Evaluation of loss characteristics of high-quality varactor diodes

THE USE OF COOLED VARACTOR DIODES in low-noise amplifiers has stimulated the development of better techniques for measuring the losses of cooled diodes. Optimum design of cooled parametric amplifiers requires knowledge of the temperature dependence of the varactor parameters. Reflection-type measurements’ of cooled varactors are difficult and often ambiguous because of the appreciable length of low thermal-loss transmission line required to couple the diode to the measuring apparatus. Since the electrical losses of the transmission line are temperature dependent, it is difficult to compensate for them. The diode holder and associated tuning susceptances must have very low losses to avoid masking the losses be followed to make measurements in such a manner of high-quality diodes. Often, involved procedures must be followed to make measurements in such a manner that he instrumentation does not set an undue limit on the precision of the results. Reflection-type measurements may become difficult to interpret if the diode base-spreading resistance varies with diode bias voltage.