This invention relates to distortion reducing arrangements in mixer circuits such as for use in communication systems (e.g. radio, optical fibre or laser systems).

It is already known to reduce distortion in amplifiers by the use of so-called "feed forward" techniques whereby an input signal applied to an amplifier is derived from one output of a power splitter device and the output signal from the amplifier is fed through one branch of a directional coupler device and a first delay circuit to a first combiner device. The output signal from the other branch of the directional coupler device which has the same coupling factor as the amplifier has gain is combined in a second combiner device with a further output derived from the power splitter device and fed to the second combiner device through a second delay circuit providing a signal delay equal to the combined propagation delays of the amplifier and the directional coupler. Since the coupling factor of the directional coupler device corresponds to the gain of the amplifier the input signals to the second combiner device will be mutually cancelling by the combiner device leaving only the distortion products of the amplifier. These are then amplified by a second amplifier and applied to the first combiner device in such phase and amplitude that the distortion produced by the first amplifier is cancelled.

The present invention is specifically concerned with mixer circuits and is based upon the realisation thai the above-described "feed forward ^1' techniques applied to amplifiers when the input and

output frequencies of the circuit are the same can _t advantageously be adapted for reducing the distortion in mixer circuits where the input and output frequencies are different thereby allowing linearity improvements to be achieved with mixer circuits.

According to one aspect of the present invention there is provided a distortion-reduced mixer circuit arrangement comprising a plurality of mixer means, in which an output including a component of the input to a first mixer means and distortion products in the output of the first mixer means are derived from a second mixer means coupled to the first mixer means, in which the output signal from the second mixer means is combined with a component of the input of the first mixer means whereby the components of the input signal to the first mixer means are mutually cancelled to provide the distortion products of the first mixer means translated at the frequency of the input signal to the first mixer means, in which the distortion products signal is applied to a third mixer means which translates the distortion products signal to the intermediate frequency of the first mixer means and in which the distortion products signal at intermediate frequency is combined with the output from the first mixer means in order to cancel the distortion products of the first mixer means.

According to another aspect of the present invention there is provided a mixer circuit arrangement comprising first, second and third mixer means which are arranged to be supplied from the same or like local oscillator circuit(s), in which the input to the first mixer means is derived from one output of a power splitter device and the or each output from the mixer means is fed on the one hand to a

combiner device through a delay circuit and on the other hand through signal attenuation means having predetermined attenuation in dependence upon the gain of the mixer means to the second mixer means which is identical to the first mixer means whereby the or each output from the second mixer means comprises a component of the input signal to the first mixer means and substantially only the distortion products of the first mixer means output, in which the or each output from the second mixer means is combined by a second combiner device with another output of the power splitter fed to the second combiner device through a second delay circuit whereby components of the input signal to the first mixer means are mutually cancelled and the output from the second combiner device comprises only the distortion products of the first mixer means translated at the frequency of the input signal to the latter and in which the output from the second combiner device is applied to the third mixer means which translates the distortion products signal to the intermediate frequency of the first mixer means after which the translated distortion products are fed through attenuation means dependent upon the gain of the third mixer means to the first combiner device in which the distortion products of the first mixer means output are cancelled.

In carrying out the present invention the adverse effects of image frequency signals occurring at the output of the second mixer means of the mixer circuit arrangement may be positively avoided by the use of so-called image rejection mixer means or by the use of suitably positioned low-pass or band-pass filters.

In applying the invention to a direct conversion receiver, quadrature hybrid networks may be introduced in respect of each of the mixer means which may each comprise a pair of identical mixers having respective local oscillator quadrature signals applied thereto. However, the hybrid networks may be dispensed with when both in- phase and quadrature channels are required for processing purposes.

By way of example the present invention will now be described with reference to the accompanying drawings in which:-

Figure 1 is a circuit diagram of a distortion reducing arrangement in a mixer circuit,

Figure 2 is a circuit diagram of a distortion reducing arrangement in a mixer circuit as shown in Figure 1 , but provided with image frequency signal rejection means; and,

Figure 3 is a distortion reducing arrangement in a mixer circuit arrangement having common in-phase/quadraturε channels .

Referring to Figure 1 of the drawings, the mixer circuit arrangement illustrated which may, for example, be used in a radio receiver comprises a first mixer 1 w ^r hich receives part of a radio frequency input signal I/P through a power splitter 2. The mixer 1 is also fed with a fixed frequency drive signal from a local oscillator 3 so that the output signal from the mixer 1 comprises an intermediate frequency signal, that is to say a signal having a frequency equal to the difference between the local oscillator frequency and frequency of the input signal I/P. plus signal distortion products actually produced by the mixer 1. One part of this intermediate frequency output signal is fed through a delay circuit 4 to a combiner device 5 whilst the other part of the signal is

applied to a second mixer 6 identical to the first mixer 1 after attenuation by a variable attenuator 7. The mixer 6 is fed with the same local oscillator signal as the mixer 1 and produces an output signal at the original signal frequency of the input signal I/P plus the distortion products of the mixer 1 and the distortion products of the mixer 6. However, since the mixer 6 is operated at a low input level compared to the mixer 1, due to the interposition of the attenuator 7, the distortion products of the mixer 6 will be very much lower than those generated by the mixer 1 and for practical purposes may be regarded as negligible. The output from the mixer 6 is then combined in a combiner device 8 with the other part of the input signal I/P from the power splitter 2, the latter signal being delayed by a delay circuit 9 to take into account the propagation delays of the mixers 1 and 6. In the combiner device 8 the components of the input signal at the frequency of the original input signal I/P are mutually cancelled so that the output from the device 8 only comprises the distortion products of the mixer 1 translated to the frequency signal I/P. These distortion products are then fed to a third mixer 10 which is also supplied with the same local oscillator signal as the mixers 1 and 6. These distortion products are heterodyned in the mixer 3 to the intermediate frequency and after appropriate attenuation by the attenuater 11 are combined in the combiner device 5 with the intermediate frequency output signal from the mixer 1 whereby the distortion products at the intermediate frequency generated by the mixer 1 are cancelled by reason of the respective phases and amplitude of the distortion products of the combined signals being mutually cancelling by

appropriate adjustment of the respective gains of the mixers by the attenuators 7 and 11.

In operation of the mixer circuit arrangement according to Figure 1 an image frequency signal may occur at the output of the mixer 6 and this may give rise to problems unless this image signal is rejected. For this reason image frequency rejection filters may be provided as depicted in Figure 2.

Referring briefly to Figure 2 of the drawings, in which the circuit components corresponding to those in the Figure 1 arrangement bear the same reference numerals, it will be seen that low-pass filters 12 and 13 are provided respectively, at the input to the mixer arrangement as a whole and also in the output of the mixer 6, whilst band-pass filters 14 and 15 are provided, respectively, in the outputs of mixers 1 and 10. This arrangement is suitable for use in a high frequency communications receiver using conversion in the mixer circuit to a high intermediate frequency. The low-pass filters 12 and 13 which are identical reject signals at the image frequency and the intermediate frequency whereas the band-pass filters reject the unwanted frequency signals but allow passage of the required intermediate frequency signals.

Figure 3 shows the invention as applied to a direct conversion communications receiver.

In this mixer circuit arrangement a split input signal derived from a power splitter 16 instead of being applied to a single mixer 1 (Figure 1) is fed through a quadrature hybrid network 17 to a pair of mixers 18 and 19 supplied, respectively, with quadrature signals X and Y from a local oscillator 20. The local oscillator drive to the pair

of mixer 18 and 19 provides for the usual single side-band type of output from the mixers. The intermediate frequency quadrature outputs from the mixers 18 and 19 which will include the distortion products it is desired to remove are fed through respective delay circuits 21 and 22 corresponding to the delay circuit 4 in Figure 1 and to combining devices 23 and 24 corresponding to the single combiner device 5 in Figure 1.

The intermediate frequency outputs from the mixers 18 and 19 are mixed back to the original frequency of the input signal I/P by mixers 25 and 26 also fed with quadrature outputs X and Y from the local oscillator 20. The resulting output signals from the mixers 25 and 26 which are fed through a quadrature hybrid network 27 to a combiner device 28 are at the original signal frequency of the input signal I/P and include the distortion products introduced at the intermediate frequency by the mixers 18 and 19 translated to the input signal frequency. In the combiner device 28 the original signal components one of which is delayed by delay circuit 33 are mutually cancelled to leave only the distortion products of mixers 18 and 19. These distortion products are translated back to the intermediate frequency by quadrature mixers 29 and 30 fed through the hybrid quadrature network 31 , the mixers 29 and 30 being driven by the respective quadrature outputs X and Y from the local oscillator 20. The outputs from mixers 29 and 30 at intermediate frequency are then summed in combiner devices 23 and 24 with the intermediate frequency outputs from the mixers 18 and 19 in order to cancel distortion. The distortion-free outputs from the combiner devices 23

and 24 are then summed again in a combiner device 32 to provide single channel "in-phase" and "quadrature" output.

It may here be mentioned that if separate channels (i.e. separate in-phase and quadrature channels) are used for subsequent processing of the intermediate frequency signals instead of a single channel as depicted in the Figure 3 arrangement, the quadrature hybrid networks 17, 27 and 31 may be omitted and replaced by combiner/splitter devices of conventional type. The combiner device 32 would also omitted.

In the Figure 3 arrangement attenuators for balancing the gain of the respective mixers have been omitted for the sake of clarity but they will be required in the inputs to the mixers 25 and 26 and outputs from the mixers 29 and 30.

The delay circuits in the various embodiments described to take propagation delays into account may take any convenient form but the delay circuits in the intermediate frequency paths may conveniently comprise charge coupled devices (i.e. CCD devices).