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

Abstract

/spl Sigma//spl Delta/ modulation with integrated quadrature mixing is used for analog-to-digital (A/D) conversion-of a 10.7-MHz IF input signal in an AM/FM radio receiver. After near-zero IF mixing to a 165 kHz offset frequency, the I and Q signals are digitized by two fifth-order, 32 times oversampling continuous-time /spl Sigma//spl Delta/ modulators. A prototype IC includes digital filters for decimation and the shift of the near-zero-IF to dc. The baseband output signal has maximum carrier-to-noise ratios of 94 dB in 9 kHz (AM) and 79 dB in 200 kHz (FM), with 97 and 82 dB dynamic range, respectively. The IM3 distance is 84 dB at full-scale A/D converter input signal. Including downconversion and decimation filtering, the IF A/D conversion system occupies 1.3 mm/sup 2/ in 0.25-/spl mu/m standard digital CMOS. The /spl Sigma//spl Delta/ modulators consume 8 mW from a 2.5-V supply voltage, and the digital filters consume 11 mW.