Performance analysis of a digital feedback AGC with constant settling time

A digital feedback automatic gain control (DFAGC) with the constant settling time is presented for the stability of the Phase Locked Loop in the QPSK demodulation. The DFAGC consists of an exponential gain controlled amplifier and a logarithmic amplifier, which gets the constant gain settling time independent of the input signal amplitude. This scheme allows using the linear system theory directly analyze the DAGC performance. So the mathematical model and the backward difference equation are given to investigate the settling time, and the transfer function given to obtain the transient response and the DFAGC tracking performance. Finally, it is verified by simulations that the analysis results closely coincide with the settling time and the DFAGC characteristic in high SNR, and the linear system theory analysis method concisely gives the insight of the proposed scheme.