1966-03-16

Abstract of GB1022598 1,022,598. Data transmission. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 20, 1964 [Oct. 18, 1963], No. 41209/63. Heading H4P. To reduce the effects of noise in a receiver in which a limiter feeds a frequency discriminator, the limiter is supplied by the received signal and by a locally generated signal having the same frequency as one of those to which the discriminator responds. In one embodiment, Mark and Space signals are received in frequency shift form. One frequency, say f 2 , is filtered out; the resultant gaps in the f 1 signal contain noise which may simulate f 1 at the discriminator. To prevent this a signal of frequency f 2 is added to the filtered signal, the added signal having an amplitude less than that of the received signals f 1 but greater than the noise level. The limiter which receives the combined signal, responding to the predominating signals, provides clean square signals at f 1 and f 2 . The received signals may be amplitude modulated. Reference is also made to a twinplex system. The local oscillator.-Fig. 3 comprises a transistor oscillator fed by a stabilized voltage, derived across Zener diode 11 from a D.C. source at 10, and by a varying voltage, derived by rectification of the filter output f 1 , fed in at 6. The amplitude of the generated oscillations f 2 thus varies within the limits specified above.

0

1966-03-16

H04L27/14 ; H04L27/144 ; H04L27/14 ; H04L27/144

H04L27/14 ; H04L27/144

GB19630041209 19631018

GB19630041209 19631018

NL6412092 (A)

1	Improvements in or relating to receivers for binary-coded signals
	Inventor: CANDLER JOHN ELDRED LINTON; GARTELL TONI; (+1)	Applicant: NAT RES DEV
	EC:H04L27/14; H04L27/144	IPC: H04L27/14; H04L27/144;H04L27/14(+1)
	Publication info: GB1022598 A - 1966-03-16
2	No title available
	Inventor:	Applicant:
	EC:H04L27/14; H04L27/144	IPC: H04L27/14; H04L27/144;H04L27/14(+1)
	Publication info: NL6412092 A - 1965-04-20

1	System for the radio transmission of a binary coded signal.
	Inventor: MASUR CHRISTIAN	Applicant: AUTOPHON AG (CH)
	EC:H04L27/10	IPC: H04L27/10;H04L27/10; (IPC1-7): H04L27/10
	Publication info: EP0093819 - 1983-11-16
2	Circuit for the demodulation of a signal modulated by digital information and receiver comprising such a circuit.
	Inventor: DEHAENE JEAN-PIERRE	Applicant: THOMSON CSF (FR)
	EC:H04L27/14	IPC: H04L27/22; H04B1/10; H04B1/30(+6)
	Publication info: EP0031746 - 1981-07-08

WHAT WE CLAIM IS: -

1 A receiver for binary-coded alternating 90 electrical signals and including discriminating means for providing a different characteristic output corresponding to each of at least two frequencies applied thereto, an amplitude limiter the output of which is ap 95 plied to the discriminating means and a signal generator for generating a signal having a frequency the same as one of the frequencies for which the discriminating means provides a characteristic output, the output 100 of the signal generator being arranged to be applied to the amplitude limiter together with a received-signal.

2 A receiver as claimed in claim 1 and wherein the output of the signal generator is 105 similar to one state of the received signal.

3 A receiver as claimed in claim 1 or 2 and wherein the signal generator is arranged to generate a signal the amplitude of which is greater than the amplitude of the received 110 noise but is less than the amplitude of the received signal.

4 A receiver as claimed in claim 3 and wherein the signal generator includes an oscillator the power supply of which is par 115 tially derived from the received signal.

A receiver as claimed in any one of the preceding claims and wherein there is provided a filter arranged to feed the amplitude limiter, the filter being arranged to 120 prevent one of the states of the received signal from reaching the amplitude limiter.

6 A receiver for binary-coded alternating electrical signals substantially as hereinbefore described with reference to Figures 125 1 and 2 of the drawings filed with the provisional specification.

7 A receiver for binary-coded alternating electrical signals substantially as herein1,022,598 before described with reference to Figures 1, 2 and 3 of the drawings filed with the provisional specification.

G P CAWSTON, Chartered Patent Agent, Agent for the Applicants Abingdon: Printed for Her Majesty's Stationery Office, by Burgess & Son (Abingdon), Ltd -1966.

Published at The Patent Office, 25 Southampton Buildings, London, W C 2, from which copies may be obtained.

c

C C

PATENT SPECIFICATION 1,022,'

DRAWINGS ATTACHED.

Inventors:JOHN ELDRED LINTON CANDLER, TONI GARTELL and JOHN HOLLAND.

0 Date offiling Complete Specification: Oct 20, 1964.

Application Date: Oct 18, 1963 No 41209/63.

Complete Specification Published: March 16, 1966.

Crown Copyright 1966.

Index at Acceptance:-H 4 P 28.

It CL:-H 04 1 27/14.

COMPLETE SPECIFICATION.

Improvements in or relating to Receivers for Binary-Coded Signals.

We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute of Kinsgate House, 66-74 Victoria Street, London, S W 1, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: -

This invention relates to receivers for binary-coded alternating electrical signals.

The invention may be incorporated in receivers which have an amplitude limiting action on received signals.

Many signals used in telegraphy or telemetering are binary-coded; that is, the transmitted signals are always in one or the other of two states, which may be called the ON state and the OFF state Information is conveyed by the sequence in which these states occur Several known types of binarycoded signal may be distinguished according to the nature of the signals used to indicate the ON state and the OFF state For instance, the ON state may be indicated by an alternating signal having a distinctive frequency, or alternatively by an amplitude modulation or by a frequency modulation on a carrier signal The OFF state may be indicated merely by the absence of an ON signal, or by a specific OFF signal, which may be a distinctive signal of the same nature as the ON signal Alternatively, the ON and OFF states may be indicated by different depths of modulation on a modulated carrier The present invention is applicable to receivers for signals of any of these types.

It is an object of the invention to provide a receiver capable of receiving binarycoded transmissions which have a low signalto-noise ratio.

lPrice 4 s 6 d l According to the present invention there is provided a receiver for binary-coded alternating electrical signals and including discriminating means for providing a different characteristic output corresponding to each of at least two frequencies applied thereto, an amplitude limiter the output of which is applied to the discriminating means and a signal generator for generating a signal having a frequency the same as one of the frequencies for which the discriminator means provides a characteristic output, the output of the signal generator being arranged to be applied to the amplitude limiter together with a received signal.

According to a feature of the present invention, the output of the signal generator is similar to one state of the received signal.

According to a further feature of the present invention, the signal generator includes an oscillator the power supply of which is partially derived from the received signal.

An embodiment of the invention suitable for use in a frequency-shift keying (FSK) system will now be described, by way of example An FSK system is a system in which the ON and the OFF states of a binarycoded signal are indicated by a signals of different frequencies Reference will be made to the drawings accompanying the provisional specification, of which: -

Figure 1 is a schematic circuit diagram of a receiver for use in an FSK system; Figure 2 is a diagram of waveforms illustrating the mode of operation of the receiver shown in Figure 1; and Figure 3 is a circuit diagram of a signal generator suitable for use in the receiver of Figure 1.

Figure 1 shows an input line 1 connected to the input of a filter 2 The output of the 598 2 1,022,598 filter 2 and the output of a signal generator 3 are applied to the input of a limiter 4.

The output of the limiter 4 is applied to a discriminating unit 5.

The filter 2, limiter 4 and discriminating unit 5 are components commonly used in receivers for binary-coded signals The filter 2 obstructs irrelevant signals and noise outside the frequency range of the signal which is to be received The output of the limiter 4 is an amplitude-limited waveform which approximates to a square wave at the frequency of the strongest signal present at the input of the limiter The discriminating unit 5 distinguishes between the ON and the OFF states of the signal, and produces an output signal indicating the sequence in which these states occur The discriminating unit 5 may include two frequency-sensitive detectors, for instance filter networks, of which one is sensitive to an ON signal, and the other is sensitive to an OFF signal.

If a received binary-coded signal is contaminated with noise, so that it has a low signal-to-noise ratio at the limiter, spurious random signals due to noise may appear in the limiter output The further the bandwidth of the filter 2 is reduced, the more noise will be excluded Normally, the bandwidth of the filter 2 is adjusted to be just sufficient to pass both the ON and the OFF signals If the bandwidth of the filter 2 in a receiver of this type, as used in known systems, is further reduced so as to pass only the ON signal (for instance) and to exclude the OFF signal, the noise amplitudes may be greately reduced, but during the OFF periods, in the absence of any other signal, the noise predominates Some of the random noise pulses received during the OFF periods will resemble the ON signal sufficiently closely to operate the discriminating unit 5, creating spurious ON signals during the OFF periods This situation is illustrated by the waveforms in Figure 2 at (a), (b) and (c) These waveforms are purely illustrative, and should not be taken as an exact representation in all respects The waveform shown at (a) in Figure 2 represents an output signal from the filter 2, when the bandwidth of the filter 2 is just sufficient to pass the ON signal of an FSK transmission The corresponding outputs from the limiter 4 and discriminator 5, when the signal generator 3 is not in use, are shown at (b) and (c) respectively The action of the limiter 4 is to cut off those parts of the signal which lie outside the horizontal dashed lines in Figure 2 (a) It will be seen from Figure 2 (c) that, due to noise, spurious ON signals occur at the output of the discriminator 5 during the OFF periods This tends to make the known type of system unreliable.

In the present invention, a locally generated OFF signal is added to the incoming signal at the input to the limiter 4 This locally generated OFF signal is provided by the signal generator 3, and is adjusted so that its amplitude exceeds the noise ampli 70 tude but is less than the ON signal amplitude of the incoming signal It is a wellknown property of an amplitude limiter that the strongest signal in its input will predominate in its output, and that components 75 of the input signal other than the strongest component have relatively less significant effect on the output-signal Thus, during the ON periods, the received ON signal predominates, and the presence of the locally 80 generated OFF signal has substantially no effect During the OFF periods, the locally generated OFF signal predominates over the random noise The discriminating unit 5 recognises the locally generated OFF signal 85 as an OFF signal, and therefore correctly indicates the OFF periods This situation is illustrated in Figure 2 by the waveforms at (d) and (e) which respectively represent a typical output of the limiter 4 and the 90 corresponding output of the discriminating unit 5, when the signal generator 3 is in use and adjusted as hereinbefore described.

The locally generated OFF signal is naturally more effective when its amplitude ex 95 ceeds the noise amplitude by a larger margin, but its amplitude must obviously be less than that of the received ON signal It is therefore desirable to control the amplitude of the locally generated OFF signal accord 100 ing to the amplitude of the received ON signal One way to achieve this is to produce the locally generated OFF signal by a transistor oscillator whose power supply is derived by amplifying and rectifying the re 105 ceived ON signal However, in conditions of severn fading, w hen the ON signal is completely lost, it would be better to have the discriminating unit 5 indicate OFF rather than spurious ON pulses due to random 110 noise The amplitude of the locally generated OFF signal should therefore have a fixed minimum value exceeding the mean noise amplitude This can be arranged for instance, by adding a constant supply voltage 115 in series with the variable supply voltage derived from the ON signal, and using this series combination to energise the transistor oscillator Such as arrangement is shown in Figure 3 120 The received ON signal may be separated out by the action of the filter 2 (Figure 1), amplified, and applied to terminals 6 and 7 of the signal generator shown in Figure 3.

The amplified ON signal is then rectified by 125 a diode 8 and associated components, so that a voltage proportional to the amplitude of the received ON signal is developed across a resistor 9 and a capacitor 14 The time constant of this circuit is arranged to be 130 1,022,598 1,022,598 long compared with the switching periods of the received signal A negative supply voltage is applied to a terminal 10 and from this voltage a stabilised voltage is derived by the action of a Zener diode 11 The stabilised voltage developed across the Zener diode 11 and the variable voltage developed across the resistor 9 are connected in series additively and applied to energise a transistor oscillator comprising a transistor 12 and associated components The output of this oscillator is taken from a terminal 13 and applied to the input of the limiter 4 (Figure 1) The input and output impedances of the circuit shown in Figure 3 are such that substantially no feedback occurs between the input and the output of a circuit.

The amplitude of the oscillations at the output terminal 13 depends upon the total supply voltage applied to the oscillator and will therefore vary according to the amplitude of the received ON signal in the desired manner.

It should be clearly understood that the invention is not limited to the embodiment described above, and that many modifications and other applications of the invention will be obvious to persons skilled in the art.

For instance, it may be applied to other types of binaiy-coded signals For instance, the ON and the OFF states may take the form of different modulations on a carrier.

In that case the received signals may be passed through demodulating or heterodyning stages before being applied to the circuit of Figure 1 The discriminating unit 5 must be arranged to interpret the locally generated OFF signal as an OFF signal, but the locally generated OFF signal need not necessarily be related to the received signal except in that it must differ in frequency from the received ON signal Where, however, there is a received OFF signal differing in frequency from the received ON signal, it may be convenient to make the local OFF signal similar to the received OFF signal.

The ON and the OFF states of the received signal may be represented by different depths of amplitude modulation In that case, if the ON state has a greater depth of modulation than the OFF state, the signal generator 3 may be used to simulate the OFF state Then the amplitude of the output of the signal generator 3 must be less than the depth of modulation of the ON state and greater than the depth of modulation of the OFF state Also, the frequency of the output of the signal generator 3 must be different from the frequency of the amplitude modulation of the received signal The discriminating unit 5 is then arranged to provide a different characteristic output when the amplitude modulation signal predominates from when the output of the signal generator 3 predominates at its input.

The invention is also applicable to receivers for double frequency shift transmissions, in which two binary-coded signals are transmitted simultaneously on the same line or carrier In such systems, four distinct frequencies are used to distinguish respectively the ON and the OFF states of one signal and the ON and the OFF states of the other signal In this case, the bandwidth of the filter 2 may be reduced until only three of these four frequencies can pass to the limiter 4 The locally generated OFF signal may be similar to the signal excluded by the filter, and its amplitude may be made to vary as the mean amplitude of the received signals which pass the filter.

In all cases, it is desirable that at least the filter 2 and the signal generator 3 should be adjustable, and it may also be desirable for the limiter 4 and the discriminating unit to be adjustable, in order that they may be adjusted to obtain the best possible reception.