I. lIturouio The performance of a microwave superheterodyne receiver is largely limited by the mixer diode conversion loss, which with diodes of high conversion efficiency and near-unity noise temperature ratio is dependent on the image frequency termination (1). The image frequency is that frequency offset from the local oscillator-frequency by the same amount as the signal, but with the opposite sign. Briefly, the mixing process for conversion of signal power to the intermediate frequency gives rise to the generation of power at the image frequency. If this power is absorbed in the image termination, as in image matched circuits, this represents a serious reduction in the conversion efficiency. However, if the image power can also be converted to the intermediate frequency then improvement in the mixer conversion loss is possible. The noise temperature ratio of modern Schottky barrier diodes is very close to unity and hence any improvement in conversion loss translates directly to an improvement in the overall noise figure.This paper discusses the performance of X-band image tuned mixer circuits with particular reference to GaAs Schottky barrier diodes. The signal filter technique has been used to provide design data for a microstrip integrated double balanced mixer circuit which effects image recovery without the associated disadvantages of a high Q filter circuit. 2. Theoretical considerati The low frequency conversion loss of the GaAs Schottky barrier diodes has been calculated from their I-V characteristics (l). Three conditions of image termination have been considered, matched, short and open circuit. The calculated low frequency conversion loss values under these conditions are 3.25 diB, 1.75 dIB and 1.25 dB respectively. Thus possible improvement of up to 2 dB is indicated under image tuned compared with conventional image matched conditions.The diode i.f. impedance is also dependent on the image termination, high i.f. impedance occurring under open circuit conditions. Thus for optimum performance it is necessary to adjust the i.f. amplifier input circuit for the appropriate diode i.f. impedance.3. Imre rgecovey Image tuning can be provided by inserting a filter in front of the mixer with the bandpass at the signal frequency. The image frequency generated in the mixer is reflected by the filter and reconverted to the intermediate frequency. A practical example is a radar receiver protected by a narrow band T.R. cell, where it may be convenient to use this component as the signal filter.It is also possible to use, as the image termination,a second mixer in which conversion of the image frequency can occur. This approach reduces some of the performance restrictions applied by the use of high Q circuits, and introduces the possibility of broadband image recovery with a potential noise figure of about 3.0 dB.3.1. Sigal filter Image tuned circuit techniques have been investigated at X-band (about 9.5 GHz) using the high Q waveguide signal filter approach.

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