A 10.7 MHz IF-to-baseband ΣΔ A/D conversion system for AM/FM radio receivers

Abstract

This analog-to-digital converter digitizes a radio signal at a 10.7 MHz intermediate frequency (IF) using integrated quadrature mixing and /spl Sigma//spl Delta/ modulation. The paper shows a block diagram of a highly-integrated AM/FM radio receiver. The IF A/D conversion and digital filtering, demodulation and further signal processing can be integrated on a single CMOS IC. The radio front-end mixes both AM and FM signals to 10.7 MHz IF, so that the A/D conversion is shared. A single channel filter is used, selecting one 200 kHz FM channel. For AM, over 20 channels pass through this filter, resulting in high dynamic range of the IF signal. When the radio is tuned to a weak AM radio station, strong neighboring channels should not introduce disturbance of the weak signal. This multi-channel aspect for AM puts severe requirements on the automatic gain control (AGC) amplifier and A/D converter in terms of noise, intermodulation and crossmodulation distortion.