GB1605346A - Improvements in or relating to signal communication systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO SIGNAL COMMUNICATION SYSTEMS VWe TIE MARCOM COMPANY LIMITED formerly Marconi's Wireless Telegraph Company Limited, of English Electric House, Strand, London, W.C.2. a British Company do hereby declare the invention, for which llwe pray that a patent may be granted lo me/us, and the method by which il is lo be performed, lo be particularly described in and by the following statement:- This invention relates to signal communicalion syslems and more panicularly to such systems of the kind in which high frequency signals obtained al geographically spaced places are required lo be made available al a common point while maintaining (to a desired degree of closeness) the frequency relations or the frequency and phase relations of the signals as originally obtained. For the sake of brevity such maintenance of frequency relations or frequency and phase relations to a desired degree of accuracy will hereinafter be refereed to as maintenance of coherence.

In certain radio systems, for example in certain radio direction finding systems and in certain Radar target detection systems, signals received at geographically spaced receivers are required lo be relayed lo a central point while maintaining coherence in order that they may be compared, combined or otherwise jointly used at the central point, e.g. for phase or frequency comparison, extraction of Doppler frequency components or multiplication of one signal by another at the central point. Such maintenance of coherence is difficult lo achieve in a simple and economical manner in the great majority of cases, encountered practically, where the signal frequencies and geographical separations are such as lo render il impractical to relay the signals directly and without frequency change of the heterodyne type over land-lines or radio links. In the great majorily of eases practical considerations involve that frequency changing of the heterodyne type must take place for the purpose of relaying the signals and the present invention seeks to solve, in a simple and salisSaclory manner, the problem of maintenance of coherence, despite the use of such frequency changing for relaying.

According to this invention a signal communication system for relaying signals from sources at geographically spaced points to a common point (which may, of course, itself be one of the geographically spaced points) comprises signal link means of predetermined pass band and extending between each of said sources which is remote from said common point to said common point; means adjacent each signal source for heterodyning the signals therefrom lo a common predetermined changed frequency which is within said pass band, said heterodyning means including a reference source from which the local oscillations for heterodyning are coherently derived and which is of a reference frequency different from the changed frequency but also within the said pass band; means for feeding changed frequency signals produced at each geographically spaced point remote from the common point lo said common point via the link means; and means for controlling the reference frequency oscillations produced at all but one of the different geographically spaced points by comparison with the reference frequency oscillations produced at said one point to maintain all the reference frequency oscillations in coherence.

The links may be radio links or cable links.

In the former case the signals to be transmitted over each link are applied lO modulate a carrier wave transmitter at the input end of the link, said link being provided with a receiver and a demodulator at its output end. In some embodiments of the invention reference frequency oscillations produced al one point are relayed to another point for comparison with reference frequency oscillations produced al said other point over the same signal link means as are employed to relay changed frequency from said one point lo the other and the error signals resulting from said comparison are either employed for controlling the reference frequency oscillator at said other point into coherence or are relayed back to said one point over an additional link provided for the purpose and are there employed for maintaining the reference oscillator there in coherence. In other embodiments of the invention changed frequency signals produced al one point are relayed lo another over one link and reference frequency oscillations produced at said other point are relayed back lo said one point for comparison with the reference frequency oscillations produced there and control of the reference frequency oscillator there for maintenance of coherence by the error signals resulting from such comparison. This type of embodimcnt is not preferred as compared to those in which only error signals are relayed over the additional links because although theoretically the additional narrow band link can be narrow band in either type or embodiment, the type in which the additional links carry only error signals lends itself much better to the use of narrow band additional links. If desired, in all eases, additional information signals, on suitable separable frequencies, may also be fed over the link means, frequency selective filtering being, of course, provided for separaling out the additional information signals as necessary.

Preferably the heterodyning means comprise, in each case, a reference frequency oscillator and at leasl one frequency multiplier driven thereby, the multiplied frequency constituting the local oscillations which are mixed with signals from the appropriale signal source to produce the changed frequency. Clearly such local oscillations will be coherent with the reference frequency as is, of course, required in order that coherence between the reference frequency sources shall result in coherence between the changed frequency signals.

Error signals may be obtained in any convenient way known per se. Thus, for example, an error signal dependent upon and representative of lack of coherence between two reference frequency signals, one from each of two reference frequency sources, may be obtained by comparing the two signals in frequency or, if required, in frequency and phase, lo produce resultant output signals which are then ulilised to control, in any manner well known per se, one of the two reference frequency sources to maintain it in coherence (frequency or frequency and phase coherence, as may be required) with the other.

In the simplest applicalions of the invention where there are only two geographically spaced points one of which is also the common point, only one link is required to relay to Lhe common point the changed frequency signals and the reference frequency oscillations (together with additional information signals, if any) from the other point. At the common point the reference frequency oscillations from said other point are separated out and fed to a comparator whose other input is taken from the reference source at said common point, the output from the comparator being employed to control the last mentioned reference source in frequency or in frequency and phase.

As already inferred, the invention is, however, not limited to such applications but is cqually well applicable to eases in which there are more than two geographically spaced signal sources. In such cases, links are provided from each geographically spaced point remote from the common point to relay at least changed frequency signals from each former point to said common point. The said links may also carry reference frequency oscillations to said common point for comparison with the reference frequency oscillations produced there and the production of error signals which are relayed over additional links back to the former points for maintenance of coherence of the reference oscillators at said former points Altematively the reference oscillations produced at the common point may be relayed over additional links to said former points for comparison with the reference oscillators there and coherence control of said reference oscillators.

The precise form of control of the reference frequency source or sources (if there is more than one to be controlled) depends on requirements. An arrangement which will give phase looking with a wide frequency "pull-in" and a good degree of "memory" comprises a frequency comparator fed with oscillations from the local reference frequency oscillator and incoming reference frequency oscillations to be compared therewith and coming from a geographically spaced point; a motor-driven frequency control device actuated by the error signal output from the frequency comparator and varying the frequency of the oscillator; an electronically controllable phase shifter which is fed with output from said oscillator and the phase shifted output from which is used as the reference signals from which heterodyning local oscillations are derived; a phase comparator fed with output from said phase shifter and with said incoming reference frequency oscillations; and means for utilising the output from the phase comparator for controlling the phase shifter and also for exercising fine control, electronically, of the oscillator frequency. With this form of control the motor, of course, provides "memory" and, in such a case (or in any other arrangement having "memory") means may be provided for cutting off temporarily and at will the supply of the aforesaid incoming signals - e.g. if the signalto-noise ratio becomes very bad - permitting temporary free running of the reference oscillator which will normally be a crystal oscillator.

The heterodyning of the source signals to the changed frequency signals relayed over the link means need not be single stage heterodyning.

Two stage heterodyning with two mixers in cascade, each fed with heterodyning oscillations, may be employed and will often be advantageous where the step-down in frequency from signal source frequency lo relayed changed frequency is required to be large. In such a ease the two heterodyning frequencies are preferably derived from the reference oscillator by means of two frequency multipliers in cascade, the fully multiplied oscillations being fed to the first mixer and the oscillations multiplied by the first multiplier only, being fed to the second mixer.

In general the multiplication factor of the second multiplier will be substantially larger than that of the first.

The invention is illustrated in the drawings accompanying the provisional specification in which figures 1, 3 and 4 illusuale in simplified schematic form, three embodiments of the invention; figures 5 and 6 illustrate, so far as is necessary to an understanding thereof, modifications; and figure 2 is an explanatory graphical figure.

Referring to figure 1, A and B represent two geographically spaced aerials constituting sources of signals which are to be rendered available at a common point while maintaining a desired degree of coherence. The signals may be of any type, e.g. modulated, unmodulated or pulsed C.W. or even random noise. They are represented as radiating from a source S, usually moving, which may be either a primary source, a transponder or a reflector. For convenience, in the following description, the station at which the aerial A is situaled will be called station A and the geographically distant station where aerial B is localcd will be called station B.

Signals received at Station A are fed to a receiver-mixer Al where they are mixed with local oscillations which are derived from a reference oscillator A2 having a frequency of, say, 5 Mc/s, by means of a frequency multiplier A3. The local oscillations from the unit A3 are mixed with the incoming signals to produce a changed (difference) frequency of, say, 3 Mc/s which is amplified by an amplifier A4 the output from which is fed to a combining network of any convenient form within the chain line block AS. Oscillations from the oscillator A2 are also fed direct lo the combining network to which may also be fed, by closing the swilch shown, other intelligence signals from a source represented by the rectangle A6. The output from the nctwork AS is fed to the input end of a link shown as a high frequency cable K which leads to station B. In a typical case the cable link pass band might be 300 c/s - 6 Mc/s and the positions of the various signal frequencies within this pass band are shown, in conventional graphical manner, in figure 2. Here, as will be seen, the changed signal band from unit A4 extends from 2 to 4 Mc/s and is marked 4S the references oscillations from A2 are represented by the line 2S and the band in which any additional intelligence signals from unit A6 lic is the 0- 1 Mc/s band marked 6S.

Station B has a mixer receiver Bl, an oscillator B2, a frequency multiplier B3 and a changed frequency amplifier B4 corresponding respectively to the units Al, A2, A3 and A4 at station A, cxccpt that the oscillator B2 is subjected to frequency or frequency and phase control to maintain it in coherence with the oscillator A2. At the output end of the cable K the oscillations provided by the oscillator A2 are separated out by a filter FB2 and fed as one input to a frequency or frequency and phase comparator B7 whose second input is supplied by the oscillator B2. The comparator gives a control output signal which is utilised in any suitable manner known per se to control a control unit B8 of suitable known form and which automatically and in known manner controls the oscillator B2 to maintain it in the desired degree of coherence with the oscillator A2.

The changed frequency signals from station A at the output of cable K are separated out by the filter FB4 and are available for utilisation al terminal AIF; the changed frequency signals of station B - these signals will be coherent with those from station A - are available for utilisation at terminal BIF; and the additional intelligence signals (if any) from unit A6 at station A are separated out by the filter FB6 and are available for utilisation at terminal AIS.

Figure 3 shows, in manner similar to that adopted in figure 1, a three station installation, the station C being the master station by means of which the reference frequency oscillators A2 and B2 at the stations A and B are controlled.

The master station C has a receiver-mixer C1, and an l.F. amplifier C4 and local oscillations for the mixer are derived from the reference frequency oscillator C2 by means of the frequency multiplier C3. Each of the other two stations A and B has a mixer Al or B1, I.F.

amplifier A4 or B4, and frequency multiplier A3 or B3 which provides local oscillations for the mixer Al or Bl by multiplying the oscillations from the reference frequency oscillators A2 and B2. These oscillators are controlled so as to maintain coherence with the oscillator C2 at the master station by means of a control unit A8 or B8 controlled by the output from a comparator A7 or B7 as already described in connection with the arrangement of figure 1. Cables KCA and KCB with pass bands as already described with reference to figure 1 connect the stations A and C and B and C respectively. Changed frequency signals from station A are fed to station C over cable KCA, suitable separating filters both marked IFAC being provided in the changed frequency channel at bolh ends of the cable.

Reference oscillations from station C are fed to the comparator A7 through the same cable, suitable separating filters, both marked RAC being provided in the reference signal channel at both ends of the cable. Cable KCB links stations C and B in the same way, the changed frequency channel filters being marked IFBC and the reference frequency channel filters being marked RBC. At station C coherent changed frequency signals are available for utilisation at the lerminals AIF, BIF and CIF the signals at these terminals being those produced at stations A, B and C respectively.

Figure 4 shows a modification of the arrangemenl of figure 3. In this modification error correcting signals instead of reference oscillations are sent out from the "master" station. This has ccrtain advantages because the bandwidlh occupied by error correcting signals will normally be quile small, a bandwidth of 1 kc/s being ordinarily sufficient.

Referring to figure 4, each of the three stations A, B and C has a receiver-mixer Al, B1 or Cl, a changed frequency amplifier A4, B4 or C4; a reference frequency oscillator A2, B2 or C2; and a frequency multiplier A3, B3 or C3; all as already described. The stations A and B have combining networks A5 and B5 respectively which bring together changed frequency signals and reference oscillations for transmission over links to station C. These links might be cables, but are shown as radio links RLAC and RLBC each with a modulator and transmitter unit at one end and a receiver and demodulator unit at the other in the usual way.

The former units are all marked T and the latler are all marked R. At station C the changed and reference frequency signals are separated by filters IFAC, IFBC and RAC, RBC, the changed frequency signals being made available for utilisation at terminals AIF and BIF. The separated reference oscillations are fed lo comparators AC7 and BC7 al station C, the second inputs to these comparators being taken from the oscillator C2 at that station.

Oscillations from oscillator C2 are also multiplied in frequency by frequency multiplier C3 to provide local oscillations for the receivermixer Cl whose changed frequency output, amplified by the changed frequency amplifier C4 is available for utilisation al terminal CIF.

The error signals from comparators AC7 and BC7 are either fed directly to modulate the transmitter units Tl of additional narrow-band radio links NRCA, NRCB or are first fed into units (indicated by dotted blocks to show they are oplional) CCA, CCB to convert them into preferred signal forms for transmission. These converter units might be, for cxample, subcarrier generators or analogue-digital converters.

The narrow band links transmit the error signals (whether converted or not, as the case may be) to the stations A and B where they are received by demodulation receivers Ri and fed lo control units A8, B8 to control the oscillators A2 and B2 into the desired coherence with oscillator C2. The units Ri may, of course, include signal re-conversion means (not shown) if the optional conversion units CCA and CCB are provided.

Although cable links are indicated in figures 1, 2 and 3 and radio links are indicated in figure 4, it is to be understood that, in all cases, a cable link may be replaced, if desired, by a radio link adapted to give the required one way or two way communication (as the case may be) or vice versa. In figure 4 the links RLAC and RLBC are wide band, e.g. 300 c/s to 6 Muck, and the links NRCA and NRCB can be quite narrow band, e.g. 1 kc/s bandwidth.

Although, in figures 1 lo 4, the conversion of received signals into changed frequency signals is in all eases shown as effected by single-stage heterodyning, the invention is, of course, not limited to this and in all cases lwo-sLage heterodyning with two frequency changes may be used. Figure 5 illusuates this, showing, by way of example, part of station A modified for two-stage heterodyne reception. In figure 5 the aerial A feeds into a first mixer IAI followed by a first amplifier IA4 which in turn feeds into a second mixer 2A1 operating at a second, lower, frequency eonstituting the changed frequency.

Oscillations from the reference oscillator A2, multiplied by a first multiplier IA3 having a multiplication factor of, for example, 6, are, when the switch SS is in the position shown, fed as local oscillations to the mixer 2A1. These multiplied oscillations, further multiplied by a second multiplier 2A3 having a multiplication factor of, say 100, are fed to the mixer lAl. To quote typical, nonlimiting practical figures, if the signals at aerial A were centred on 3033 Mc/ s, the oscillator A2 might be 5 Mc/s giving local oscillations of 30 Me/s to mixer 2A1 and local oscillations of 3000 Me/s to mixer 1Al. The frequency at amplifier IA4 would then be centred on 33 Me/s and the frequency at amplifier 2A4 would be centred on 3 Mc/s as in the already described case of figure 1. If, at any time, exact phase and frequency coherence was not required, the swilch SS would be thrown into its other position and local oscillations fed to the second mixer 2AI from a separate local oscillator SLO of 30 Mc/s which was not locked lo the oscillator A2 but was a crystal or similarly controlled oscillator of required stability.

The comparators and control units employed in carrying out this invention may be of any suitable form known per se and will be of a design depending upon operating requirements and the degree of coherence required. Figure 6 shows a phase and frequency controlling arrangement which has a degree of "memory" so that, should the supply of controlling reference oscillations from another station fail at any time or be deliberately cut off for any reason (e.g. the presence of excessive noise), there will be minimum disturbance. Figure 6 illustrates the arrangement now to be described as applied to the controlled station B of figure 1.

In figure 6, B3 is the multiplier B3 of figure 1, the filters FB4, FB6 and FB2 are the filters so referenced in figure 1, and B2 is the reference oscillator B2 of figure 1. It is shown conventionally as including a motor driven tuning condenser 1B8 which is adjustable by an electric motor EM. Between the units B2 and B3 is interposed an electronically controllable phase shifter PH. When the relay switch RS is in the posilion shown reference oscillations (in this case from station A - see figure 1) are fed as one input lo a first comparator 1B7 which is a frequency comparator and the second input to which is taken from the oscillator B2. Output from this comparator controls the motor EM, the control being diagrammatically represented by the lead FMC. There is a second comparator 2B7 which is a phase comparator and receives one reference oscillation input (from station A, figure 1) via relay RS and the other from the phase shifter PH. The output from the comparator is fed through suitable time constant filter circuits TCF to effect, in known manner and electronically, fine control of the frequency of the oscillator B2 by means of an electronic frequency control unit 2B8, and also of the phase shift introduced by the phase shifter pH.

If the supply of reference oscillations from station A fails, or if il is deliberately interrupted by actuating the relay RS, there will be a minimum of frequency disturbance because the motor providers a substantial element of "memory". An arrangement as shown in figure 6 may, of course, be used wherever control of a reference frequency oscillator is required in any of the embodiments illustrated.

In order not to complicate the drawings only figures 1 and 5 are shown with means (A6) for superimposing exua intelligence signals. Such means, with suitable associated filtering, may, of course, be provided wherever required.

As will now be appreciated it is important that, in all embodiments of this invention, the frequencies of the reference oscillators and of the received changed frequency signals shall all lie within the low frequency passbands of the cables or radio links (as the case may be) which interconnect the various sites. When radio links are employed the combined signals are fed lo the modulator input terminals of each link equipment to form a frequency division multiplexed baseband signal. Since frequencies appearing at the demodulator output situated at the receiving terminal of a radio link are identical with those fed into the link modulator at the transmitting terminal, this method of transmission results in frequency coherence being maintained between corresponding frequencies within the system independently of the radio frequencies used in the links or of heterodyne processes to which the signals are subjected in passing from one station lo another.

WHAT WE CLAIM IS: 1. A signal communication system for relaying signals from signal sources at geographically spaced points lo a common point said system comprising signal link means of predetermined pass band and extending between each of said sources which is remote from said common point to said common point; means adjacent each signal source for heterodyning the signals therefrom to a common predetermined changed frequency which is within said pass band, said heterodyning means including a reference source from which the local oscillations for heterodyning are coherently derived and which is of a reference frequency different from the changed frequency but also within the said pass band; means for feeding changed frequency signals produced at each geographically spaced point remote from the common point to said common point via the link means; and means for controlling the reference frequency oscillations produced at all but one of the different geographically spaced points by comparison with the reference frequency oscillations produced at said one point to maintain all the reference frequency oscillations in coherence.

2. A system as claimed in claim 1 and including one or more links constituted by a cable link or links.

3. A system as claimed in claim 1 and including one or more links constituted by a radio link or links, the signals to be relayed being applied to modulate a carrier wave transmitter at the input end of the link which is provided with a receiver and a demodulator at its output end.

4. A system as claimed in any of claims 1 to 3 wherein reference frequency oscillations produced at one point are relayed to another point for comparison with reference frequency oscillations produced at said other point over the same signal link means as are employed to relay changed frequency from said one point to the other and the error signals resulting from said comparison are employed for c

Claims (16)

**WARNING** start of CLMS field may overlap end of DESC **. one reference oscillation input (from station A, figure 1) via relay RS and the other from the phase shifter PH. The output from the comparator is fed through suitable time constant filter circuits TCF to effect, in known manner and electronically, fine control of the frequency of the oscillator B2 by means of an electronic frequency control unit 2B8, and also of the phase shift introduced by the phase shifter pH. If the supply of reference oscillations from station A fails, or if il is deliberately interrupted by actuating the relay RS, there will be a minimum of frequency disturbance because the motor providers a substantial element of "memory". An arrangement as shown in figure 6 may, of course, be used wherever control of a reference frequency oscillator is required in any of the embodiments illustrated. In order not to complicate the drawings only figures 1 and 5 are shown with means (A6) for superimposing exua intelligence signals. Such means, with suitable associated filtering, may, of course, be provided wherever required. As will now be appreciated it is important that, in all embodiments of this invention, the frequencies of the reference oscillators and of the received changed frequency signals shall all lie within the low frequency passbands of the cables or radio links (as the case may be) which interconnect the various sites. When radio links are employed the combined signals are fed lo the modulator input terminals of each link equipment to form a frequency division multiplexed baseband signal. Since frequencies appearing at the demodulator output situated at the receiving terminal of a radio link are identical with those fed into the link modulator at the transmitting terminal, this method of transmission results in frequency coherence being maintained between corresponding frequencies within the system independently of the radio frequencies used in the links or of heterodyne processes to which the signals are subjected in passing from one station lo another. WHAT WE CLAIM IS:

1. A signal communication system for relaying signals from signal sources at geographically spaced points lo a common point said system comprising signal link means of predetermined pass band and extending between each of said sources which is remote from said common point to said common point; means adjacent each signal source for heterodyning the signals therefrom to a common predetermined changed frequency which is within said pass band, said heterodyning means including a reference source from which the local oscillations for heterodyning are coherently derived and which is of a reference frequency different from the changed frequency but also within the said pass band; means for feeding changed frequency signals produced at each geographically spaced point remote from the common point to said common point via the link means; and means for controlling the reference frequency oscillations produced at all but one of the different geographically spaced points by comparison with the reference frequency oscillations produced at said one point to maintain all the reference frequency oscillations in coherence.

2. A system as claimed in claim 1 and including one or more links constituted by a cable link or links.

3. A system as claimed in claim 1 and including one or more links constituted by a radio link or links, the signals to be relayed being applied to modulate a carrier wave transmitter at the input end of the link which is provided with a receiver and a demodulator at its output end.

4. A system as claimed in any of claims 1 to 3 wherein reference frequency oscillations produced at one point are relayed to another point for comparison with reference frequency oscillations produced at said other point over the same signal link means as are employed to relay changed frequency from said one point to the other and the error signals resulting from said comparison are employed for controlling the reference frequency oscillator at said other point into coherence.

5. A system as claimed in any of claims 1 to 3 wherein reference frequency oscillations produced at one point are relayed to another point for comparison with reference frequency oscillations produced at said other point over the same signal link means as are employed to relay changed frequency from said one point to the other and the error signals resulting from said comparison are relayed back to said one point over an additional link provided for the purpose and are there employed for maintaining the reference oscillator there in coherence.

6. A system as claimed in any of claims 1 to 3 wherein changed frequency signals produced at one point are relayed to another over one link and reference frequency oscillations produced at said other point are relayed back to said one point for comparison with the reference frequency oscillations produced there and control of the reference frequency oscillator there for maintenance of coherence by the error signals resulting from such comparison.

7. A system as claimed in any of the preceding claims wherein additional information signals, on separable frequencies, are also fed over the link means, frequency selective filtering being provided for scparaling out the additional information signals.

8. A system as claimed in any of the preceding claims and wherein the heterodyning means comprise, in each case, a reference frequency oscillator and at least one frequency multiplier driver thereby, the multiplied frequency constituting the local oscillations which are mixed with signals from the appropriate signal source lo produce the changed frequency.

9. A system as claimed in any of the

preceding claims wherein error signals dependent upon and representative of lack of coherence between two reference frequency signals, one from each of two reference frequency sources are obtained by comparing the two signals to produce resultant output signals which are then utilised to control one of the two reference frequency sources to maintain it in coherence with the other.

10. A system as claimed in any of the preceding claims wherein there are only two geographically spaced points one of which is also the common point and a single link is employed to relay to the common point the changed frequency signals and the reference frequency oscillations from the other point and wherein at the common point the reference frequency oscillations from said other point are separated out and fed to a comparator whose other input is taken from the reference source at said common point, the output from the comparator being employed to control the last mentioned reference source.

11. A system as claimed in any of the claims I to 9 wherein there arc more than two geographically spaced signal sources and links are provided from each geographically spaced point remote from the common point lo relay at least changed frequency signals from each former point to said common point said links also carrying reference frequency oscillations to said common point for comparison with the reference frequency oscillations produced there and the production of error signals which are relayed over additional links back to the former points for maintcnance of coherence of the refereiice oscillators at said former points.

12. A system as claimed in any of the claims I to 9 wherein there are more than two geographically spaced signal sources and links are provided from each gcographically spaced point remote from the common point to relay at Icast changed frequency signals from each former point to said common point, reference oscillations produced at the common point being relayed over additional links lo said former points for comparison with the reference oscillators there and coherence control of said reference oscillators.

13. A system as claimed in any of the preceding claims 1 to 4, 6 to 10 or claim 12 and including a reference frequency source controlling arrangemcnt comprising a frequency comparator fed with oscillations from the local reference frequency oscillator and incoming reference frequency oscillations to be compared therewith and coming from a geographically spaced point; a motorZriven frequency control device actuated by the error signal output from the frequency comparator and varying the frequency of the oscillator, an elecuonically conuollable phase shifter which is fed with output from said oscillator and the phase shifted output from which is used as the reference signals from which heterodyning local oscillations are derived; a phase comparator fed with output from said phase shifter and with said incoming reference frequency oscillations; and means for utilising the output from the phase comparator for controlling the phase shifter and also for exercising fine control, clecuonically, of the oscillator frequency.

14. A system as claimed in any of the preceding claims 1 to 4, 6 to 10, or claims 12 or 13 and including a reference frequency source controlling arrangement having "memory" wherein means are provided for cutling off at will incoming reference frequency oscillators so as to permit temporary free running of the reference oscillator in question.

15. A system as claimed in any of the preceding claims wherein heterodyning of source signals to changed frequency signals relayed over a link is effected in two stages and the two heterodyning frequencics are derived from the reference oscillator by means of two frequency multipliers in cascadc, the fully multiplied oscillations being fed to the first mixer and the oscillations multiplied by the first multiplier only, being fed to the second mixer.

16. Signal communication systems substantially as hercin described and illustrated in the drawings accompanying the provisional specification.