A superhet receiver in which an input signal is frequency down-converted in a first mixer (14) to produce a first IF signal comprising a wanted channel signal and an image channel signal. The first IF signal, without image rejection filtering, is frequency down-converted in a second mixer (24) to produce a second, lower IF signal comprising at least the wanted channel signal having a bandwidth corresponding to a predetermined passband of a channel filter (32) coupled to the output of the second mixer. The local oscillator signals used in the first and second frequency down conversion processes are varied in order to produce successively different image channel signals in the first IF signal and to maintain the wanted channel signal in the second IF signal within the passband of the channel filter (32). By rapidly hopping the local oscillator signals in the frequency domain the wanted signal can be made to remain stationary within the bandwidth of the channel filter whilst other unwanted frequencies are spread across a wider bandwidth and have their amplitudes averaged in the filter (32).

The present invention relates to improvements in or relating to superheterodyne receivers and to a method of image rejection filtering.

In traditional superheterodyne (generally referred to as superhet) receivers it is generally impractical to mix an incoming narrow band, RF signal directly down to a low IF. This is usually because of the impossibly high Q requirements that would be imposed on the front-end image rejection filters. In order to help circumvent this problem a second stage of mixing is often introduced creating a second, lower IF. The main problem with this type of receiver is that it requires the use of two separate image rejection filters, one before each mixer. Also, since the selectivity required from each filter is usually quite high, in practice they tend to be realised as discrete off-chip designs. This makes a double conversion receiver inherently difficult to integrate fully and therefore relatively expensive to produce.

It is an object of the present invention to provide an effective integratable double conversion receiver.

According to one aspect of the present invention there is provided a superhet receiver comprising a first mixer having a first input for an input signal, a second input for a first local oscillator signal and an output for a first frequency converted signal, a first source of local oscillator signals coupled to the second input of the first mixer, a second mixer having a first input coupled to the output of the first mixer, a second input for a second local oscillator signal and an output for a second frequency converted signal, a second source of local oscillator signals coupled to the second input of the second mixer, a bandpass filter having a predetermined passband coupled to the output of the second mixer, means for varying the first source of local oscillator signals to produce a first frequency converted signal including a wanted channel signal and successively different image channel signals, and means for varying the second source of local oscillator signals such that the spectrum of the wanted channel signal in the second frequency converted signal lies within the passband of the bandpass filter.

According to a second aspect of the present invention there is provided a method of image rejection filtering, comprising subjecting an input signal to a frequency down conversion process to produce a first IF signal comprising a wanted channel signal and an image channel signal, subjecting the first IF signal to a second frequency down conversion process to produce a second IF signal comprising at least the wanted channel signal having a bandwidth corresponding to a predetermined passband of channel filtering means, wherein local oscillator signals used in the first and second frequency down conversion processes are varied in order to produce successively different image channel signals in the first IF signal and to maintain the wanted channel signal in the second IF signal within the passband of the channel filtering means.

By varying the first local oscillator frequency to get successively different image channel signals or interferers with the wanted signal and adjusting the second local oscillator frequency to keep the wanted signal plus the current interferer within the passband of the low frequency bandpass filter not only can the filter be integrated with other components of the receiver but also the amplitudes of unwanted image signals are smeared across a wide bandwidth and are averaged by the channel filter. The continuous changing of unwanted image signals means that each unwanted signal is present only briefly. Hence with a large amplitude image signal its contribution to the average is diminished. Additionally processing gain can be obtained in the event of some of the image channels being vacant or containing only low amplitude signals. Another benefit is that it is no longer necessary to have image rejection filters prior to the first and second mixers.

Conveniently the first and second local oscillators are varied in accordance with respective random frequency hopping sequences. The variation of the frequency hopping sequences may be related and be moved randomly in equal but opposite directions so that wanted signal in the second IF signal remains within the bandwidth of the bandpass filter whilst the current image or interfering signal changes.

In the drawings the same reference numerals have been used to represent corresponding features.

The double conversion superhet receiver shown inFIG. 1comprises an antenna10coupled to a first input12of a first mixer14. A source16of first local oscillator signals LO1, such as a frequency synthesizer, is coupled to a second input18of the mixer14. An output20of the first mixer14comprising a first IF signal is coupled to a first input22of a second mixer24. A source26of second local oscillator signals LO2, such as a frequency synthesizer, is coupled to a second input28of the mixer24. An output30of the second mixer24comprising a second IF signal is coupled to a channel filter comprising a bandpass filter32, the bandwidth FBWof which is such as to pass the signals present in the wanted signal channel. A demodulator34is coupled to an output of the filter32.

The first and second local oscillator signals LO1and LO2are varied, for example frequency hopped, as will be described with reference toFIGS. 2to7. A hopping sequence function generator36has outputs38,40coupled respectively to the sources16,26for controlling the selection of frequencies comprising the first and second local oscillator signals LO1and LO2.

The principle of operation of the illustrated double conversion superhet receiver is to use spread spectrum techniques, employed inside the receiver, to help resolve the wanted signal from its image. This is effected by frequency hopping the local oscillators in a suitable manner so that the wanted signals are preserved as narrowband signals whilst unwanted images which in practice are of variable amplitudes are spectrally spread in the amplitude averaging carried out in the bandpass filtering thereby reducing their effect on the wanted signal. The key steps to explain the concept will be described below with reference toFIGS. 2to4andFIGS. 5to7.

Referring initially toFIGS. 2to4,FIG. 2shows a plurality of contiguous frequency channels L1to L11, R1to R8, W, R10and R11, where channel W, shown cross-hatched, represents the wanted channel. Although for convenience the amplitudes of the channel signals have been shown to be equal, in reality the amplitudes will vary and also there will be occasions when one or more of the channels will have no signal. Thus signals received at the antenna10could be present in some or all of these frequency channels and be present on the first input12of the first mixer. A local oscillator signal LO1having a frequency between the channels R3, R4is applied to the second input18of the first mixer. The channel frequencies are frequency down-converted and the first IF signal on the output20of the first mixer14is as shown in FIG.3.

The frequency spectrum has been folded about the first local oscillator frequency LO1and as a result an unwanted interferer having an image frequency L3is also mixed down to a frequency coincident with that of the wanted channel W. As indicated above the amplitudes of the wanted signal W and the unwanted interferer are likely to be different. The first IF signal, without image rejection RF filtering, is present on the first input22of the second mixer. A second local oscillator signal LO2having a frequency lower than that of the signal LO1, is applied to the second input28of the second mixer24. The choice of frequency for the signal LO2is determined such that the wanted channel and the unwanted interferer, in this instance the image frequency L3, lie within the bandwidth FBWof the filter shown in dotted lines in FIG.4. The second IF signal is as shown in FIG.4. The spectrum has been folded about the frequency of the second local oscillator signal LO2. The wanted channel signal W and the interfering image signal L3are still coincident in frequency and therefore unresolvable. However the bandpass filter32(FIG. 1) is a low frequency filter which effectively averages the wanted and unwanted signals. As the filter32is a low frequency filter it can be fully integrated with other parts of the receiver.

FIGS. 5to7effectively correspond toFIGS. 2to4with the difference that the first and second local oscillator signals LO1and LO2have been changed as a result of frequency hopping. InFIG. 5the signal LO1has been decreased in frequency by the equivalent of three channel spacings and now lies between channels L1and R1. The first IF signal shown inFIG. 6now shows the wanted channel signal W having a higher frequency than in FIG.3and being coincident with an interfering channel L9which corresponds to a new image frequency.

In order to keep the wanted signal W within the passband response FBWof the channel filter32after the second stage of mixing, the frequency of the local oscillator signal LO2has to be increased in frequency by also three channel spacings. The results of this operation can be seen inFIG. 7where the wanted channel W and the interferer L9have been isolated by FBW. By comparingFIGS. 4 and 7it can be seen that although the wanted signal W effectively remains stationary within the passband FBWof the channel filter32, the coincident interferer changes from being L3in the first case to L9in the second case. The frequencies of the first and second local oscillator signals LO1and LO2can be varied, for example hopped, according to any suitable sequence. Such a sequence, for example, may be generated by a frequency hopping sequence generator or may be a pre-stored pseudo-random sequence. For example, looking atFIG. 2or5, if the signal LO1is changed by the equivalent of one channel, the next but one unwanted channel coincides with the wanted signal W. The averaging process in the bandpass filter32will tend to reduce the mean average of the interferers whilst leaving the wanted signal W substantially unaffected. By frequently varying the frequencies of the first and second local oscillator signals, each unwanted signal is present only briefly. Hence the contribution of a large interferer to the average is diminished.

If the frequencies of the signals LO1and LO2are moved randomly in equal but opposite directions then, the wanted signal will remain stationary while other unwanted frequencies will be smeared across a wider bandwidth.

The rate at which the frequencies of the first and second local oscillator signals are varied may be chosen to give a desired averaging period. For example the rate may be rapid as used typically for frequency hopped spread spectrum systems (FHSS).

If all the adjacent channels have large signals in them (equivalent to having lots of simultaneous users in a normal FHSS system) then the processing gain diminishes to zero. However, if some channels are vacant or only have low amplitude signals in them then the processing gain will increase.

Optionally the rate at which the frequencies of the first and second local oscillator signals are varied may itself be varied, to reduce the time spent on frequencies where large interfering image signals are present.

In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed.