GB2187907A - AM interference suppression arrangement - Google Patents

Abstract

A method of reducing or eliminating the interference caused to a wanted radiocommunication signal comprising an AM carrier by at least one concurrent unwanted such signal in an adjacent channel. The carrier of at least one of the concurrent signals is extracted from the concurrent signals, and from it is generated a carrier of the same frequency but of different amplitude and/or opposite phase. The generated carrier is then combined with the original concurrent signals and the combination subjected to a conventional detection step in such a way as to enhance the wanted detected signal relative to the unwanted detected signal. Finally, and if necessary, the difference-frequency signal between the concurrent signals is filtered out. <IMAGE>

Description

SPECIFICATION Improvements in or relating to interference suppres sion This invention relates to interference suppression methods and circuits, in particularforthe reduction or elimination ofthe interference caused, after detection, by the concurrence of wanted and unwanted amplitude-modulated (AM) signals of generally symmetric al sideband structure in adlacentchannels (ie in channels having a separation equalto ormorethan the nominal maximum signal modulation bandwidth).

Such interference can make comprehension of the wanted detected signal difficult.

According to the present invention a method of reducing or eliminating the interference caused to a received, wanted, radio-communicationsignal comprising an AM carrier by at least one concurrent, unwanted, such signal in an adjacent channel (as hereinbefore defined) comprises: extracting the carrier of at least one of said signals; generating from said extracted carrier a carrier of thesamefrequencybutofdifferentamplitudeand/or opposite phase; combining the original concurrent signals with said generated carrier and detecting the combined signals in such a way as to enhance the wanted detected signal relative to the unwanted detected signal; and, optionally, filtering-out the difference-frequency signal between said concurrent signals.

Where the concurrent signals have different carrier strengths and the wanted carrier is strongerthan the unwanted carrier the method may comprise: extracting the carrier of at least one of said signals; generating from said extracted carrier a carrier of thesamefrequencybutofamplitudelargerorsmaller than said extracted carrier depending on whether said one signal carrier is stronger orweakerthan the other said signal carrier respectively; adding to or subtracting from the original concurrent signals said generated carrier depending on whether said one signal carrier is stronger or weaker than the other said signal carrier respectively; detecting said added or subtracted signals; and, optionally, filtering-outthe difference-frequency signal.

The method may comprise extracting the carriers of both said signals; generating from said extracted carriers two carriers of the same two frequencies but of respective amplitudes largerorsmallerthan said extracted carriers depending on whether each signal carrier is stronger orweakerthan the other said signal carrier; respectively adding to and subtracting from the original concurrent signals said two generated carriers depending on whether each generated carrier is derived from the stronger or weaker ofthe original two signal carriers respectively; detecting said added and subtracted signals; adding togetherthetwo signals resulting from said addition and subtraction; and, optionally, filtering out the difference-frequen- cy signals.

Where the concurrent signals have different carrier strengths andthe wanted carrier is stronger than the unwanted carrier the method may alternatively comprise: extracting the carrier of the wanted signal; combining said extracted carrier with the concur rent signals in such a way as to produce a version of the concurrent signals in which the carrierofthe wanted signal has its phase reversed but its amplitude unchanged; detecting separatelythe original concurrent signals and said version of the concurrent signals; subtracting the two detected signals; and, optionally, filtering-out the difference-frequency signal.

Said version ofthe concurrent signals may be produced by doubling the amplitude and inverting the phase ofthe extracted carrier and adding the result to the original concurrent signals.

Prior to separately detecting the original concurrent signals and said version ofthe concurrent signals as aforesaid, the carrier amplitudes of the wanted and/or unwanted signals may be respectively increased and/or decreased by extracting the relevant carrier or carriers, generating therefrom a carrier orcarriers of largerorsmalleramplitudedepending upon whether the respective carrier or carriers are the stronger orthe weaker, and adding or subtracting said generated carrier or carriers to orfrom the original concurrent signals depending upon whether their respective carrier or carriers are the stronger or the weaker.

Where the concurrent signals are of approximately equal carrier strengths the method may comprise: extracting the carrier of one of the two signals; generating from said extracted carrier a carrier of thesamefrequencybutoflargeramplitude; adding tothe original concurrentsignalssaid generated carrier to thereby produce two concurrent signals in which one carrier is effectively stronger than the other; and thereafter performing a method as aforesaid for concurrent signals whereofthe wanted signal carrier is stronger than the unwanted signal carrier.

Where the wanted concurrent signal carrier is weakerthan the unwanted concurrent signal carrier the method may comprise: extracting the weaker signal carrier; generating from said extracted carrier a carrier of the same frequency but of larger amplitude; adding to the original-concurrentsignals said generated carrier to thereby produce two concurrent signals wherein the carrierwhich was originally the weaker is now effectively the stronger; and thereafter performing a method as aforesaid for concurrentsignals whereof the wanted carrier is strongerthanthe unwanted carrier.

Where the concurrent signals have different carrier strengths and both carriers are extracted, the stronger extracted carrier may be subtracted from the concurrentsignals priortothe extraction therefrom of the The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

weaker carrier.

Where there are two unwanted signals each having a weakercarrierthanthewanted signal and each in a separate adjacent channel, the interference may be reduced or substantially eliminated by a method as aforesaid for one unwanted signal carrier weaker than the wanted signal carrier.

The detecting steps may use either linear or square-law detection.

In the present invention the step of extracting the carrier of a signal may be performed by a conventional phase-locked loop (PLL) comprising a phase-detector, low-pass filter and voltage-controlled oscillator (VCO). However, the present invention also provides, for performing the aforesaid step, an improved method of extracting the carrier comprising:: applying the concurrentsignalsto a PLLas aforesaid to generate a VCO output ofthe desired carrierfrequency and in approximate phase quadra turewiththe desired carrier; applying the DC error signal generated in the PLL to derive from the VCO output a modified VCO output substantially in phase with the desired carrier; mixing the modifedVCO outputwiththe concurrent signals and detecting the product; further mixing the DC detected product, after low-passfiltering,with said modifiedVCO output and detecting the product of said further mixing to produce the extracted carrier.

The present-invention also provides circuits for use in methods as aforesaid.

The present invention will now be described with reference to the accompanying drawingswherein: Fig 1 is a diagram showing vectorial addition of strong and weak signals; Fig 2 is a modification of Fig 1 with carrier phase-inversion ofthestrong signal; Fig 3 is a further modification of Fig 1 showing vectorial multiple-signal addition; Fig 4 is a block schematic circuit diagram, byway of example, of a carrier-extraction circuit for use in the present invention; Fig 5 is a block schematic diagram illustrating one method ofthe present invention; Fig 6 is a block schematic circuit diagram illustrating another method of the present invention; Fig 7 is a block schematic circuit diagram illustrating the combination ofthe methods of Figs 5 and 6;; Fig 8 is a block schematic circuit diagram illustrating a yetfurther method ofthe present invention.

This description will first deduce mathematically the level of interference obtained from an adjacent channel, via a conventional linear detection process, when the wanted signal is stronger, ie greater in amplitude, than the unwanted signal. The result accounts for what is often referred to as the "capture effect" whereby the detected amplitude of the strong signal is enhanced relative to that ofthe weak. (Forthe avoidance of doubt, it is noted that the so-called "linear detector" is really a non-linear element which produces an output proportional to the amplitude of the input signal. The so-called "square-law detector" is a non-linear element which produces an output proportional to the square of the input signal.Howev- er, the same element may function as a lineardetector for large-amplitude signals and a square-law detector for small-amplitude signals; eg a diode can behave thus on account of the shape of its voltage-current characteristic.) The result of such a conventional process will then be related to that of an ideal synchronous detection process whose capture effect is theoretically perfect, although the latter's performance as regards the suppression of beat (difference) frequencies remains comparable with the conventional linear detection process.

The "carrierfrequencies" referred to in thefollow- ing description will normally be the difference (IF) frequencies produced by a superheterodyne receiver after mixing the received, propagated (RF) frequencies with its local oscillator, although the description is equally applicableto receivers handling signals atthe propagated frequencies.

Capture effect using conventional lineardetection Fig 1 shows the vector diagram arising from the two AM signals a = a sin wit = atl + km cos wmt) sin wit c = c sin w2t = c(l + kn cos wnt) sin w2t where a = a(l + km cos wmt) c = cel + kn cos wnt) wl and (1)2 are the radio angularfrequencies of the two carriers, and a and ctheir respective unmodulated amplitudes.

w, and w, are fhe modulation angtilarfrequencies with appropriate modulation indices km and kno Leta c If b is the resu ltantvector, then by vectorial analysis - - - -- # b t (a2 + C2 + 2ac cos where G =w2t - #t, the phase of beat frequency w2 - wb.

be removable by filtering action after linear detection.

Thusthe productoflineardetection is

Thus, ignoring terms containing knkm, kn2 etc, there is produced a wanted signal akm cos wmtwith the main unwanted term as

ie the interference is down by ca as a result of the capture effect. The beat4requency component, ccos 6, is not reduced by the capture effect, and must be reduced if desired by filtering.

Such filtering will usually be desirable unless the beat-frequency component has a frequency outside the range 15 of the ultimate receiver, eg above the range audible to the human ear, but is limited in practice by the difficulty and/or expense of providing post-detection filters having very sharp cut-offs relative to the pass-band. The effect of these beat components can be serious. Under some circumst ances the residual beat left after less-than-ideal filtering can cause a lack of comprehension comparable with that of any modulation interference left after capture.

Perfect capture using ideal synchronous detection In ideal synchronous detection, the received signals are multiplied by a carrierwhich is in synchron- ism with the wanted signal. Such a synchronised carrier may be already available or may be derived using, eg, a phase-locked oscillator.

The output after ca rrier-frequency filtering can then be shown to be proportional to a+ccos6 Thus (the detected component ofthe wanted signal) can be obtained completely free of c (the detected component of the unwanted signal). Howeverthere is seen to be a beat component involving cos 6.

In practice, perfect multiplicative detection is difficult if not impossible overwide signal dynamic ranges; also, imperfections in the multiplicative characteristics quickly introduce unwanted terms. A form of pseudo-synchronous detection can be obtained using a balanced detector arrangement in which the synchronising carrier is of very great strength. This arrangement, however, suffers from problems arising from any noise sidebands on this carrier, from distortions introduced when the signal bandwidths become comparable with the carrier frequency and from dynamic range limitations; also, it does not alleviate the beat problem. The present invention achieves results approaching the ideal synchronous case while using conventionally operated detectors, and also allows improvement as regards the beat4requency content.

Enhancement of capture effect using linear detection The enhancement ofthe known capture effect, and reduction of the beat component, by means ofthe present invention will now be described, assuming that the carriers of both signals can be extracted.

(Circuits for achieving such extraction will be described later, with reference to Fig 4.) The carrier a sin wlt is extracted from equation (1) and replaced by agl sinwlt whereg1 > 1 then equation (1) becomes

This will then give a wanted signal of

Acircuitforobtainingthisresultisshown in Fig 5, which is self-explanatory. As explained earlier, the final beat (#r(#i) filter may sometimes be omitted.

It will be seen that the beat term, a cos 6, is not reduced by this technique. Also, when the received carrier is strong, and can vary over a wide dynamic range, the values of g1 may have to be severely limited byamplifierdynamic-range restrictions and any noise levels associated with the replacement carrier; any such generated noisewill be proportional to the strength of the carrier. If either or both ofthese difficulties are present, the situation can be improved by derivation and replacement ofthe carrier ofthe unwanted signal as follows.

The signal represented by equation (2) can be replaced by

ie a reduction of g2 overthe simple capture effect has been obtained without an increase in the value of a.

The value of the beatterm now becomes

which tends to Ckn cos o)nt.cos 6 as g2 tends to zero, iethe interference reduces towards that arising from the information content (ie the sideband content) only. This should be compared with the beat value c(1 +kncos#nt)cos# forthe previous techniques. Thus with g2 < 1, some improvement is obtained.

A circuitfor obtaining this result is shown in Fig 6, which is again self-explanatory; the final beat filter may again be omitted.

The use of a value of zero for 92 would apparently reduce the modulation interference to zero and the beattermsto that of modulation sideband level only.

However, there are lower amplitude terms, arising from a more accurate expansion of equation (2), which are independent of gb eg

Thus some residual modulation interference remains, although this can be reduced albeit by increasing a. However, the net effect is not only a sizeable reduction in the beat interference, but also a reduction in the amount of increase of wanted signal carrier, via agr, required for a given level of modula- tion interference.It follows from the foregoing that derivation and replacement of both carriers can be used in combination to produce a new value of In as follows:

ie the overall enhancement of capture effect com g2 pared with equation (3A) is ~, g1 or, expressed otherwise, the interference is reduced g2 C by - x - gt a as a result of the enhanced capture effect.

A circuit arrangement for obtaining this combined result is shown in Fig 7 (neglecting the interruptedline additions) which is also self-explanatory. As in Figs Sand 6, the final beat filter may sometimes be omitted.

Enhanced reduction ofinterference using a balancing technique Theforegoing has shown howthe modulation interference can be reduced by predictable amounts.

These amounts are limited in general by the level and dynamic range ofthe received strong signal carrier.

By means of a slightly more complicated balancing technique, this interference can be yet further reduced as follows.

Consider equation (1 ) to be changed by carrier extraction and its replacement by - a sin wst.

then a = a sin wtt = - a(l - km cos wmt) sin wjt The vector diagram then becomes that given by Fig 2 bthen becomes

The equivalent expression representing the expansion ofthe results of normal linear detection becomes

By subtracting (3B) from (9) the output becomes

+ modulation interference terms having amplitudes dependent

The latter expression for the output can be made more general by replacing a by ag1, c by cg2, km by kn and kn by ~ g2 in accordancewith the theory given previously.Such replacement not only increases the carrier content of the wanted signal (whether it is stronger of weaker than the unwanted signal) but also allows elimination ofthe unwanted carrier in orderto achieve reduction ofthe beat term.

Thustheoutputmaymoregenerallybewritten as Output =

+ modulation terms having amplitudes dependent on

the beat interference terms 2cg2 cos 6 and 2ckn cos rent cos 6.

All terms involving g2 can be reduced to zero by making g2 equal to zero.The modulation interference term not involving g2 is small but can be reduced further by using a suitably large value of g1.

A circuit which utilises all the aforesaid techniques is shown in Fig 8, in which it will be seen that two balancing inputs are subtracted to produce the final output. The right-hand input is provided by a Fig 7 circuit (minus its final beat filter which, when required, is preferably connected in the final output connection as shown in Fig 8 ratherthanfollowing each ofthetwo detector units; thusthe right-hand detector in Fig 8 can be the detector preceding the final beat filter in Fig. 7). The left-hand input is provided by a detectorfed from appropriate points in the same Fig 7 circuit.

Itwill be understoodthat, as described above,the replacement of a by agl and/or c by cg2, as in Fig 8, may not always be essential. In the latter case the in puts to the two detector units in Fig 8 may respectively be simply the original concurrent signals (iea sin 1t + c sin co2t + wanted and unwanted signal sidebands) as one input, and the concurrent signals with a sin (i)1t replaced by- a sin (o# as the other input.

This other input can be obtained byfeeding the concurrentsignalsto a Fig 4circuitfollowed by an amplifierwith a gain of followed by an inverter, and adding the inverter output to the original concurrent signals.

As will be understood by those skilled in the art, the amplifiers, attenuators, adders, inverters and any combinations thereof in Figs 5to 8 can betransfor mers. It will also be understoodthatmany ofthe combinations shown can themselves be replaced by single components; egthe amplifierand phase inverter in Fig 6 can be replaced by an amplifier or transformerof gain g-1. Also, the additionscan be replaced by subtractions or combinations of addition and subtraction or combinations of addition and subtraction with appropriate use of phase-inverters.

Almost-equal and weak signals using linear detection So far it has been assumed that a strong (a) a weak signal (c) are present. Where the signals are equal or nearly so, the known capture effect described earlier does notapplyand mathematical analysis is very difficult However, by extracting onecarrierand replacing it by a carrier ofthe same frequency but much greater strength, it is possible to make one signal dominate and so again achieve a simple capture effect. Moreover, the methods ofthe present invention, already described for unequal signals, become applicable.

Where it is desired to reduce interference, with a weaksignal, due to a stronger signal in an adjacent channel, the carrieroftheweaker signal can be extracted, as previously decribed, and replaced by a much stronger carrier of the same frequency so that, in effect, the initially weaker (wanted) signal now becomes the stronger and it is possible to use the enhanced capture-effecttechniques already described. In particularthe technique of Fig 7 now becomes more effective, provided the carrier of the initially weaker (wanted) signal is increased (by suitable choice of g1) to exceed the modulation sideband strength oftheoriginallystronger(un- wanted) signal or at leastthestrength of the latter's carrier.

Multiple signals So far, only interference dueto one signal in an adjacent channel has been considered. Fig 3 shows the situation of a strong signal in the presence of two weakersignals. Using similar notation as hitherto, the second weaker signal is expressed as:

Following the earlier analysis of simple capture effect it can be shown likewisethat

This can be resolved into: wanted term (a); inter c2 d2 ferenceterms ~ and ~ which can be reduced a a oreliminatedasalreadydescribedforone interfering signal; beat terms 61 and 62 which can be removed by filtering; and a beatterm 6i - 62 which can only be removed by filtering provided both c and dare each in sepa rate adjacent channels, ie one above and one belowthe wanted signal channel. In practice it is likelythat,with higher numbers of multiple signals, morethan onewould beinthesameadjacent channel; thus the present invention is only applicable where there is a wanted signal and either one ortwo interfering signals.

The description so far has assumed thatthe detection steps use linear detection. The effect of using square-law detection will now be described, starting with its influence on the known capture effect.

Normal square4awdetection From Fig 1 the vectorial addition oftwo AM signals can be expressed, as shown after equations (1 ) and (2), by

After square-law detection and assuming that the cos (2#1t + 6) terms are filtered off,

The amplitude ratio of the modulation interference as regards the w, and #n terms is c2 kn a km which is identical to that previously determined for linear detection (equation (3)). The ratio of the wanted modulation to the beat content also remains the same. In addition, however, there are second-harmo- nicterms of Zamcfhat are not present intheformer detection and would usually be highly undesirable It isthesetermsthattendtodiscouragethegeneral use of square-law detection, but in the present invention both interference and harmonic problems can be suppressed by the balancing technique. Before describing this usage however, its effect on the enhancement ofthe capture effect will be described.

Enhancement ofcapture effect using square-law detection As for linear detection, equation (1) is replaced, via extraction of the coi carrier, by equation (4).

Then the amplitude ratio of interference is then

ie, reduction of 1 G1 as for linear detection enhancement.

Likewise, by extraction and replacementofthe carrier wa, the same enhancements described previously for linear detection can be obtained, allowing the value of ag1 to be reduced as previously as well as reducing or eliminating the carrier heatcontentproduced by the 2ac cos 6 term.

Enhanced reduction ofinterference using the balancing technique and square-law detection.

In line with the procedure described forthe enhanced reduction of interference using linear detection, equation (1 ) is changed, via the process of carrier extraction and replacement, to produce the vector diagram of Fig 2. Equation (1)then becomes modified to:

Thus by subtracting equation t15) from equation (12) using the circuit of Fig 8, all butthe wanted term 4a2km cos c1)mt and the unwanted heat term 4accos 6 disappear, ie perfect capture is obtained.Because the retained signal is now proportional to a2 ratherthan to a, in order to avoid a dynamic-range problem in post-detection amplifiers it may be necessaryto use a form of AVC control in which the long-term amplitude a2 is detected and fed forward to control an attenuator whose output is proportional to @; The carrier factorofthebeatterm,4ac,can be reduced by the technique of unwanted carrier signal extraction and its replacement bycg2 in accordance with the foregoing descriptions.

Almost equal and weak signals using suare-law detection The immediatelyforegoing description ofthe enhanced reduction of modulation interference, using square-law detection in the circuit of Fig 8, is in theory independent of relative signal levels. Thus there are no limitations in this respect, and the technique becomes equivalentto ideal synchronous detection in effectiveness. However, restriction of the dynamic range to that over which the square-law characteristic will apply limits the overall effective ness ofthetechnique, andthus the linear detection arrangements as previously described will usually be preferable.

Carrier extraction and processing Fig 4 shows a suitable carrier-extraction circuit which produces an output directly related to the original phase and amplitude; however, alternative circuits forth is function may be used. It comprises a conventional phase-lock loop (PLL) formed by a balanced mixer (also known as a multiplier) 1 which includes a detector and acts as a phase-detector in a known manner and has the concurrentwanted and unwanted signals as one input.The post-detection outputfrom mixer 1 is fed via a low-pass filter 2to control a voltage-controlled oscillator (VC0)3 whose outputistheotherinputto mixer 1. (Detailed descriptions of suitable PLL circuits are 30 given in eg "Phase-lockTechniques" by F M Gardner, Second Edition, John Wiley and Sons, 1979.) VCO3 is tuned to the proximity of the carrier it is desired to extract, and the tuning can be either manual or automatic as no co-channel interference (ie interference from a signal within the modulation bandwidth) is assumed.

Manual tuning can be assisted in a known manner by an IF panoramic display, but this is not essential. The PLL locks on to the carrier, or in effectthe mean frequency of the signal, and holds this frequency within a bandwidth defined bythe inverseofthe loop memory time-constant. After lock, the output phase of VCO3 will re reasonably close to being in quadrature with that of the carrier and no further correction may be necessary, ie a sinwlt or a sin within Figs 5 to 8 maybe derived directlyfrom VCO3.Amplitude control can be by manual setting, but asthis may be rather awkward it is preferred to obtain automatic amplitude control, together with more exact phase control, by means ofthe remainder of the circuit shown.

These are obtained by feeding the output of VCO3 to a voltage-controlled phase-shifter (VCPS) 4 controlled by the output offilter 2. VCPS 4 operates over about j 20% and its output is fed through a 90 phase-shifter 5 to a balanced mixer (multiplier) 6.

Mixer 6 has as its second input the original concurrent signals and its output is fed to a second balanced mixer (multiplier)7 whose second input is derived directly from VCPS 4 and which provides the final replacement carrier output. The filtered outputfrom the PLL detector included in mixer 1 is proportional to the phase-shiftbetweenthetruecarrierandthe locked VCO output, and is arranged to reduce the phase-shift in VCPS 4 so that the VCo output enters phase-shifter Sin nearly exact quadrature with the carrier phase. The DC post-detection output from mixer6 is equal to the amplitude ofthe original carrier and is thus used to set the amplitude ofthe VCO-produced replacement carrier by means ofthe mixer 7, whose second input is obtained from phase-shifter 5.

The operation ofthis circuit can be described mathematically as follows. Let the carrier4requency output from phase-shifterS be expressed as A sin wlt wheeA > > a.(A may include the effective gain produced by VCO 3).

In balanced mixer 6 this output is added to and subtracted from the concurrent signals expressed by equations (1) and (2) to give two new versions of these equations, viz

Assuming lineardetection in mixer6andtheir subsequent addition therein, the output of mixer 6 is DC terms + o, or w, terms + IF#1 or #2 terms. The DC terms are mainly (from inspection of equation (3))

Thus the ratio of wanted (a-dependent) to unwanted (c-dependent) DC terms is 2A C , which is very large if a < c A. The effect ofthe unwanted signal isthus small even if cis large compared with a, ie the DC output of mixer 6 is sensibly proportional to a and independent of c.After passing through the low-pass filter 8 to eliminate o)# and #m, as well as beat and IF terms, the output of mixer 6 is multiplied in mixer 7 byAsin w1tfrom phase-shifter 5 to generate a carrier D sin wlt whose amplitude Disproportional to the original carrier amplitude a.

Using the arrangement of Fig 7, any difficulty in enabling the carrier extraction circuit of Fig 4to lock on to the weaker (unwanted) signal c sin Wgt can be obviated by including in the preceding RF amplifier (not shown) a channel filter tuned to be more receptive ofthe #2 frequency channel as well as a channel filtertuned to acceptthe cai frequency channel. This modified arrangement is shown by interrupted lines in Fig 7, where the channel filter 8 is assumed to be the usual IF filter of a superhet receiver tuned to accept the #1 signal, and there isfurther included a second lFfilter9tuned, at the same receiver local-oscillator setting, to accentuate the wz signal.Instead of the concurrent signals being applied directly to both carrier extraction circuits, separate outputs are now taken from the two IF filters to the respective carrier-extraction circuits, as shown.

Fig 7 also shows another arrangement, again in interrupted lines, for reducing the strong carrier component, a sin wilt, which would otherwise be fed to the carrier-extraction circuit handling the weaker carrier component, c sin oa2t. In this case the strong extracted carrier is subtracted from the input to the carrier-extraction circuit handling the weaker carrier component, as shown at 11 and 12.

It will be understood that either of these arrangements shown by interrupted lines in Fig 7 can be adopted alone or in combination. There is, however, an economic advantage in using only the simple carrier-extraction and subtraction arrangement, as shown at 11 and 12 iewithout using the Iffilters 8 and 9, which are expensive. Although this carrier-extraction and subtraction arrangement can only alleviate carrier and not sideband interference, in most cases this will suffice. However, if required, it can be extended to alleviate sideband interference also. This is achieved by using the final detected output of Fig 7 to amplitude-modulatethe input to phase-inverter 11; this latter option is not shown in Fig 7.

Any difficulty, with the unmodified arrangement of Fig 7 (ie without items 8,9,11 and 12), in enabling the carrier 30 extraction circuit of Fig 4 to lock onto a weakerwantedsignal, ie where a < < c, can be obviated by including an IF stop-filter 10 preceding the (wanted) wl carrier-extraction circuit as shown, and tuned, at the same receiver local-oscillator setting,to reduce the ( 2 signal as received in the c), channel.Alternatively or additionally, the arrangement shown in Fig 7 for reducing thestrong-carrier component (there a sinco1t) at 11 and 12 as applied to the lower ("Fig 6") arm of Fig 7, can be applied to the upper ("Fig 5") arm thereof in order to reduce the strong carrier component (here c sin u2t) fed tothe wl carrier-extraction circuit; this last option also is not shown in Fig 7.

Forms ofthe invention which use balancing inputs, eg as in Fig 8, have a disadvantage which they share with conventional synchronous detection methods, viz thattem porary failure ofthe carrier-extraction circuit(s), eg Fig 4, in the circuit(s) which supply the two inputs will cause total failure of detection. This effect does not occur in forms which do not use balancing inputs, eg those of Figs 5-7, and forth is reason the use of balancing inputs is nota preferred form ofthe invention.

Claims (14)

1. A method of reducing or eliminating the interference caused to a received, wanted, radiocommunication signal comprising an AM carrier by at least one concurrent, unwanted, such signal in an adjacent channel (as hereinbefore defined) comprising: extracting the carrier of at least one of said signals; generating from said extracted carrier a carrier of the same frequency but of different amplitude and/or opposite phase; combining the original concurrent signals with said generated carrier and detecting the combined signals in such awayastoenhancethewanted detected signal relative to the unwanted detected signal; and, optionally,filtering-outthe difference-frequency between said concurrent signals.

2. A method as claimed in claim 1 where the concurrent signals have different carrier strengths and the wanted carrier isstrongerthan the unwanted carrier, said method comprising: extracting the carrier of at least one of said signals; generating from said extracted carrier a carrier of the samefrequency but of amplitude larger or smaller than said extracted carrier depending on whether said one signal carrier is strongerorweakerthan the other said signal carrier respectively; adding to or subtracting from the original concurrent signal said generated carrier depending on whethersaid one signal carrier is stronger or weaker than the other said signal carrier respectively; detecting said added or subtracted signals; and, optionally, filtering-outthe difference-frequency signal.

3. A method as claimed in claim 2 comprising: extracting the carriers of both said signals; generating from said extracted ca rrierstwo carriers of the same two frequencies but of respective amplitudes largerorsmallerthan said extracted carriers depending on whether each signal carrier is stronger orweakerthan the other said signal carrier; respectively adding to and subtracting from the original concurrent signals said two generatedta rriers depending- on whether each generated carrier is derived from the stronger or weaker ofthe original two signal carriers respectively; detecting said added and subtracted signals; adding togetherthetwo signals resulting from said addition and-subtraction; and, optionally, filtering out the difference-frequency signals.

4. A method as claimed in claim 1 where the concurrent signals have different carrier strengths and the wanted carrier is strongerthan the unwanted carrier, said method comprising: extracting the carrier ofthe wanted signal; combining said extracted carrier with the concurrent signals in such a way as to produce a version of the concurrent signals in which the carrier of the wantedsignal has its phase reversed but its amplitude unchanged; detecting separately the original concurrent signals and said version of the concurrent signals; subtracting thetwo detected signals; and optionally, filtering-out the difference4requen- cy signal.

5. A method as claimed in claim 4wherein said version of the concurrentsignals is produced by doubling the amplitude and inverting the phase of the extracted carrier and adding the result to the original concurrent signals.

6. A method as claimed in claim 4 or claimS wherein, prior to separately detecting the original concurrentsignals and said version oftheconcurrent signals, the carrier amplitudes of the wanted and/or unwanted signals are respectively increased and/or decreased by extracting the relevant carrier or carriers, generating therefrom a carrier or carriers of larger or smaller amplitude depending upon whether the respective carrier or carriers are the stronger or the weaker, and adding or subtracting said generated carrier or carriers to or from the original concurrent signals depending upon whether their respective carrieror carriers are the stronger orthe weaker.

7. A method as claimed in claim 1 wherethe concurrent signals are of approximately equal carrier strengths, said method comprising: extracting the carrier of one ofthe two signals; generating from said extracted carrier a carrier of the same frequency but of larger amplitude; addingtotheoriginalconcurrentsignalssaid generated carrierto thereby producetwo concurrent signals in which one carrier is effectively stronger than the other; and thereafter performing a method as claimed in any of claims 2 to 6for concurrent signals whereofthe wanted signal carrier is stronger than the unwanted signal carrier.

8. A method as claimed in claim 1 where the wanted concurrent signal carrier is weaker than the unwanted concurrent signal carrier, said method comprising: extracting the weakersignal carrier; generating from said extracted carrieracarrierof the same frequency but of larger amplitude; adding to the original concurrent signals said generatedcarrier to thereby produce two concurrent signals wherein thecarrier which was originallythe weaker is now effectivelythestrnnger; and thereafter performing a method as claimed in any of claims 2to 6for concurrent signalswhereofthe wanted carrier is stronger than the unwanted carrier.

9. A method as claimed in any of claims 1 to 6 or 8 where the concurrent signals have different carrier strengths and both carriers are extracted, wherein the stronger extracted carrier is subtracted from the concurrentsignals priorto the extraction therefrom ofthe weaker carrier.

10. Amethod as claimed in claim 1 where there are two unwanted signals each having a weaker carrierthan the wanted signal and each in a separate adjacent channel, wherein the interference is reduced or substantially eliminated by a method as claimed in any of elaims 2 to 6 for one unwantedsignal carrier weakerthanthewanted signal carrier.

11. A method as claimed in any preceding claim wherein the step of extracting the carrier of a signal is performed by a method comprising: applying the concurrentsignalsto a phase-locked loop (PLL) to generate a voltage-controlled oscillator (VCO) output of the desired carrier frequency and in approximate phase quadrature with the desired carrier; applying the DC error signal generated in the PLLto derive from the VCO output a modified VCO output substantially in phase with the desired carrier; mixing the modified VCO outputwith the concurrent signals and detecting the product; further mixing the DC detected product, after low-passfiltering, with said modified VCO output and detecting the product of said further mixing to producethe extracted carrier.

12. An electronic circuit for use in a method as claimed in any preceding claim

13. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

14. An electronic circuit as claimed in claim 12 and substantially as hereinbefore described with referenceto the accompanying drawings.