Stability Considerations in Lossless Varactor Frequency Multipliers

One of the principal limitations to efficient wideband harmonic generation with varactor diodes is the generation of spurious signals.1- 2 - 3 The origin of these signals is usually thought1 to be due to a parametric "pumping up" of some signal in the multiplier passband, or to a parametric up-conversion process,1 or a variation in the average capacitance of the diode at input frequency.3 A multiplier which contains these spurious signals is considered to be unstable,4 and it is this type of instability that is investigated in this paper. At the present time, much is not known about the stability of harmonic generators, even though it is a widely-known experimental fact that this is a serious problem in high-efficiency varactor multipliers. 2,4 Very little is also known about the conditions imposed by stability on the available circuit configurations. Consequently, present design procedures leave the problem to be solved experimentally, and this is often done at the expense of efficiency. Very often isolators are used 2035