which c is the speed of propagation of waves in the transmission unit (2).

10. Frequency measurement device as claimed in claim 9, wherein the transmission unit (2) is composed of a transmission line (200, 230' at the first and second ends of which the first and second input signals are respectively applied, the said first and second ends constitu ting the first and second initial propagation points oe the input signals.

11. Frequency measurement device as claimed in claim 9, wherein transmission unit (2) is composed of a first and a second transmission line (210, 220) which are parallel, loaded at their first end by their respective characteristic impedance (Z1, Z2) and fed at their second end, constituting-respectively the first and second initial propagation points (A,B), by the first and second input signals respectively, the first end of one transmission Line (210, 220) being arranged opposite the second end of the other line (220, 210) so that the said input signals propagate in opposite directions.

12. Measurement device as claimed in either of claims 9 or 10, wherein the second initial propagation point (B) is short circuited so-that the second input signal is the first input signal propagating in the opposite direction.

13. Measurement device as claimed in any of claims 9 to 12, wherein a plurality of N samples of the resultant wave existing in the transmission unit (2) having been respectively taken by a plurality of N means of coupling (201, 202, ..., 20N), the envelope generation means (3) of the resultant wave comprise a plurality of N means of detection (301, 302, ..., 30N) fed respectively by a sample taken by coupling means (201, 202, ..., 20N).

14. Frequency measurement device as claimed in claims 11 and 13, wherein the plurality of N means of coupling (201, 202, ..., 20N) each take a first sample of the first input signal on the first transmission line (210) and a second sample of the second input signal on the second transmission line (220) and each perform the sum of the first and second samples, the points for taking the first and second samples being arranged opposite each other for each means of coupling (201, 202, ..., 20W).

15. Frequency measurement device as claimed in claim 13 or 14, wherein the means (3) for generating the envelope of the resultant wave comprise in addition means of space-time conversion (31) which, from the plurality of N detected samples, generate a sampled curve (Sp) showing the variations of the envelope of the resultant wave as a function of time.

16. Measurement device as claimed in claim 15, wherein the space-time conversion means (31) comprises an N input commutator fed respectively by the N detected samples which are successively switched to the output of the commutator at a rate given by the frequency (fe) of a pulsed exterior control signal.

17. Frequency measurement device as claimed in claim 16, wherein the means (4) for measurement of the spatial frequency (Fs) of the resultant wave envelope comprise a measurement circuit for the frequency (F) of the sampled curve (Sp) generated by space-time conversion means (31) and a circuit for spatial frequency (Fs) calculation, such that Fs = F/(Fe. A s); b sbeing the interval separating the means of coupling (.201, 202, ..., 20N) from each other.

18. Frequency measurement device as claimed in claim 17, wherein the measurement circuit for the frequency (F) of the sampled curve (Sp) is a Fourier analyzer circuit.

1 9. Frequency measurement device as claimed in claim 13, wherein the means (4) for measurement of the spatial frequency (Fs) of the resultant wave envelope comprise a circuit for the calculation of the discrete Fourier transform fed in parallel by the N detected samples, issued by means (3) of envelope generation, and a circuit (408 which determines the spatial frequency (Fs) of the enve'.ope of the resultant wave corresponding to the highest level of the discrete Fourier transform.

20. Frequency measurement device as claimed in claim 19, wherein the circuit for calculating the discrete Fourier transform comprises a plurality of M adder circuits (41, ..., 4M) whose respective positive and negative inputs are fed by a sum of the N detected samples, issued by means (3) of envelope generation and each weighted by a weighting resistance (411, ..., 41N; ...; 4M1, ... 4MN) and whose respective outputs (401, ..., 40M) feed circuit (40) for determining the spatial frequency (Fs) of the envelope.

21. Frequency measurement device as claimed in any of claims 13 to 20, wherein the means of detection (301, 302, ..., 30N) are quadratic detectors.

22. A frequency measurement device substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

22. A process for the r:;easurelnent of the frequency of a signal substantially as hereinbefore described With reference to, and as illustrated in, this accompanying drawings.

23. A frequency measurement device substantially as hereinbefore described with reference to, and as illustrated in, tlJe accompanying drawings.

AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS 1. A process for measurement of frequencies of a signal, which comprises successively: - a first stage for injection of the signal, called the incident signal of unknown frequencies, at input means of a transmission unit in such a way that it gives rise to a first and a second signal, each of which has the same frequency as the other and which propagate in opposite directions in the transmission unit from two initial points respectively;; - a second stage for sampling at a plurality of N points along the transmission unit either the amplitude of a standing wave which exists in the transmission unit or the amplitudes of the first and second signals and combining the latter samples at each point, the amplitude of the standing wave or the combined samples having periodic variations of spatial frequency (Fs) which are a function of the distance from a point of reference of the transmission unit for which the transmission unit for which the electrical paths traversed by the said input signals from the respective initial point of propagation are equal; - a third stage of measurement of each spatial frequency Fs of the amplitude of the standing amve or of the combined samples; and - a fourth stage of calculation of the frequency of the incident signal by multiplying each said spatial frequency by the coefficient of c/2 in which c is the speed of propagation of the waves in the transmission unit.

2. A process of frequency measurement as claimed in claim 1, wherein the detection is followed by a space time conversion which makes it possible to generate a sampled curve which represents the variations in the amplitude of the standing wave as a function of time.

3 A process of frequency measurement as claimed in claim 2, wherein the third stage comprises the computa tion of frequency. F of the sampled curve which corresponds to the incident signal of frequency f and the caLculation of the spatial frequency of the corresponding amplitude.

4. A process of frequency measurement as claimed in claim 3, wherein the frequency F of the sampled curve is determined by a Fourier analysis.

5. Aprocess of frequency measurement as claimed in claim 1, wherein the third stage comprises the calcula tion of the discrete Fourier transform and the computa tion of the spatial frequency Fs corresponding to the maximum level of the said Fourier transform.

6. A Process of frequency measurement as claimed in any of claims1 to 5, wherein the transmission unit each of comprises two transmission circuits at the input of which the two input signals are respectively applied, and in which the said input signals propagate in respective opposite directions, detection / pergformed on the plurality of samplings corresponding to the sum of two elementary samples on the one and on the other transmission circuit respectively, at points situated opposite each other.

7. A process of frequency measurement as claimed in any of claims 1 to 6, wherein the detection is quadratic.

8. A device for measurement of frequencies of a signal of unknown frequencies, which comprises: - a transmission unit having input means at which the signal, called the incident signal, is injected and which comprises means for generating a first and a second signal, called input signals, each of which has the same frequency as the other and which propagate in opposite directions in the transmission unit, from a first and a second point respectively, called the initial propagation points;; - means of sampling at a plurality of N points along the transmission unit either the amplitude of a standing wave which exists in the transmission unit or the amplitudes of the first and second signals and combining the latter samples at each point, the amplitude of the standing wave or the combined samples having periodic variations of spatial frequency (Fs) which are a function of the distance from a point of reference of the transmission unit for which the electrical paths traversed by the said input signals from the respective initial propagation 'point are equal; - means for measurement of each spatial frequency (Fs) of the amplitude of the standing wave or of the combined samples; and - means for calculating the frequency (f) of the incident signal equal to the product of each said spatial frequency (Fs) of the amplitude of the standing wave or of the combined samples and the coefficient c/2 in which c is the speed of propagation of waves in the transmission unit.

9. A frequency measurement device as claimed in claim 8, wherein the transmission unit is composed of a transmission line at the first and second ends of which the first and second input signals are respectively applied, the said first and second ends constitu ting the first and second initial propagation points of the input signals.

10. A frequency measurement device as claimed in claim 8, whereinthe transmission unit is composed of a first and a second transmission line which are paral lel, loaded at their first ends by their respective char acteristic impedance and fed at their second end; constituting- respectively the first and second initial propagation points, by the first and second input signals respectively, the first end of one trans mission line being arranged opposite the second end of the other line so that the said input signals propagate in opposite directions.

11. A measurement device as claimed in either of claims 8 and 9, wherein the second initial propagation point is short circuited so-that the second input signal is the first input signal propagating in the opposite direction.

one

12. A measurement device as claimed in any/of claims 8 to 11, wherein a plurality of N samples of the standing wave existing in the transmission unit having been respectively taken by a plurality of N means of coupling, the means of sampling the standing wave comprise a plurality of N means of detection fed respectively by a sample taken by coupling means.

13. A frequency measurewent device as claimed inclaim 12 when appended to claim 10, wherein the plurality of Di- means of coupling - each take a first sample of the first input signal on the first transmission Line and a second sample of the second input signal on the second transmission line and each perform the sum of the first and second samples, the points for taking the first and second samples being arranged opposite each other for each means of coupling.

14. A frequency measurement device as claimed in claim 12 or 13, wherein the means for sampling ~~~~~~~~~~~~~ the the standing wave comprise in addition means of space-time conversion which, from the plurality of N detected samples, generate a sampled curve (Sp) showing the variations of the amplitudeof the standing wave as a function of time.

15. A measurement device as claimed in claim 14, wherein the space-time conversion means comprises an N input commutator fed respectively by the N detected samples which are successively switched to the output of the com mutator at a rate given by the frequency (fe) of a pulsed exterior control signal.

16. A frequency measurement device as claimed in claim 15, wherein the -means for measurement of the spatial frequency (Fs) of the standing wave amplitude comprise a measurement circuit for the frequency (F) of the sampled curve (Sp) generated by space-time conversion means and a circuit for spatial frequency (Fs) calculation, such that Fs = F/(Fe. A s), a sbeing the interval separating the means of coupling from each other.

17. A frequency measurement device as claimed in claim 16, wherein the measurement circuit for the frequency 'F) of the sampled curve (Sp) is a Fourier analyzer circuit.

18. A frequency measurement device as claimed in claim 12 , wherein the means for measurement of the spatial frequency (Fs) of the standing wave amplitude comprise a circuit for the calculation of the discrete Fourier transform fed in parallel by the N detected sampLes, issued by means of sampling, ~~~~~~~~~~~~ and a circuit which determines the spatial frequency (Fs) of the amplitudeof the standing wave corresponding to the high est level of the discrete Fourier transform.

19. A frequency measurement device as claimed iri claim 18, wherein the circuit for calculating the discrete Fourier transform comprises a plurality of M adder cir cuits whose respective positive and negative inputs are fed by a sum of the N detected sampLes, issued by means of sampling ~~~~~~~~~~~~ and each weighted by a weighting resistance and whose respective outputs feed circuit for determining the spatial frequency (Fs) of the amplitude.

20. A frequency measurement device as claimed in any one of claims 12 to 19, wherein the means of detection are quadratic detectors.

21. A process for the measurement of thefre quencies of a signal substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.