Determination of the pump modulation factor of varactor diodes under operating conditions

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/s phase locked with a pump frequency at 11.7 Gc/s. The input impedance of the amplifier varies with relative phase of pumped elastance (θ
p
) and signal current (θ
s
) according to
Z{\upsilon} = R_{s} + \frac{S_{0}}{j\omega_{s}} + \frac{mS_{0}}{\omega_{s}} e^{i(\theta_{p} - 2\theta_{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
m
. The results show a noticeable difference between the behavior of Si epitaxial and various types of GaAs diodes. The silicon diodes exhibit a large increase in
m
with increasing diode forward current, while little is gained by changing the bias voltage. GaAs diodes show little increase in
m
with increasing current, but there is a steady increase with increasing negative bias when the forward current is fixed. It can be concluded from the data that the gain-bandwidth products can be improved at the expense of increased shot noise-which must be expected at larger forward currents-when using Si epitaxial diodes by pumping slightly into the forward conduction region, whereas little can be gained by using the same technique with GaAs diodes.