A frequency-domain stability criterion for periodically driven varactor circuits

In this paper we show how input-output stability theory bears on the problem of obtaining a frequency-domain stability criterion that can be used to design periodically driven varactor circuits with guaranteed stability. The design criterion requires that the locus of jωZ(jω) for -∞<ω<∞ avoids a critical disk in the complex plane, where Z(jω) is the Thevenin equivalent driving-point impedance presented to the varactor at frequency ω. The location and size of the disk is a function only of the minimum and maximum incremental capacitance of the varactor. We present an example of how a varactor frequency doubler with guaranteed stability may be designed using this criterion