GB2120052A - Receiver - Google Patents

Abstract

A receiver capable of distinguishing between signals at frequencies equally offset above and below a local oscillator frequency includes a mixer receiving the signals and the local oscillator signal which is frequency modulated by a low frequency symmetrical waveform or a low frequency asymmetrical waveform which serves to distinguish if the deviation of an input signal is above or below the carrier frequency. The frequency of the modulating signal is greater than the bit rate but less than the offset. Circuits are disclosed for deriving the data signal waveform from the difference term at the output of the mixer.

Description

SPECIFICATION Receiver The present invention relates to a reciever capable of distinguishing between signals originating atfrequencies equally offset above and below a local oscillatorfrequency. The receiver has particular, but not exciusive, application low bit rate signalling systems, such as paging.

In signalling systems such as paging there is a requirementfora small, lightweight, reliable inexpensive receiver and this had led to the development of an integrable (or single chip) receiver having the minimum ofexternal components.

Known types of integrable receivers for use with F.M. tone signalling, as disclosed for example in British Patent Specifications 1,517,121 and 2,032,737A and generally referred to as "direct conversion" or "zero i.f." f n receivers, include a quadrature pair offront end mixers, a local oscillator at the carrier frequency, a pair of channel filters, a pair of limiting amplifiers and a D-typeflip-flop. The needforthis complexity arises from having to decidewhetherthe directfrequencymodulation transmission is deviated above or below the local oscillatorfrequency. In both cases the mixer output is at the deviation frequency. This is because the deviation is offset equally above and belowthe local oscillator frequency.Consequently it is customaryto use the phase relationship between the outputs ofthetwo mixers to determinethetransmitted data.

Drawbackstosuch complexityarethatthe R.F.

components in themselves are relatively expensive and also have a high power consumption which is a disadvantageforany battery-powered equipment operating on standbyfor long periods oftime.

A proposal for avoiding the need for a quadrature pair of front end mixers is disclosed in British Patent Application 8132181 (PHB 32819) wherein in orderto distinguish whetherthe direct frequency-modulation transmission is above or below the local oscillator frequency, the local oscillatorfrequency is offset slightly from the carrierfrequency so that signals deviated equally above and belowthe carrier emerge from a single R.F. mixer at differentfrequencies. In orderto distinguish between which frequency is present and to decide whether a "1 " or "0" signal is being transmitted, various conventional techniques can be used.British Patent Application 8132181 discloses using a pair of bandpass filters to identify which ofthetwo frequencies is being produced and so recover the data being transmitted. Becausethe receiver has been found to require tight control over theoffsetfrequency, AFC systems have had to be developed that can maintain correcttuning overthe full operating temperature range ofthe receiver. The provision of an AFC system introduces an undesired complexitytoan otherwiseverysimple receiver.

Its an object of the present invention to overcome the disadvantages of the known receivers.

According to the present invention there is provided a receiver in which an input signal is mixed down using a local oscillator signal comprising the carrier frequency which has been frequency modulated in orderto distinguish frequencies equally offset above and belowthefrequency of the local oscillator.

The effect of modulating the carrierfrequencyfrom the local oscillator is that although the signals corresponding to a "1 " or a "0" emerge from the front end mixer at the same frequency, the sign of the modulation is reversed in the case of one of them, for example the "0". The signals can therefore be distinguished and the data recovered.

An embodiment of a data receiver made in accordance with the present invention comprises a mixer having a first input for a frequency shifted data signal and a second input coupled to a local oscillator, means forfrequency modulating the local oscillatorwith a signal having a frequency greater than the bit rate and less than the deviation frequency, and means for recovering the data from the difference term in the mixer output.

The modulating means may comprise an asymmet ricwaveform source in which case the data recovery means may comprise a frequency to voltage converter coupled to the output of the mixerfor recovering the modulating waveform, a differentiating circuit coupled to the output of said converter for differentiating the recovered waveform, a limiterto limitthe differentiated waveform signal and smoothing means for deriving the data signal waveform.

Alternatively, the modulating means may comprise a symmetrical source. Such a source is conveniently a sinewave source which has the advantage overthe asymmetrical source and certain other types of symmetrical sources such as a square wave source or atriangularwavesourceofnotbeing high in harmonic contentwhich could result in spurious responses. In the case of using a sinewave source the data recovery means may comprise a frequecyto voltage converter for deriving a voltage in response to the instantaneous frequency in said difference component, and means for comparing the output from the converter with the output of the modulating means to form the data output.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure lisa block schematic circuit diagram of an embodiment of a data receiver made in accordance with the present invention in which the modulating means is a symetrical waveform source, Figures 2 and 3 are diagrams of the R.F. mixer inputs and outputs, respectively, during a single bit period and serve for explaining the operation ofthe circuit shown in Figure 1, Figure 4 is a block schematic circuit diagram of another embodiment of a data receiver made in accordance with the present invention in which the modulating means is an asymmetrical waveform source, and Figure 5 and Figures 6A and 6B are diagrams of the R.F. mixer inputs and outputs, respectively, and serve for explaining the operation ofthe circuit shown in Figure 4, Figures 7 and 8 are block schematic circuit diagrams of two frequency to voltage converters suitable for use in the data receiver shown in Figure 1 or4.

The data receiver shown in Figure 1 comprises an R.F. mixer 10 having a first input 12 for receiving a low bit rate (512 bits/sec) frequency modulated signal fc # #f where fc is the carrier frequency and #f is the deviation, typically 4.5 kHz. A second input 14 ofthe mixer 10 is connected to a local oscillator 16 which produces a signal at the carrier frequency fc.The local oscillator signal is frequency modulated by a symmet rical signal source, such as a sinewave source 18 having a modulating frequencyfrn. The modulating frequencyfrn should be a singlefrequencygreater than the bit period and less than the deviation frequencyAf. In the present example cm could lie between 600 Hzand 4 kHz.

The output of the R.F. mixer 10 is applied to a low pass filter 20 which selects the difference term from the mixer output and serves to provide the required adjacent channel selectivity. It is then necessary to extractthe modulating frequency fm and this is done by applying the output of the low pass filter 20 to a discriminatorformed by a frequency to voltage converter 22. In order to determinewhetherthe recovered modulation isin-phaseorantiphase relativetothemodulation appliedtothe local oscillator 16 the output from the converter 22 is multiplied in another mixer 24 by a reference comprising the modulationfrequencyfrn which is derived from the sinewave source 18.A delay device 26 in the form of an all pass network may be connected between modulation signal source 18 and the mixer 24. The time delay, for example typically 100 S, introduced by the device 26 compensates forthe delay introduced in the signal path bythe channel selectivity filter 20 and the frequencyto voltage converter 22. The output from the mixer 24 comprises the data signal waveform which is recovered using a low pass filter 28 and a limiter 30.

The operation ofthe data receiver shown in Figure 1 will now be described with referenceto Figures 2 and 3. In this explanation itwill be assumed that a "1" is represented by -Af and a "0" by +Af. Thus the signal on the input 12 ofthe mixer 10 will be eitherf-Af or fc+Af. The local oscillator signal is the carrierfrequen cry which has been frequency modulated byfm,thus the instantaneous deviation of cm relative to the carrier frequency fc is Afrn cos fm(t-to). The effect of modulating the local oscillator frequency is shown in Figure 3,the N"1 " signal is 1800 out of phase relative to the "0" signal.This can be explained by considering the instantaneous frequency difference between the local oscillator signal frequency, the sinewave referenced 32, and the respective tone frequencies fc + Af and fc - #f. At time to the local oscillator signal, the sinewave 32, is at a greater distance from the "1 " transmission frequency than it is from the "Os transmission frequency. This distance corresponds to Af+ Afmfora "1" signal and Af- Afmfor a "0" signal.

By means of the low pass filter 20 the difference term is always taken from the output of the mixer 10. Thus what happens can be summarised by saying that for a signal above the carrier frequency f,,ahe difference term decreases with an increase in the modulated local oscillatorfrequency,the sinewave 32, and increases with a decrease in the modulated local oscillatorfrequency, and for a signal belowthe carrier frequencyfcthe difference term increases with an increase in the modulated local oscillatorfrequency and decreases with a decrease in the modulated local oscillatorfrequency. This is shown by the broken line waveforms in Figure 3.Mathematically, this can be shown as follows: Difference term = finput - Lo where finput is the signal on the input 12 ofthe mixer 10 and fLO is that on the input 14, for a tton, finput =fc + Af, difference "0" = Af+ fc - fLo, and for a n 1 ", finput = fcAf, difference " 1 " = - Af + fc fLo.

Rewriting difference "0" = [Af- (fLo - fc)] and difference "1" = - [Af + (fLo - fc)], (fLo - fc) being the modulation applied to the local oscillator.

As far as the subsequent signal processing is concerned onlytheterms insidethesquare brackets are of interest and for practical purposesthe sign outside the square brackets can be ignored.

The data contained in the difference term is recovered in the embodiment shown in Figure 1 by extracting the modulation frequency. This is done in two steps, firstly the instantaneous frequency of the signal is measured using thefrequencytovoltage converter 22. Secondly, the output ofthe converter is applied to the mixer 24 where it is mixed with the modulating signal fm. f the two signals are in-phase then a positive output is obtained which in the described example indicates a "1 ". Conversely if the two signals are antiphase then a negative output is obtained which in the described example indicates a "0". By applying the output from the mixer 24to the low pass filter 28 which is setto pass signals around the bit rate and limiting the filtered signal in the limiter 30, a date waveform is obtained.

Figure 4 illustrates another embodiment ofthe data receiver in which the modulating means is an asym metrical waveform source 40 which can conveniently comprise a ramp waveform generator. An advantage of using an asymmetrical waveform source over a symmetrical one is that the recovered modulation's sign can be obtained directly as will be explained.

Figure 5 shows the inputs to the mixer 10,thefirst input 12 will either be atfc + Af orfc - #f where as the ramp waveform 42 is on the second input 14. The alternative differenceterms selected by the low pass filter 20 are shown in Figures 6Aand 6B,theformer representing a "1" and the latter a "O".The data waveform is recovered by measuring the frequency using the frequencyto voltage converter 22. The outputtherefrom is differentiated in a differentiator 44, a "I" signal producing positive going pulses and a "0 " signal negative going pulses. The outputfrom the differentiator 44 is applied to a limiter 46 and then to a low passfilter48 in orderto produce a positive signal for a "1" and a negative signal for a "0". Whilst the data receiver of Figure 4 is simpler in its construction than that of Figure lit is anticipated that the harmonics produced by the asymmetrical waveform source could cause spurious receiver responses.

Figures7 and 8 illustrate two frequency to voltage converters which can be used in the illustrated and described embodiments of the data receiver and which enable a more rapid measurement of frequency and thus a more clearly defined result compared with a known type of converter comprising a phase shifter and a multiplier.

Considering Figure 7 the difference term from the low pass filter 20 (Figures 1 and 4) is applied to a first input of a mixer 50 to a second input of which a reference signal conveniently having the same frequency as the deviation Af in the input signal to the mixer 10 (Figures 1 and 4), namely 4.5 kHz, is supplied by a reference source 52. The sum term is selected from the output of mixer 50 using a high pass filter 54, for simplicity of representation the sum term is 2Af + Afm, however it is to be understood that ±4fm includes the frequencies between Afrn and +at,.

The sum term is applied to a conventional frequency discriminator 56 which in the illustrated embodiment comprises a phase shifter 58 connected between the high pass filter 54 and a first input of a mixer 60, a second input of the mixer 60 being connected to the output ofthe filter 54. The phase shifter 58 is arranged to produce a 90 phase shift at 2Af, i.e. 9kHz. The sum term from the mixer 60 may be removed using a low pass filter 62 which passes the difference term although the low pass filter 28 will also preventthe high frequencies from appearing at the data output.

Operating at a higherfrequency, namely 2Af enables the frequency to be measured rapidly and gives a more clearly defined result.

Figure 8 differs from Figure 7 in respect of the fact that the reference source 52 has been omitted and the difference term is connected to both inputs of the mixer 50. The effect of doing this is that the sum term passed by the filter 54 is 2Af + 2afro.

The stability of the modulation signal sources 10 (Figure 1) and 40 (Figure 4) is not critical as the whole system moves together. The linearity ofthe data receivers is limited by spurious responses.

Although the symmetrical source 18 has been described as being a sinewave source, othersymmetrical sources, e.g. a triangular wave source or a square wave source, may be used subject to the safeguard that their outputs should not be so high in harmonic content which could result in spurious responses.

Claims (10)

1. A receiver in which an input signal is mixed down using a local oscillator signal comprising the carrier frequency which has been frequency modulated in orderto distinguish frequencies equally offset above and below the frequency of the local oscillator.

2. A data receiver comprising a mixer having a first inputfor a frequency shifted data signal and a second input coupled to a local oscillator, meansforfrequency modulating the local oscillator with a signal having a frequency greaterthan the bit rate and less than the deviation frequency, and means for recovering the data from the difference term in the mixer output.

3. A data receiver as claimed in Claim 2, wherein the modulating means comprises an asymmetric waveform source.

4. A data receiver as claimed in Claim 3, wherein thedata recovery means comprises a frequency to voltage converter coupled to the output ofthe mixer for recovering the modulating waveform, a differentiating circuit coupled to the output of said converterfor differentiating the recovered waveform, a limiterto limit the differentiated waveform signal and smoothing means for deriving the data signal waveform.

5. A data receiver as claimed in Claim 2, wherein the modulating means comprises asymmetric waveform source.

6. A data receiver as claimed in Claim 5, wherein the symmetric waveform source is a sinewave source.

7. A data receiver as claimed in Claim 2,5 or 6, wherein the data recovery means comprises a frequencyto voltage converter for deriving a voltage in response to the instantaneous frequency in said difference term, and meansforcomparing the output from the converter with the output ofthe modulating means to form the data output.

8. A data receiver as claimed in Claim 4 or7, wherein thefrequencyto voltage converter comprises another mixer having an input for receiving the difference term from the first-mentioned mixer and another input for receiving a reference frequency, means for selecting the sum term in the output ofthe another mixer, and a discriminator operating at the frequency of the sum term.

9. A data receiver as claimed in Claim 4 or7, wherein thefrequencyto voltage converter comprises another mixer whose inputs are connected to receive the difference term from the first-mentioned mixer, means for selecting the sum term in the output ofthe another mixer, and a discriminator operating at the frequency of the sum term.

10. Areceiverconstructed and arranged to operate substantially as herein before described with reference to and as shown in the accompanying drawings.