A brief historical survey is made of the use of semiconductor devices as frequency changers or mixers in radar applications, with particular reference to factors that led to the design and performance of the types CV2154-5 silicon mixer crystals. The factors that led to the choice of a coaxial construction and the part played by the insulator and its effect on admittance is discussed. A mixer crystal incorporating a germanium wafer and titanium whisker is described. This crystal has a noise factor better by 2 dB than the silicon and tungsten combination of the types CV2154-5. The necessity for positive bias, its effects on r.f. admittance and noise factor and methods of obtaining the bias when required are considered. The superior bandwidth and small spread in admittance between samples is attributed to the choice of material for the insulator. Burn-out performance is discussed and measurement of this parameter indicated. Finally, it is shown that, contrary to general belief, the behaviour of germanium mixer crystals as a function of temperature is not significantly different from silicon devices, at least over the temperature range ÿ80 to + 100ðC.
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