The pump modulation factors m of various GaAs and Si varactor diodes have been measured under operating conditions. The device used for the measurements was a phase sensitive degenerate parametric amplifier with a signal frequency of 5.85 Gc phase locked with a pump frequency at 11.7 Gc. The input impedance of the amplifier varies with relative phase of pumped elastance (

) and signal current (

) according to Z
V= R
S+

The locus of the impedance is a circle which can be plotted directly on a Smith chart. The radius of this circle yields the modulation factor.