Nonlinear Modelling of Automatic Gain Control Loops Considering Loop Dynamics and Stability

Abstract

This work presents modelling aspects of automatic gain control (AGC) loops based on linear-in-dB variable gain amplifiers (VGAs). In these loops, the VGA control voltage is also an excellent received signal strength indicator (RSSI). The VGA gain is however nonlinearly related to the control voltage. Moreover, VGAs and detectors undergo nonlinear compression under high input amplitudes during settling transients. The main contribution in this work is a proposed nonlinear model based on simple and readily available components from the "analogLib" and "functional" libraries in CADENCE design environment -making it very easy and fast to build and simulate-that captures the nonlinear effects of AGC loops. The model is capable of verifying the AGC loop stability and capturing the loop dynamics with high accuracy compared to time consuming circuit level simulations. This provides insights into system level parameters such as AGC loop bandwidth, phase margin, settling time as well as estimating the AGC range and RSSI voltage vs. input power. Measurement results from a fabricated AGC prototype are in good agreement with simulation and modelling results thus validating the proposed modelling approach.