A bandpass version of sigma-delta modulation is presented, with simulation results for second and fourth-order converters. For the fourth order converter operating at 8MHz, simulations demonstrate a resolution of 16 bits over an 8kHz band centred at 1MHz. Applications may include analog to digital conversion for AM radio, and digital to analog conversion for narrow-band RF systems.
The traditional low-pass ZA analog-to-digital converter is extended to the bandpass case. For input signals with small relative bandwidths, bandpass !ZA converters offer high signal-to-noise ratios at significantly lower sampling rates tban are required for low-pass XA converters. A sixth-order single-ended switched-capacitor circuit, clocked at 3 MHz, is designed to convert bandpass signals centered at 455 kHz with 20-kHz bandwidth. Time-domain circuit simulations find that this modulator realizes a 94-dB signal-to-noise ratio for a half-scale input, giving roughly 16-bit performance.
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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