GB2263554A

GB2263554A - Instantaneous digital frequency meter with spatial sampling

Info

Publication number: GB2263554A
Application number: GB9017032A
Authority: GB
Inventors: Jean-Luc Montardy; Eric Chamouard; Daniel Jahan
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1989-08-04
Filing date: 1990-08-03
Publication date: 1993-07-28
Also published as: IT1242719B; DE4024438C2; GB9017032D0; SE9002654D0; IT9067580A1; SE513753C2; FR2686702B1; SE9002654L; FR2686702A1; DE4024438A1; IT9067580A0

Description

.DTD:

2263554 Instantaneous digital frequency meter it spatial sampling The present invention relates to a frequency meter operating from a spatial frequency measurement of an envelope of a standing-wave pattern generated by the signal whose frequency is to be measured in a short- circuited transmission line.

.DTD:

In this kind of frequency meter, the frequency measurement is performed through spectral analysis by searching for the maximum significative of the mod131us of the Fourier transform of Lhe envelope of the standingwave pattern detected by means of a set of diodes distributed along the transmission line. As, in practice, only a discrete Fourier transform with a limited number of points is available, this measurement is performed through a determination of the point of the discrete Fourier transform corresponding to the maximum in modulus and through an interpolation about this point.

.DTD:

In the state of the art, the interpolation is performed with accuracy only in digital manner, which requires to digitize the points of the dlscrete Fourier transform or possibly the voltages detected along the transmission lines, the spectral analysis being then also performed in the digita d3main.

.DTD:

A purpose of the present invention is to avoid thzs analog-to-digital conversion in order to achieve a frequency meter simple to implement and requiring only little hardvare.

.DTD:

According to one aspect of the present invention there is provided an instantaneous digital frequency meter.with spatial sampling, comprising:

.DTD:

- a transmission line which is short-circuited at its output and which receives at its input a signal of unknown frequency to be measured, said signal generating a standing-wave pattern in the transmission line; detectors distributed along the transmission line and sampling the amplitude of the envelope of the standing-wave pattern established within the transmission line; - first spatial-frequency discriminating means in a number equal to m, m being an integer greater than one, which have their inputs connected in parallel to the outputs of the detectors and whose frequency responses are periodic with alternate passbands and stopbands separated by transitions which are regularly distributed over the overall frequency band of the frequency meter and whose number doubles when passing from a first spatial-frequency discriminating means to the next. this number starting from a single transition in the vicinlty of the middle of the overall frequency band of the frequency meter for a first spatial-frequency discriminating means to m-1 reach 2 transitions in another; - second spatial-frequency discriminating means in a number equal to m which have their inputs connected in parallel the outputs of the detectors and whose frequency responses are complementary of tose of the first spatial-frequency criminating means; and - a set of m comparators, each of which compares the output signal of a first spatial-frequency discriminating means,it that of the second spatial-frequency discriminating means whose frequency response is complementary of its own response, and that together deliver a m-bit word characteristic of the frequency to be measured.

.DTD:

0 UI 0 I o 0,, :") c c O. o I c- - m. O":3 1-11 o - 0 0 r -< rr 0 1"I o. -- z- 3" CD 3-.- 3" F-, F-, --..::) I=1 I"I o 0 r-s- I. =:7 -- i- -'i 0) (I) t I o m {I) "1o.- I.J i-.1 "- CL ('0 I-- c c 4c c- o o I o:3- o.:3 "7 C). 0 - (0 I-. m - i... t I ==i 0 .. "-1 0 0.) n:3 "0 I I i I i-.- I u O) I.

I..

.DTD:

o I - Figure ? shows another embodiment consisting in cascading two frequency meter arrangements shown in the preceding Figures.

.DTD:

Referring to Figure 1, a transmission line 1 is shown with a shortcircuited end 2 and an end 3 that receives a signal sin wt whose frequency is to be measured. This transmission line 1 exhibits a standing- wave pattern having a spatial frequency Fs reIated to the frequency f of the incident signal by the relationship:

.DTD:

f = c Fs/2 (1) where c is the propagation velocity of the waves in the transmission line. N square-law detectors, each represented by a diode 11,!2, 13, 14, 15, 16, respectively, followed by a lowpass filter 21, 22, 23, 24, 25, 26, respectively, have their input connected to taps regularly distributed along the transmission line 1. The pitch of these taps corresponds, to satisfy the sampling theorem, to the minimum spatial halfiperiod of the envelopes of the standing waves capable of developing in the transmission line I in the presence of a signal to be measured having the maximum frequency permitted by the frequency meter. The knowledge of the distance from the shortcircuit to the closest square-law detector allows to benefit from the mirror effect caused by the short-circuit and to simulate a double-length transmission line with spatial sampling receiving at both ends the signal whose frequency is to be measured.

.DTD:

The N outputs of the square-law detectors are connected to the N conductors of a bus connected in turn to the inputs of a bank of m spatial-frequency discriminators 31, 32, 33 and 41, 42, 45. The first bank of spatial-frequency discriminators)I, 32, 33 is made up of m spatial-frequency discriminators having periodic frequency responses with alternate passbands and stopbands separated by transitions which are regularly distributed over the overall frequency band of the frequency meter and whose number doubles when passing from a spatia1-frequency discriminator to the next, starting, in a spatial-frequency discriminator 51, from a single transition located in the vicinity of the middle of the overall frequency band of the frequency meter to reach at the limit in another 2m-I spatial-frequency discriminator)3, transitions and going through all the intermediates stages of 21 22 ..., 2m-2 tran- sitions. The second bank of of m spatial-frequency discriminators 41, 42, 45 is made up of spatial-frequency discriminators having frequency responses complementary of those of the spatial-frequency discriminators)1, 32, 53 of the first bank. The outputs of each pair of spatial-frequency discriminators 31,41; 32, 42;)), 43, respectively, of the first and second banks with complementary frequency responses are connected to the inputs of a comparator 51, 52, 53, respectively, which generates at its output a binary signal with a logic level or the other depending on the frequency of the signal to be measured being within the passband or the stopband of the spatial-frequency discriminator belonging to the first bank in the pair of interest. The outputs of the comparators 51, 52, 5) deliver a m-bit word which is characteristic of the relative position of the frequency to be measured within the overall frequency band of the frequency meter and which is applied to an encoder 60 performing a firanscoding of this m-bit word into pure binary code.

.DTD:

Referring to Figure 2, a diagram is shown with curves illustrating the frequency responses of the first and second banks of spatial-frequency discriminators plotted as a function of the temporal frequency of the signal appIied to the transmission line 1 which can be derived from the spatial frequency of the standing-wave pattern present in the transmission line 1 by means of the relationship (1).

.DTD:

The curve V1 corresponds to the frequency response of the spatialfrequency discriminator 31 of the first bank. This curve exhibits a single transition in the vicinity of the middle of the overall frequency band of the frequency meter, separating a stopband occupying the ange of lower frequencies from a passband occupying therange of upper frequencies.

.DTD:

The curve V' corresponds to the frequency response of1 the spatial-frequency discriminator 41 of the second bank. This curve has s shape complementary of that of curve VI. It exhibits a single transition in the vicinity of the middle of the overall frequency band of the frequency meter, separating a passband occupying the range of the lower frequencies form a stopband occupying the range of the upper frequencies.

.DTD:

The combination of these two shapes of frequency response V1 and V'1 at the additive and subtractive inputs of tne comparator 51 allows to obtain at the output thereof a binary signal with the logic leval 0 if the frequency of the signal to be measured is within the stopband of the spatial-frequency discriminator)1 of the first bank, in the range of tie lower frequencies of the frequency meter, or with the logic level 1 if the frequency of the signal to be measured is within the passband of the spatial-frequency discriminator 51 of Lhe first bank, in the range of the upper frequencies of the fequency meter.

.DTD:

The curve V2 corresponds to the frequency response of the I0 spatial-frequency discriminator}2 of the first bank. This curve exhibits two transitions located in the vicinity of the middle of the ranges of lower and upper frequencies of the frequency meter and delimiting a central passband flanked by two stopbands, the width of the passband corresponding to that of both stopbands.

.DTD:

The curve V' corresponds to the frequency response of2 the spatial-frequency discriminator 42 of the second bank. Its shape is complementary of that of curve V2.

.DTD:

The combination of these two shapes of frequency response V2 and V'2 at the additive and subtractive inputs of the comparator 52 allows to obtain at the output thereof a binary signal with the logic level 0 if the frequency of the signal to be measured is within a stopband of the spatial-frequency discriminator)2, in the first half of the range of the lower frequencies or in the second half of the range of the upper frequencies of the frequency meter, or with the logic Ievel I if the frequency of the signal to be measured is within the passband of the spatial-frequency discriminator)2, in the second half of the range of the lower frequencies or in the first half of the range of the upper frequencies of the frequency meter.

.DTD:

The iogic level of the binary signai at the output of the comparator 52 allows to locate the frequency of the signal to be measured wit respect to an alternate sequence of I+2 ranges dividing the overall frequency band of the frequency meter. Combined with the logic level of the binary signal at the output of the comparator 51, it allows to locate the frequency of the signal to be measured with respect to a succession of 22 ranges of equal width covering the overall frequency band of the frequency meter.

.DTD:

The curve V corresponds to the frequency response of the m last spatial-frequency discriminator 33 of the first bank. This curve exhibits 2m-1 transitions between passbands and stopbands, regularly distributed over the overall frequency band of the frequency meter.

.DTD:

The curve V' corresponds to the frequency response ofm the last spatial-frequency discriminator 4) of the second bank.

.DTD:

Its shape is complementary of that of curve V m The combination of these two shapes of frequency response at the additive and subtractive inputs of the comparator 5) allows to obtain at the output thereof a binary signal with the logic level 0 if the frequency of the signal to be measured is within a stopband of the patial-frequency discriminator 33, or with the logic level I if the frequency of the signal to be measured is within a passband of the spatial-frequency discriminator)), thus allowing to locate the frequency of the signal to be measured with respect to an alternate sequence of I+2m-1 ranges dividing the overall frequency band of the frequency meter. This binary signal supplemented by the set of signals from the other comparators 51, 52, allows to determine the frequency of the signal to be measured by locating it with respect to a succession of 2m ranges of equal width covering the overall frequency band of the frequency meter and constituting a sort of graduation, the more finer the greater the number m is.

.DTD:

The synthesis of the spatial-frequency discriminators is performed by combining in an adequate manner the responses of the narrow-band Frequency discriminators regularly distributed over the overall frequency band of the frequency meter which are constituted by the points of the discrete Fourier transform computed From the voltages delivered by the N square-law detectors.

.DTD:

As a matter of fact, these points of the discrete Fourier I I C v O.

-- Iz N - 1.0 - This last expression shows that the unknown frequency response X(f) can be obtained through a linear combination of the detected voltages V weighted by real coefficients C with n n max = Z X(k Cn N k the values:

.DTD:

) cos[2TF p(n)].

.DTD:

In the foregoing, the effects of sampling which are significant have not been taken into account because the number N of the detectors is generally small, so one must in practice optimize the weighting coefficients as a function of the performance characteristics which are to be maximized.

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Referring to Figure 3, a circuit diagram of the digital frequency meter is shown in which each pair of spatial-frequency discriminator connected at the input of a comparator 51', 52', 53' is implemented by means of a single differential amplifier 71, 72, 7) receiving at its inputs the voltages V n delivered by the detectors and transmitted through resistors implementing weighting coefficients. Each resistor reprssents a weighting coefficient C" which positive if it is connected n to the non-inverting input of the differential amplifier, or negative if it is connected to the inverting input of the differential amplifier, and which corresponds to the sum of the weighting coefficients Cn, C' of same order of the discrimi- n nators in the pair:f interest.

.DTD:

Referring to Figure 4, there are shown the output voltages V71 through V73 of the differential amplifiers 71 to 73 of Figure 3 which implement directly the difference of the output signals of the spatial-frequency discriminators complementary of the type shown in Figure 2. The output comparatocs 51' to 53' have then their reference equal to zero and operate then as polarity detectocs.

.DTD:

t I I cond individual stages with inverting operational amplifiers 714, 715, 716 with their inputs connected through resistors to the output of the first stage, and of the various detectors. This arrangement amounts in practice to use as second discriminator in each pair a single spatialfrequency discriminator whose frequency response is of the all-pass type and that delivers at its output a signal whose level corresponds to the output average level of the first spatial-frequency discriminator in each pair.

.DTD:

Referring to Figure 7, there is shown the circuit diagram of another embodiment consisting in cascading two frequency meter arrangements similar to those which have just been described, with a common encoder, using transmission lines with different lengths. The first arrangement 100 operates with a transmission line 101 having taps whose spacing is chosen as a function of the oecall operating frequency band, and delivers a mLbit word representative of the frequency to be measured. The second arrangement 200 operates with a tansmission line 201 having taps with a wider spacing chosen for an operating frequency band 2m' times narrower and delivers a m"-bit word representative of the frequency to be measured with an accuracy 2m' times greater than that of the bLnary word of the first arrangement 100, but with ambiguities resolved by the binary word of the first arrangement. The mLbit binary word delivered by the first arrangement 100, and the m"-bit word delivered by the second arrangement 200 ace applied to a common encoder 300 that delivers a measure of the frequency in pure binary code with m' m" bits.

.DTD:

The accuracy of these two cascaded arrangements is that of an arrangement providing m'+m" bits of frequency requiring about 2m'm'' detecto[-s, whereas they comprise only_?m' 2m" of them.

.DTD:

In practice, the effects of sampling in these two arrangements in cascade result in the sensitivity of the second arrangement 200 becoming periodically zero as a function of the frequency. The function of the comparator 400 is to determine whether the second arrangement 200 operates in a range of low sensitivity. If so, the frequency measured by the second arrangement must be replaced by that corresponding to the range of low sensitivity. In the case where the short-circuit placed at the end of the trasnmission line 200 is distant from the first detector by half the spacing between successive detectors, this value to be substituted is O. This substitution is performed by logic gates of te ANO type 500 controlled by the comparator 400 and inserted between the second arrangement and the encoder 300.

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.CLME:

Claims

Claims .CLME:

I0 1. An instantaneous digital frequency meter with spatial sampling, comprising:

.CLME:

- a transmission line which is short-circuited at its output and that receives at its input a signal whose frequency is to be measured, said signal generating a standing-wave pattern in said transmission line; detectors distributed along said transmission line and sampiing the amplitude of the envelope of said standing-wave pattern established within the transmission line; - first means of spatial-frequency discrimination in a number equal to m, m being an integer greater than one, which have their inputs connected in parallel to the outputs of said detectors and whose frequency responses are periodic with altsrnate passbands and stopbands separated by transitions which are regularly distributed over the overall frequency band of the frequency meter and whose number doubles when passing from a first filtering means to the next, this number starting from a single transition in the vicinity of the middle of sa'id oerall frequency band of the frequency meter for a first spatialfrequency discriminating means to reach 2m-1 transitions in another; - second spatial-frequency discriminating means in a number equal to m which have their inputs connected in parallel to the outputs of said detectors and whose frequency responses are complementary of those of said first spatial-frequency dsicriminating means; and - a set of m comparators, each comparing the output signal of a first spatial-frequency discriminating means with that of the second spatial-frequency discriminating means whose frequency response is complementary of its own, and which together deliver a m-bit word characteristic of the frequency to be measured.

.CLME:

I0 2. A frequency meter as elamed in clalm I, comprising in addition an encoder disposed at the output of said set of m comparators, that transcodes into pure binary code said m-bit word delivereo by tne comparators.

.CLME:

5. A frequency meter as cla{mpd in clam 1, -rein said first and second spatial-frequency discriminating means are implemented together by means of differential amplifiers with their inputs connected to said detectors through weighting resistors, the set of m comparators comparing then each output signal from said differential amplifiers with a zero voltage to extract its sign.

.CLME:

4. A frequency meter as claimed ino.lam3, aN.reineaehof said differential amplifiers includes two successive inverting operational amplifier stages, each provided with a feedback resistor.

.CLME:

5. An instantaneous digital frequency meter with spatial sampling, comprising:

.CLME:

- a transmission line which is short-circuited at its output and which receives at its input a signal of unknown frequency, said signal generating a standing-wave pattern in said transmission line; - detectors distribJted along said transmission line and sampling the amplitJde:F the envelope of said standing-wave pat- I0 tern established within the transmission line; - first spatial-frequency discriminating means in a number qual to m, m being an integer greater than one, which have their inputs connected in parallel to the outputs of said detectors and whose frequency responses are periodic with alternate passbands and stopbands separated by transitions which are regularly distributed over the overall frequency band of the frequency meter and whose number doubles when passing from a first spatiaI-frequency discriminating means to the next, said number starting from a single transition in the vicinity of the middle of said overall frequency band of the frequency meter for a first spatal-frequqcy discriminating means to reach 2m-1 transitions in another; - a second spatial-frequency discriminating means which has its inputs connected in parallel to the outputs of said detectors and whose frequency response is of the all-pass type wit an output signal level corresponding to the average level of the output signals of said first spatial-frequency discrlm,nating means; and - a set of m comparators, each of which comparing the o,tput signal of a first spatial-frequency discriminating means wlt the output signal of said second spatial-frequency discr,m:nating means, and which together delivers a m-bit ord charteristic of the frequency to be measured.

.CLME:

6. A frequency meter as cla-=d in claim 5, to,prising in addition an encoder.Jispsed at the output of said set tf m comparators and transcod[ng into pure binary code said -oit 4oro deli'ered by the compar=tors.

.CLME:

7. A frequency meter as clampd in claim5, wherein said first and second spatial-frequency discriminating means comprise differential amplifiers which are connected to the outputs of said detectors through weighting resistors and which are made up of two successive stages of inverting operational ampIifiers, the first stage being common to all of said differential amplifiers, which amplifiers have individual second stages.

.CLME:

8. A frequency meter arrangement as clamQ in any of clalm I and 5, comprising at least two frequency meters, the first with a transmission line having taps whose spacing is chosen as a function of the overall operating frequency band of the arrangement, delivering a m'-digi,t binary word, m' being an integer greater than one, representative of the frequency to be measured, and the second with a transmission line having taps whose spacing is chosen as a function of an operating frequency band 2m' times narrower than that of said arrangement, delivering a m"-digits binary word precising the m'digit binary word deliverd by said first frequency meter.

.CLME:

9. An instantaneous digital frequency meter with spatial sampling and substantially as hereinbefore described and as shown in Figures I and 2 or Figures 3 and 4 or Figure 5, or Figure 6 or Figure 7 of the accompanying drawings.

.CLME:

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.CLME:

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.CLME:

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.CLME:

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.CLME:

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.CLME:

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.CLME:

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.CLME:

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.CLME:

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.CLME:

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