Distortion Analysis of a Digitally Implemented FM Receiver

High performance FM voice and data radios are widely used for military communications. With increased demands for smaller circuitry and multi-function equipment, digital implementation of conventional demodulation systems becomes attractive. This paper analyzes the distortion in a digitally implemented FM receiver. The receiver structure analyzed estimates the phase between in-phase and quadrature channels and then performs a differential operation with filtering to obtain an estimate of the modulation. Mathematical formulas are derived to show the individual effects of A/D converter quantization, phase angle resolution, and sample rate on the output signal-to-distortion ratio. These formulas indicate the trade-off relationship between hardware complexity and sample rate. A computer simulation model was implemented to analyze the combined effects of A/D quantization, phase resolution, and sampling rate. Results from the simulation indicate that good signal-to-distortion ratios can be achieved with relatively coarse quantization and phase resolution at moderate sampling rates (approximately 3 to 10 times the Nyquist rate).