WO1999038253A1

WO1999038253A1 - Mixer circuit

Info

Publication number: WO1999038253A1
Application number: PCT/GB1998/003827
Authority: WO
Inventors: Philip Richard Bellamy
Original assignee: The Secretary Of State For Defence
Priority date: 1998-01-21
Filing date: 1998-12-18
Publication date: 1999-07-29
Also published as: GB9801123D0

Abstract

A mixer circuit directs input signals to an amplifier (16) and a delay unit (18) from which amplified and delayed signals pass to a signal mixer (20) and a correction mixer (24) respectively for mixing with a common local oscillator signal. The signal mixer output signal is connected to an attenuator (30). A subtractor (26) subtracts the correction mixer output signal from the attenuator output signal, which provides a difference signal consisting largely of distortion introduced by amplification and mixing. The difference signal is used to counteract distortion in the signal mixer output signal.

Description

1

MIXER CIRCUIT

This invention relates to a mixer circuit.

The problem of capturing a signal, or band of signals, from a radio spectrum was solved in the early part of this century by the superheterodyne technique. Instead of using a tunable filter, this technique uses a band-pass filter with a fixed centre frequency to which the signal frequencies to be received are shifted or mixed. Good receiver performance in terms of selectivity, image rejection etc may require several mixing stages.

The density of radio signals in the electromagnetic spectrum is increasing, and the dynamic range of receivers is becoming a more critical parameter. This is especially so in the case of wide-band receivers used in electronic systems where spurious responses generated by large amplitude in-band signals cannot be distinguished from real low-level signals, and therefore must be treated as being the latter, which increases the signal analysis burden.

The dynamic range of a receiver is limited by the additive noise within the receiver defining the smallest detectable signal, and by the non-linear characteristics of individual electronic components which generate harmonics and intermodulation products between multiple signals.

Good design practices can optimise the achievable receiver dynamic range, but only as far as the performance of individual components allows. Mixers are a significant cause of distortion which adversely affects dynamic range, since the mixing action often depends on the non-linear behaviour of components such as diodes.

In the prior art, attempts have been made to address the problem of mixer distortion by using field-effect transistors (FETs) as switches operating without electrical bias like voltage controlled resistors. Examples of this are described in "A comparison of GaAs transistors as passive mode mixers", Schindler, Chu, Binder, IEEE MTT-S Digest, 1994; "A GaAs MESFET balanced mixer with very low intermodulation", Maas, IEEE MTT-S 2

Digest, 1987; and "A GaAs MESFET mixer with very low intermodulation", Maas, IEEE MTT, Vol. MTT-35, No. 4, Use of FET switches in this way produces a useful improvement in performance over mixer diodes, but they remain the dominant source of distortion in receiver design at the present time.

US Patent No. 4,383,334 describes an intermodulation distortion cancellation circuit in which distortion introduced by a high power mixer is reduced by subtraction of a signal produced using a low power mixer. A small proportion of the input signal is amplified to provide the signal to be fed to the low power mixer, whereas the high power mixer receives virtually all the input signal. Because the mixers are dissimilar, the spectra of their distortions do not match one another and cannot provide good cancellation performance over a broad bandwidth. Moreover, noise introduced by amplification is not cancelled in subtraction.

US Patent No. 4,408,352 discloses a further intermodulation distortion cancellation circuit using an input coupler to produce two versions of an input signal, ie attenuated and substantially unattenuated versions. These versions are mixed with a local oscillator (LO) signal by low and high level mixers respectively to provide two intermediate frequency (IF) signals; an IF coupler produces a reduced amplitude equivalent of the high level IF signal, which is used to cancel the non-distortion component (ie the required input signal contribution) in the low level IF signal leaving largely distortion only. The resulting distortion signal is used to counteract distortion in the high level IF signal. However, in this arrangement, the input coupler attenuates the input signal without attenuating the associated noise, which is passed on to the low level mixer and thence to IF circuit components. This contributes directly to the noise figure of the IF circuitry and therefore also to the overall noise figure of the circuit, which negates its distortion cancellation benefits (noise figure is defined as the degradation of output signal to noise ratio compared to that for the input signal).

It is an object of the present invention to provide an alternative form of mixer circuit.

The present invention provides a mixer circuit including input means connected to amplifying means for amplifying an input signal to provide an amplified signal, mixing 3 means connected to the input means and to the amplifying means for mixing the amplified signal and the input signal with an LO signal to produce IF signals, combining means for combining the IF signals to produce a distortion signal substantially lacking input signal components, and correcting means for counteracting distortion in one of the IF signals by means of the distortion signal, characterised in that the mixing means comprises main and subsidiary mixers (20, 24) arranged to provide main and subsidiary IF signals respectively, the amplifying means (16) is connected to the main mixer (20), the combining means (22, 26, 30) is arranged to combine a reduced amplitude version of the main IF signal with the subsidiary IF signal and the correcting means (32, 34, 36) is arranged to provide for the distortion signal to have appropriate amplitude to counteract distortion in the main IF signal.

The invention provides the advantage that it is capable of providing improved noise performance compared to the prior art; this is because amplification provides for the main IF signal to be at a higher level than the subsidiary IF signal, and so it is not necessary to produce an attenuated version of the input signal for use in generating the subsidiary IF signal. Degradation of the circuit's overall noise figure due to such attenuation is avoided. Unwanted signal components introduced by amplification can be tolerated because they appear in the distortion signal and are therefore counteracted in the main IF signal by the correcting means. Moreover, it is not necessary to use different types of device for the main and subsidiary mixers, because the circuit does not rely on these mixers having different mixing characteristics. This makes it possible to implement the invention in miniaturised form using millimetre wave integrated circuit technology (MMIC) if required.

In a preferred embodiment, the correcting means includes a second amplifying means for amplifying the distortion signal to achieve substantial equality between the magnitudes of the mixer distortion components in the distortion signal and the main IF signal to enable distortion in the latter to be counteracted.

The mixer circuit of the invention may include means for delaying the input signal prior to mixing with the LO signal for appropriate relative phasing of the subsidiary and main 4

IF signals at the combining means and the correcting means may include means for delaying the amplified main IF signal for phasing as appropriate to enable distortion therein to be counteracted. The combining means may include attenuating means arranged to attenuate the main IF signal in order to achieve substantial equality between the magnitudes of the input signal components in the main and subsidiary IF signals to facilitate removal of such component from the distortion signal. It may be arranged to produce a distortion signal containing contributions from the amplifying means and the main mixer to be counteracted in the main IF signal by the correcting means.

In one embodiment, the mixer circuit of the invention includes input coupling means for connecting the input means to the subsidiary mixing means and for providing a substantially unattenuated version of an input signal thereto. The input coupling means is preferably arranged to couple a reduced amplitude version of an input signal to the amplifying means. The invention may include combining means incorporating a main IF coupling means to provide a reduced amplitude version of the main IF signal and a subsidiary coupling means to combine the said signal version with the subsidiary IF signal.

In another aspect, the invention provides a method of mixing signals including amplifying an input signal to provide an amplified signal, mixing the amplified signal and the input signal with an LO signal to produce IF signals, combining the IF signals to produce a distortion signal substantially lacking their input signal components, and counteracting distortion in one of the IF signals by means of the distortion signal, characterised in that the IF signals are main and subsidiary IF signals respectively, the main IF signal is derived from the amplified signal, a reduced amplitude version of the main IF signal is combined with the subsidiary IF signal and the distortion signal has appropriate amplitude to counteract distortion in the main IF signal.

The subsidiary IF signal may be derived by mixing a substantially unattenuated version of the input signal with the LO signal, and the amplified signal may derived from a reduced amplitude version of the input signal. 5

Amplitude reduction of the main IF signal and its combination with the subsidiary IF signal may be achieved by means of signal coupling.

The distortion signal may contain contributions arising from amplifying the input signal and from mixing the amplified signal, distortion in the main IF signal corresponding to both such contributions being counteracted. The distortion signal may be amplified to have appropriate amplitude to counteract distortion in the main IF signal.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic block diagram of a mixer circuit of the invention;

Figure 2 is a schematic block diagram of low frequency, computer simulated mixer circuits of the invention and of the prior art;

Figure 3 graphically illustrates the respective outputs of the circuits of Figure 2 as a function of frequency; and

Figure 4 shows a modified version of the circuit of Figure 1 with signal couplers for signal division, attenuation and subtraction.

Referring to Figure 1, a mixer circuit of the invention is indicated generally by 10. The circuit 10 has an input terminal 12 connected to an input splitter coupler 14, which is in turn connected to an input amplifier 16 and an input delay unit 18 giving a time delay tl. The amplifier 16 provides output to a signal mixer 20 connected to a signal splitter 22. The delay unit 18 is connected to a correction mixer 24, which in turn is connected to a non-inverting input 26a of a subtractor 26. A local oscillator (LO) 28 provides 2.006 GHz LO input signals of like phase and amplitude to the mixers 20 and 24 via an LO splitter 29. The signal splitter 22 is connected via an attenuator 30 to an inverting input 26b of the subtractor 26, and to an adder 32 via an output delay unit 34 giving a time delay t2.

The adder 32 also receives input from the subtractor 26 via a correction amplifier 36, and 6 generates an output at 38. The input amplifier 16 is a GaAs millimetre wave integrated circuit (MMIC) device for use at frequencies in the region of 2 GHz. The correction amplifier 36 is a wideband operational amplifier suitable for use up to 11 MHz. As will be described later in more detail, the attenuator 30 provides an attenuation to equalise the amplitudes of signals reaching the subtractor 26 by different paths. The signal and correction mixers 20 and 24 are as nearly identical as manufacturing tolerances will allow to enable them to have matched distortion characteristics. They are diode ring mixers employing Schottky barrier diodes and are suitable for use at 2 GHz. The delay units 18 and 34 may be implemented as lengths of cable.

The circuit 10 operates as follows. A signal from a radio receiver intermediate frequency circuit (not shown) with 2 GHz centre frequency and 10 MHz bandwidth is applied to the circuit input 12. It is divided by the input coupler 14 into two signals of unequal amplitude which are in phase with one another. One of these coupled signals is amplified by the input amplifier 16 and fed to the input mixer 20. The other coupled signal is delayed by tl in the input delay unit 18 and fed to the input mixer 24. In order that the noise figure of the implementation is not degraded unacceptably compared to that for a single mixer alone, the coupling loss between the circuit input 12 and the delay 18 should be as small as possible. The delay tl is selected so that signals reaching the subtractor inputs 26a and 26b are correlated over the operating bandwidth of the circuit.

The signal and correction mixers 20 and 24 receive the same LO signal of 2.006 GHz from the local oscillator 28 and generate sum and difference signals centred at 4.006 GHz and 6 MHz.

The generation of sum and difference signals in most mixers relies on the non-linear characteristic of diodes. In general, this non-linear characteristic can be described by the following equation for the mixer output signal V(v)>

V(v) = k₀ +k_lv+k₂v² +k₃v³ +... = ∑k_nvⁿ (1)

0

Where v represents input signal and k„ (n = 0, 1, 2 ∞) are constants. 7 If v = V_j + v₂ representing two simultaneous input signals,

v_l = Isinω_lt (2)

and v₂ = /sino₂t , (3)

where I is input signal amplitude and α>ι and ω₂ are angular frequencies,

then the third order term ΛjV³ in Equation (1) expressed as H „₌₃ gives rise to the following components:-

V »=3^" = kλl³ sin³ o^t + 1³ sin³ ω₂t + 3I³ sin² ω t sin<2)₂/ + 3/³ sinωJ sin² ω₂t \ (4)

The third term (containing sin²ωjt) in Equation (4) can be expanded to yield:-

3k₃I³ sin² ω t sin ω₂t - 3k₃I³ sin ω₂t(l - cos² ω_xt\

= 3k₃I sinty-J — (sinω₂/cos2ty,/ + sinω₂t)

3k I³ l

= — - — {sin ω₂t — [sin(2fl), + ω₂ )t - sin(2βj_! - ω₂ )/] } (5)

Similarly the fourth term (containing sin²β>₂t) in Equation (4) expands to

3k₃

{sin ω, t — [sin(2fi;₂ -τ-ω^t - sin(2fi)₂ - ω )t\ (6)

The terms at 2ω_λ +ω₂ zxiά 2ω₂ +ω. axe at frequencies far removed from those of interest and are therefore unimportant. However the terms at 2ω, - ω₂ and 2ω₂ - ω are at frequencies very close to those of wanted signals and cannot easily be filtered out. These terms are third order intermodulation products, and they are usually the limiting factor in the overall dynamic range of a receiver. Equations (5) and (6) show that the amplitudes of these third order product terms are proportional to I³. 8

In a mixer, the second order term in Equation (1) is exploited to shift the wanted RF signal to a different (intermediate) frequency IF.

If the mixer input consists of an RF signal VRF and a local oscillator signal V _O, ie if

v_RF = I_RF sinω_RFt , and

v_LO = I_Lσ smω_LOt

where IRF and ILO are input signal amplitudes and CORF and O> O are angular frequencies,

then the second order term k₂\? in Equation (1) expressed as Ft „₌₂ gives rise to the following components :-

V\_n-ι = k₂ ]l² sin² ω^t + I , sin ² α;_i0t + 2/_ΛF/_LO[cos(ω_LO -ω^t - cosiω^ +ω_RF)t^^'

(7)

The desired frequency downshifted component is at frequency ω_w - ω^ . If ω^ has two or more components then intermodulation products are generated by the third (and higher) order terms in Equation (1) and appear in the frequency shifted mixer output.

If the input signal to the correction mixer 24 has amplitude Ii then it will produce an output Ioc at the shifted frequency which can be described by:-

/_α. * tf_I +< _J (8)

where k is a constant which accounts for the conversion loss of the mixer and d₃ is the amplitude of the third order distortion components.

The input to the input amplifier 16 is also I] because it receives the same signal as the input delay unit 18. Since the amplitude of the third order distortion components are 9 proportional to the third power of the signal amplitude, as shown by equations (5) and (6), the amplitude of the output signal Ios from the signal mixer 20 can be described by:-

l_os = \0kl_x +kδ + \0³d₃ (9)

where the voltage gain of the amplifier 16 is 10 and£ is the distortion added by the input amplifier 16.

The output from the signal mixer 20 is divided by the signal splitter 22 into two equal components each with l/V2 of the signal mixer output amplitude (i.e. 3 dB amplitude reduction). The attenuator 30 receives one of these components and attenuates it sufficiently to provide for its amplitude (ignoring distortion) at the subtractor 26 to be equal to that of the output signal Ioc from the correction mixer; i.e. changes in signal amplitude due to amplification at 16 and splitting at 22 are removed.

The subtractor 26 therefore receives respective input signals hs and he from the attenuator 30 and correction mixer 26 which are as follows;

/ - Io _{= kIl} +— + l0²d₃ ⁵⁵ 10 ' 10 ³

*sc ~ *oc ⁼ kl₁ +a₃

The subtractor output Iso is given by:-

I_so = Isc -I_Ss = d₃ -^-l0²d₃ (10)

Thus the original signal component 7/ does not appear in the subtractor output, which contains predominantly third order distortion components from the signal mixer 20. The subtractor output is amplified at 36 to become equal in amplitude to the distortion in the signal received at the adder 32 from the output delay unit 34. This unit is adjusted so that the third order distortion terms in signals from itself and from the correction amplifier 36 are correlated with one another but opposite in sign when received by the adder 32, and in consequence the distortion in the signal from the former is largely cancelled by the 10 signal from the latter. The signal I_OD at the output of the correction amplifier 36 is

I_OD = \0I_so = \0d₃ -kδ -\0³d₃ (11)

where, for the purpose of this analysis, the voltage gain of the amplifier 36 is 10,

and the output of the adder 32 is therefore:-

I_OA = I_os + I_OD = 10Λ . + kδ + \0³d₃ + 10d₃ -kδ - 10³d₃ = I0kl_x +I0d₃ (12)

Equation (12) shows that the output of the mixer circuit 10 of the invention consists of the input RF signal, magnified by a factor of 10k, and a distortion term 10d₃. The distortion δ produced by the amplifier 16 is also cancelled. Note that from equation (9) the level of third order distortion was 10³d₃ and so the feed-forward cancellation has achieved a suppression of a factor of one hundred or 40 dB.

In general it can be shown that third order distortion terms are suppressed by a factor of G² where G is the gain of the input amplifier 16. Higher odd-order distortion terms which also produce intermodulation products close to the wanted signals are suppressed to greater degrees.

Additional noise contributed by the amplifier 16 has not been discussed, but it can be treated in the same manner as distortion components in the above analysis. This amplifier noise is not present in the signal from the correction mixer 24 and is not cancelled by subtraction of that signal; it therefore appears in the output signal from the subtractor 26. This output signal consequently counteracts amplifier noise from the main IF signal at 32 together with unwanted intermodulation products. However, there is no cancellation of noise components generated in the subsidiary signal path via the delay unit 18, correction mixer 24, subtractor 26 and amplifier 36, because they are not derived from signals in the main path via the amplifier 16 and signal mixer 20; these components appear undiminished at the output 38. This is the reason for the noise figure in the subsidiary path being made as low as possible. The noise figure of the circuit 10 is dominated by that of the correction mixer 24 (typically 7 dB) but additional attenuation between the input 12 and mixer 24 adds to this value and should be avoided as far as possible. 11

In order to test the circuit of the invention, its operation was investigated and compared with a prior art circuit by means of a computer simulation. Referring now to Figure 2, there are shown a circuit 100 of the invention and a prior art mixer circuit 200 as simulated by a computer in both cases. In this drawing, parts equivalent to those described earlier are like-referenced with prefixes 100 and 200 respectively.

The circuit 100 is intended to be equivalent to the circuit 10 subject to simulation constraints and simplifications, and the description will concentrate on aspects of difference. For convenience, simulation is carried out at low frequencies near 1 MHz, circuit delays are ignored and there are no equivalents of the delay units 18 and 34.

The circuit 100 comprises an amplifier 116 and signal mixer 120 simulated by a combination of an ideal or perfect multiplier 120a and a soft limiter 120b. Similarly, a correction mixer 124 is simulated by an ideal or perfect multiplier 124a and a soft limiter 124b. The limiters 120b and 124b saturate at an input voltage of ±2 V and have a gain of 1, as indicated by numerals 2, -2 and 1 therein. The circuit 100 includes signal splitters 114, 122 and 129 indicated by black dots, together with an attenuator 130 shown as an amplifier with inverse gain {1/gain}.

For convenience of simulation and calculation, the splitters 114, 122 and 129 are assumed to pass on all of their respective input signals to each of the components to which they are connected, although (as described earlier) strictly speaking division of a signal between two components results in each receiving a 3 dB reduction in signal level even if an ideal splitter is used. The circuit 100 also includes a local oscillator 128, a subtractor 126, an adder 132, a correction amplifier 136 and an output 138.

Noise inputs in the circuit 100 are simulated by adders 141a, 141b and 141c connected to noise sources 143a, 143b and 143c respectively, of which the first two represent amplifier noise (equivalent to a noise figure of about 10 dB) and the third thermal noise. Within the bandwidth of operation of the circuit 100, noise added by the input amplifier 116 is cancelled in the same way as distortion products. The same is not true of noise from the correction amplifier 136, and in a real circuit where low noise is important the noise contribution from the subsidiary signal path must be minimised. 12

The prior art circuit 200 comprises an amplifier 216 and signal mixer 220 simulated by a combination of an ideal or perfect multiplier 220a and a soft limiter 220b. Here again the limiter 220b saturates at an input voltage of ±2 V and has a gain of 1. The circuit 200 also includes a local oscillator 228 and an output 138. Noise introduced by the amplifier 216 is represented by an adder 241 connected to a noise source 243.

The simulated circuits 100 and 200 receive a common input signal from a source 245, which incorporates two generators 245a and 245b connected to an adder 247. The source 245 provides two frequencies fl and £2 produced by respective generators 245a and 245b. The use of two frequencies for test purposes in this way is conventional in distortion testing, and is referred to as a two-tone stimulus. It enables investigation of the degree to which a circuit produces unwanted cross product terms from two input signals differing only slightly in frequency.

Referring now to Figure 3, the performance of the simulated circuits 100 and 200 is shown by respective graphs 320 (bold line) and 310 (narrow line). For each circuit the respective graph represents the circuit output plotted on a logarithmic scale against frequency on a linear scale. The graphs were produced by computer simulation based on a local oscillator frequency fLO of 1 MHz and two input frequencies fl and f2 of 1.1 MHz and 1.11 MHz respectively. Ignoring higher order frequencies near or above 1 MHz, mixing of these frequencies in the signal mixers 120 and 220 produces the frequencies f3, f4, f5 and f6 as follows:-

O = fl - fLO = 100 kHz (13)

f4 = £2 - fLO = 110 kHz (14)

f5 = 2fl - f2 - fLO = 90 kHz (15)

f6 = 2f2 - fl - fLO = 120 kHz (16)

In a practical mixer circuit associated with a radio receiver, f3 and f4 would be intermediate frequency (IF) equivalents of spectral components of the required input signal, whereas £5 and f6 would be unwanted spurious cross-product terms introduced by 13 the downconversion or mixing process; £5 and f6 cannot be removed by filtering because they are too close to £3 and f4.

As indicated by an arrow 325, in graph 320 the signal strength at f6 is about 38 dB below (nearly 80 times weaker than) the equivalent for graph 310, with a similar effect at £5. This indicates that the simulated circuit 100 of invention has introduced a rejection of unwanted spurious mixer terms by nearly two orders of magnitude, a very substantial improvement over the prior art mixer circuit 200.

Referring now to Figure 4, there is shown a further embodiment of a mixer circuit of the invention indicated generally by 400. It corresponds to the circuit of the mixer 10 of Figure 1 with removal of the couplers 14 and 22, subtractor 26 and attenuator 30, and their replacement by an input coupler 414 and main and subsidiary IF couplers 451 and 453. Parts equivalent to those described earlier are like referenced with a prefix 400. The components and operation of the circuit 400 are very similar to those of the circuit 10 and will not be described in detail. The differences are threefold: firstly, the input coupler 414 connects the input 412 directly to the correction mixer 424 (ignoring the intervening input delay unit 418); in this context "directly" means without attenuation, splitting, coupling or other substantial amplitude reduction processes. The correction mixer 424 consequently receives a largely unattenuated version of the input signal. This reduces the scope for degradation of the overall circuit noise figure. There is no such direct connection to the amplifier 416; instead, the input coupler 414 provides coupling in this respect so that a reduced magnitude version of the signal from the input 412 passes to the amplifier 16. Secondly, instead of using an attenuator 30, the main IF coupler 451 produces a reduced magnitude version of the main IF signal for combination with the subsidiary IF signal; thirdly, the subtractor 26 is replaced by the subsidiary IF coupler 453, which combines the reduced magnitude main IF signal with the subsidiary IF signal. Design of couplers to produce reduced magnitude versions of signals (ie input coupler 414 and main IF coupler 451) and to subtract one signal from another (ie subsidiary IF coupler 453) are well known in the art of electronics and will not be described.

Claims

14CLAIMS

1. A mixer circuit including input means (12, 14) connected to amplifying means (16) for amplifying an input signal to provide an amplified signal, mixing means (20, 24) connected to the input means (12, 14) and to the amplifying means (16) for mixing the amplified signal and the input signal with an LO signal to produce IF signals, combining means (22, 26, 30) for combining the IF signals to produce a distortion signal substantially lacking their input signal components, and correcting means (30, 32, 36) for counteracting distortion in one of the IF signals by means of the distortion signal, characterised in that the mixing means comprises main and subsidiary mixers (20, 24) arranged to provide main and subsidiary IF signals respectively, the amplifying means (16) is connected to the main mixer (20), the combining means (22, 26, 30) is arranged to combine a reduced amplitude version of the main IF signal with the subsidiary IF signal and the correcting means (32, 34, 36) is arranged to provide for the distortion signal to have appropriate amplitude to counteract distortion in the main IF signal.

2. A mixer circuit according to Claim 1 characterised in that the correcting means (32, 34, 36) includes a second amplifying means (36) for amplifying the distortion signal to achieve substantial equality between the magnitudes of the mixer distortion components in the distortion signal and the main IF signal to enable distortion in the latter to be counteracted.

3. A mixer circuit according to Claim 2 characterised in that it includes means (18) for delaying the input signal prior to mixing with the LO signal for appropriate relative phasing of the subsidiary and main IF signals at the combining means (22, 26, 30), and the correcting means (32, 34, 36) includes means (34) for delaying the amplified main IF signal for phasing as appropriate to enable distortion therein to be counteracted.

4. A mixer circuit according to Claim 1, 2 or 3 characterised in that the combining means (22, 26, 30) includes attenuating means (30) arranged to attenuate the main IF signal in order to achieve substantial equality between the magnitudes of 15 the input signal components in the subsidiary and main IF signals to facilitate removal of such component from the distortion signal.

5. A mixer circuit according to Claim 1 characterised in that the main and subsidiary mixers (20, 24) are of like construction and the circuit is implemented as a millimetre wave integrated circuit.

6. A mixer circuit according to Claim 1 characterised in that it includes input coupling means (414) for connecting the input means (412) to the subsidiary mixing means (424) and arranged to provide a substantially unattenuated version of an input signal thereto.

7. A mixer circuit according to Claim 6 characterised in that the input coupling means (414) is arranged to couple a reduced amplitude version of an input signal to the amplifying means.

8. A mixer circuit according to Claim 1 characterised in that the combining means incorporates a main IF coupling means (451) arranged to provide a reduced amplitude version of the main IF signal and a subsidiary coupling means (453) arranged to combine the said signal version with the subsidiary IF signal.

9. A mixer circuit according to Claim 1 characterised in that the combining means (22, 26, 30) is arranged to produce a distortion signal containing contributions from the amplifying means (16) and the main mixer (20), and the correcting means (32, 34, 36) is arranged to counteract distortion in the main IF signal corresponding both such contributions.

10. A method of mixing signals including amplifying an input signal to provide an amplified signal, mixing the amplified signal and the input signal with an LO signal to produce IF signals, combining the IF signals to produce a distortion signal substantially lacking their input signal components, and counteracting distortion in one of the IF signals by means of the distortion signal, characterised in that the IF signals are main and subsidiary IF signals respectively, the main IF signal is derived from the amplified signal, a reduced amplitude version of the 16 main IF signal is combined with the subsidiary IF signal and the distortion signal has appropriate amplitude to counteract distortion in the main IF signal.

11. A method according to Claim 10 characterised in that the subsidiary IF signal is derived by mixing a substantially unattenuated version of the input signal with the LO signal.

12. A method according to Claim 10 characterised in that the amplified signal is derived from a reduced amplitude version of the input signal.

13. A method according to Claim 10 characterised in that amplitude reduction of the main IF signal and its combination with the subsidiary IF signal are achieved by means of signal coupling.

14. A method according to Claim 10 characterised in that the distortion signal contains contributions arising from amplifying the input signal and from mixing the amplified signal, and distortion in the main IF signal corresponding to both such contributions is counteracted.

15. A method according to Claim 10 characterised in that the distortion signal is amplified to have appropriate amplitude to counteract distortion in the main IF signal.