On the Use Of Passive Circuit Measurements for the Adjustment of Variable Capacitance Amplifiers

Although variable capacitance amplifiers have been built and operated successfully, their design has been an art practiced by the individual designer rather than a systematic construction. The designer generally provides various adjustable components in his amplifier and obtains the final optimum result on a trial and error basis. However, the designer can never be free from the fear that just one more adjustable component may considerably improve the noise figure. Furthermore, when the measured noise figure is not as good as expected from some previous 361 362 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1962 experience, there is no simple way to determine which circuit is the main cause of the poor noise figure, i.e., the signal, the idler or the pump circuit. The poor result might also be due to a poor diode. The cold test procedure goes a long way towards resolving this uncertainty. In microwave tubes, such as the klystron and magnetron, the cold test plays an important role. However, no attempt in this direction has been made for the variable capacitance amplifiers. It is the purpose of this paper to present both the theory of a cold test procedure for parametric amplifiers and some experimental results on a 6-kmc degenerate amplifier showing the validity and the limit of applicability of the theory. It can be shown 1 that the minimum noise figures of lower-sideband idler-output and circulator type amplifiers, degenerate amplifier and upper-sideband up-converter are all basically determined by a dynamic quality factor of the diode and that, for optimum noise figure operation, each type of amplifier requires certain values of Ra/R0 and R,,/Rl , where Rs is the series resistance of the diode, R,, the internal resistance of the generator and R L the load resistance.