مرکز منطقه ای اطلاع رساني علوم و فناوري - Determination of the pump modulation factor of varactor diodes under operating conditions

Abstract :

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 (θp) and signal current (θs) 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.