The application of automatic gain control to microwave radiometers

An automatic gain control system for stabilizing the gain of microwave radiometers is described. System analysis sets forth the requirements for gain and bandwidth of the AGC loop. Two systems are described. One is a continuous AGC system that maintains a constant detector voltage and, as a result, the system gain is a function of the input signal. For input signals which are small compared to the system noise temperatures the nonlinearity due to the AGC system is negligible. In the second system, the output of the detector is sampled when the receiver comparison switch is connected to the reference termination. Therefore, the gain of the system is not affected by the signal and no nonlinearity is caused by the AGC system. The effect of noise in the AGC loop is analyzed and it is shown that the time constant of the AGC system can be made shorter than the final system output filter without increasing the over-all system noise.