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

The pump modulation factors 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 (θp) and signal current (θs) according to . 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 . 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 with increasing diode forward current, while little is gained by changing the bias voltage. GaAs diodes show little increase in 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.