GB2113930A

GB2113930A - Frequency discriminator

Info

Publication number: GB2113930A
Application number: GB08202164A
Authority: GB
Inventors: Percy John Fish
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1982-01-26
Filing date: 1982-01-26
Publication date: 1983-08-10
Also published as: GB2113930B

Abstract

A frequency discriminator measures rate of change of phase of an input signal 4 by calculating the phase angle ??? of samples of the signal, and taking the difference of successive phase angles. Quadrature versions of the input signal are generated 1-6 and sampled 13- 16, and the function log ¦tan ???¦ is calculated by taking logarithms 20, 21 and subtracting 24. The argument ??? is found 25 e.g. by lock-up tables, and assigned to the correct quadrant 32. The difference between successive phase angles is then taken 34-37. <IMAGE>

Description

SPECIFICATION Frequency discriminator This invention relates to frequency discriminators.

More especially it relates to frequency discriminators which utilise digital signal processing.

The frequency f, of a signal may be expressed as the rate of change of phase of that signal i.e.

d# f= dt and d# dt may be considered to be approximated by ## #t where 616 is the phase difference between successive samples taken at times spaced by the interval St, provided that 1 #t is larger than the bandwidth of the signal under consideration.

The present invention seeks to provide a frequency discriminator which compares the phase of successive signal samples utilising digital signal processing techniques thereby to provide an indication of the frequency of a signal.

According to the present invention a frequency discriminator comprises a pair of mixers arranged to receive input signals at a frequency to be discriminated, a reference frequency source, the frequency of which is in the same frequency band as the input signals and which is used to provide a local oscillator signal for the mixers of the pair, phase quadrature means effective to produce a phase quadrature relationship between the input signals fed to the mixers or between the local oscillator signals fed thereto whereby a pair of phase quadrature related signals are produced by the mixers, low pass filter means to which the phase quadrature related signals produced by the mixers are fed, clock pulse generator means, signal processor means responsive to clock pulses produced by the clock pulse generator means for contemporaneously taking successive samples of the filtered phase quadrature related signals and for producing successive pairs of corresponding quadrature related digital signals characteristic of the logarithm of the samples, subtractor means operative to subtract the signals of each pair thereby to produce a resultant signal, further processor means effective to take the tan -' antilog of the resultant signal so as to produce a signal characteristic of the phase of the sample, storage means to which signals characteristic of the phase of samples are fed and stored for a period corresponding to the interval between samples, further subtractor means responsive to respective signals fed contemporaneously to the storage means and from the storage means and derived from successive samples for providing a signal indicative of frequency, and sign storage means effective to store the signs of each pair of quadrature related signal samples and phase correction means responsive to data appertaining to the sign of successive samples for applying as necessary a correction factor to the said signal indicative of frequency thereby to provide an output signal indicative of the frequency of the input signals.

The signal processor means to which the phase quadrature related filtered signals are fed may comprise two analogue-to-digital converters responsive to the clock pulses for sampling respective quadrature related signals thereby to produce quadrature related digital signal samples and two logarithmic processors to which respective quadrature related digital signal samples are fed thereby to produce the said pairs of corresponding quadrature related digital signals characteristic of the logarithms of the samples.

Alternatively the signal processor means may comprise an analogue-to-digital converter having a logarithmic conversion law.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a block schematic diagram of a frequency discriminator.

Referring now to the drawing a frequency discriminator comprises a pair of mixers 1, 2 which are fed, via a splitter 3 which is fed from an input signal line 4, with input signals the frequency of which is to be discriminated. The mixers 1 and 2 are fed with local oscillator signals from a reference oscillator 5 which is arranged to feed the mixer 1 directly and which is arranged to feed the mixer 2 via a phase quadrature device 6 thereby to produce on output lines 7 and 8 from the mixers 1 and 2 respectively phase quadrature related output signals. In one practical case the frequency of the input signal applied to the input line 4 may lie somewhere within a 20 Mhz band between 60 Mhz and 80 Mhz and in this particular case the reference oscillator 5 might be set to 70 Mhz.

The phase quadrature related output signals from the mixers 1 and 2 are fed via the lines 7 and 8 respectively to a pair of low pass filters 9 and 1 0. The low pass filters serve to remove from the phase quadrature related signals on the lines 7 and 8 the upper side band components as will hereinafter be described.

Filtered phase quadrature related signals on lines 11 and 12 from the filters 9 and 10 respectively are fed to analogue-to-digital converters 1 3 and 14 which operate responsively to clock pulse signals fed thereto on a line 1 5 from a sample clock 1 6 which operates under the control of a master clock 17.

Digitised signals from the analogue-to-digital converters 1 3 and 1 4 are fed via lines 1 8 and 1 9 respectively to logarithmic processors 20 and 21. Digitised phase quadrature related logarithmic signals are fed from the logarithmic processors 20 and 21 on lines 22 and 23 respectively to a subtractor unit 24. An output signal from the subtractor unit 24 characteristic of the difference between signals on the lines 22 and 23 is fed to a further processor 25 via line 26.As will hereinafter be described the signal on the line 26 is characteristic of the logarithm of tha tangent of the phase angle of the signal samples and accordingly if the processor 25 is arranged to perform the function tan -1 antilog an output signal from the processor 25 on the line 27 will be produced which is characteristic of the phase angle of the samples. It will be appreciated that there may be some phase ambiguity present in the signal on line 27 and in order to obviate this ambiguity the sign of the signals on lines 1 8 and 1 9 is determined and stored in sign stores 28 and 29 respectively.Signals characteristic of the signs of the quadrature related signals are fed from the sign stores 28 and 29 on lines 30 and 31 respectively to a compliment unit 32 which in dependence upon the signals present from time to time on the lines 30 and 31 modifies the signal on the line 27 whereby an output signal is provided on line 33 which is characteristic of the-phase angle measured and takes account of the quadrant in which the phase angle is measured. The signal samples on the line 33 are fed to a store 34 which has a storage or delay time corresponding to the sampling period and thus signals on a line 34 which communicates directly with the output from the compliment unit 32 and signals on a line 36 from the output of the store 34 represent the phase angle of consecutive signal samples.

These signals on lines 35 and 36 which represent the phase angle of consecutive signal samples are subtracted in a subtraction device 37 thereby to provide a signal on line 38 representative of the phase change S) in a period St which corresponds to the period between signal samples. It will be appreciated that with the arrangement so far described there is the possibility of ambiguity when a zero phase crossing occurs between consecutive signal samples and in order to avoid this ambiguity a transit or zero crossing sensing device 39 is provided responsive to the sign of consecutive signal samples which is transferred from the compliment unit 32.If a zero crossing is sensed by the unit 39 in due to the sign of the signals received on lines 40 and 41 then a correction factor is applied to the signal on line 38 by means of a correction unit 42 so as to provide an output signal on line 43 which is truly characteristic of the frequency applied to the input line 4.

The system as just before described can be realised in hardware using an microprocessor or hardwired logic processing in dependence upon the rate of data processing required. Look up tables are suggested as the most effective means of function generation. The system operates in effect to take the in phase and quadrature components of the incoming signal with respect to the local oscillator signal provided by the oscillator 5. This is a standard signal processing technique and produces the signals on the lines 11 and 12 respectively.

Thus the incoming signal on line 4 may be represented by the expression S,(t)=A(t)Cos(cl)lt+s(t) } where w is the mid frequency or local reference frequency, which is mixed with quadrature local oscillator signals Cosco1t and Sinw1t giving A(t)[CosI2w,t+(t) +Cos4(t)l and A(t) [SinI2w,t+(t)+Sin I--(t)J1 the 2w1 terms are removed by low pass filtering which does not affect the other terms. A(t) Cosj(t) and -A(t) Sin(t).

This is then digitised and converted to the logarithm giving logjA(n) Cos(n) ii and logjA(n) I Sin(n) I signals characteristic of the phase of samples are fed and stored for a period corresponding to the interval between samples, further subtractor means responsive to respective signals fed contemporaneously to the storage means and from the storage means and derived from successive samples for providing a signal indicative of frequency, and sign storage means effective to store the signs of each pair of quadrature related signal samples and phase correction means responsive to data appertaining to the sign of successive samples for applying as necessary a correction factor to the said signal indicative of frequency thereby to provide an output signal indicative of the frequency of the input signals.

2. A frequency discriminator as claimed in claim 1 wherein the signal processor means to which the phase quadrature related signals are fed comprises two analogue-to-digital converters responsive to the clock pulses for sampling respective quadrature related signals thereby to produce quadrature related digital signal samples and two logarithmic processors to which respective quadrature related digital signal samples are fed thereby to produce the said pairs of corresponding quadrature related digital signals characteristic of the iogarithms of the samples.

3. A frequency discriminator as claimed in claim 1 wherein the signal processor means comprises an analogue-to-digital converter having a logarithmic conversion law.

4. A frequency discriminator substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

signals characteristic of the phase of samples are fed and stored for a period corresponding to the interval between samples, further subtractor means responsive to respective signals fed contemporaneously to the storage means and from the storage means and derived from successive samples for providing a signal indicative of frequency, and sign storage means effective to store the signs of each pair of quadrature related signal samples and phase correction means responsive to data appertaining to the sign of successive samples for applying as necessary a correction factor to the said signal indicative of frequency thereby to provide an output signal indicative of the frequency of the input signals.

2. A frequency discriminator as claimed in claim 1 wherein the signal processor means to which the phase quadrature related signals are fed comprises two analogue-to-digital converters responsive to the clock pulses for sampling respective quadrature related signals thereby to produce quadrature related digital signal samples and two logarithmic processors to which respective quadrature related digital signal samples are fed thereby to produce the said pairs of corresponding quadrature related digital signals characteristic of the iogarithms of the samples.

3. A frequency discriminator as claimed in claim 1 wherein the signal processor means comprises an analogue-to-digital converter having a logarithmic conversion law.

4. A frequency discriminator substantially as hereinbefore described with reference to the accompanying drawings.

where the n indicates the value at the nth sampling cycle

(t=n . At) with the sign of Cos#(n) and Sinj(n) stored separately. Subtracting log{A(n){Sin#(n) i 1-log A(n) t cos#(n) l l=iogt ! tan0(n) ii which is converted to çs(n) in the first quadrant 0-7t/2 by the look up table, and the stored signs enable 0(n) to be placed in the full 0-2.

In determining

the possibility of # jumping by 2# must be allowed for and the sampling rate must be sufficiently high so that 0(t) cannot change by more than # between samples, i.e. the input signal should be contained within an angular frequency band

which is the same condition as the proviso that

is larger than the bandwidth of the signal under consideration.

In a preferred implementation twos complement notation is used for the digital numbers, and the sensing of crossing the zero of 2# phase condition is taken care of automatically in the arithmetic subtraction.

An alternative realisation which would involve the use of one additional function generator or look up table, and would provide a logarithmic output log A(t) is to generate (look-up) also logf | Sin#(n) i } and log I Cos#(n) l } from log( | tan#(n) 11.

This can be done using a function generator to obtain log {| I Sin#(n) I } from log 11 | tan#(n) II } and then using -log { | tan#(n) | }=log{ | I Cot#(n) I I in the same function generator to obtain log 1 I Cos#(n) | 1 if a check is required.

Iog(A(n)J can then be obtained from log{A(n) | Sin(n) I }-log{ Sin(n) II and/or log{A(n)|Cos#(n)|}-log{|Cos#(n)|}.

Claims (Filed on 12/1/83)

1. A frequency discriminator comprising a pair of mixers arranged to receive input signals at a frequency to be discriminated, a reference frequency source, the frequency of which is in the same frequency band as the input signals and which is used to provide a local oscillator signal for the mixers of the pair, phase quadrature means effective to produce a phase quadrature relationship between the input signals fed to the mixers or between the local oscillator signals fed thereto whereby a pair of phase quadrature related signals are produced by the mixers, low pass filter means to which the phase quadrature related signals produced by the mixers are fed, clock pulse generatore means, signal processor means responsive to clock pulses produced by the clock pulse generator means, signal contemporaneously taking successive samples of the filtered phase quadrature related signals and for producing successive pairs of corresponding quadrature related digital signals characteristic of the logarithm of the samples, subtractor means operative to subtract the signals of each pair thereby to produce a resultant signal, further processor means effective to take the tan -' antilog of the resultant signal so as to produce a signal characteristic of the phase of the sample, storage means to which