Two second-order bandpass delta-sigma A/D modulators have been implemented in a 0 . 8~1 BiCMOS [2] process to demonstrate the feasibility of converting a 10.7MHz radio IF signal to digital form. The circuits, based on switched-C biquads, are clocked at 42.8MHz and demonstrated 55dB S N R in a 200kHz bandwidth while dissipating 6OmW of power from a 5V supply. htroductianThere is industrial interest in A/D conversion at the intermediate-frequency (F) stage of a radio [ 11, because that allows demodulation and IF filtering to be done digitally. The robustness of the digital circuitry gives manufacturing advantages and allows the use of sophisticated algorithms, which are especially useful with digitally coded transmissions. A/D conversion can be done in quadrature at baseband, using a "zero-IF' receiver, but that requires precision matching and is susceptible to a variety of DC instabilities and electromagnetic compatibility problems. Alternately, a bandpass signal can be directly converted using a video or AZ converter. AI conveners consist of a filter and comparator (or other low-resolution A/D) in a feedback loop, and are oversampled. Monolithic fourth-order bandpass deltasigma (BPAX) modulators have been reported [31,[41 with centre frequencies of 455kHz and 2MHz and bandwidths of lOkHz and 2OOkHz respectively. Another device obtained a 55dB S N R at a 6.5MHz centre fre-
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