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

Sigma 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 Sigma Delta modulators. A prototype IC includes digital filters for decimation and the shift of the near-zero-IF to de. 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 AID converter input signal, Including downconversion and decimation filtering, the IF A/D conversion system occupies 1.3 mm(2) in 0.25-mum standard digital CMOS. The Sigma Delta modulators consume 8 mW from a 2.5-V supply voltage, and the digital filters consume 11 mW.