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

In the field of microwave tubes, the cold test plays an important role. However, no attempt in this direction has been made for the variable capacitance amplifier. It is the purpose of this paper to present the theory of a cold test procedure for parametric amplifiers. The cold test is essentially the measurement of the impedance locus of the input, the output and the pump circuits under the no-pump condition, with the diode bias voltage as the variable parameter. From these impedance loci one can evaluate all the important circuit parameters of the equivalent circuit of the parametric amplifier, including the value of the dynamic quality factor of the diode. Using these data, it is relatively easy to design or adjust the circuit so as to give the best noise performance. Since the theory of the cold test procedure neglects the effects of (1) higher harmonics, (2) any parallel conductance of the diode, and (3) circuit losses, the validity of the theory can be established only by experiment. For this reason, this paper also presents some of the experimental results obtained with a 6-kmc degenerate amplifier. The correlation between theory and experiment has proved to be excellent.