Abstract: An amplitude stabilization circuit for fiber optic links that holds the RF gain of the link constant despite optical parameter variations. A voltage variable attenuator which attenuates DC and RF equally is placed between the link photodetector and a load resistor. The voltage on the resistor is compared with a reference voltage, and the comparison result is an error signal used in a feedback loop to control the attenuation of the attenuator.

Abstract: An adaptive polarization combining system automatically adjusts the polarization of a polarization diverse antenna to match that of the incoming RF signal, thereby maximizing the received signal-to-noise ratio. Signals from the orthogonally polarized ports of the antenna are passed through a variable combiner circuit which is adjusted to maximize the combined signal at a single output port. Sample signals from each antenna port are provided to a calibration circuit which obtains phase and amplitude information from the two orthogonally polarized received signals and uses this information to control the combiner circuit phase shifters to adapt the combiner circuit to the polarization of the received signals. Therefore, the combining system can rapidly adapt electronically to polarization changes in the received signals.