A bandpass ΣΔ modulator with 92 dB SNR and center frequency continuously programmable from 0 to 70 MHz

Abstract

Use of a bandpass /spl Sigma//spl Delta/ modulator permits direct conversion of an analog signal to digital form at IF frequencies. This allows the ADC to be moved closer to the receiver front end. Moving the digital interface closer to the antenna reduces receiver analog circuit complexity, eliminates DC-offset cancellation, inphase/quadrature (I/Q) gain calibration, dual I/Q mixers and improves system robustness as mixing is in the digital domain. This second-order bandpass /spl Sigma//spl Delta/ modulator is targeted for an airborne radar system but is also expected to find use in a variety of communications applications. Measurements yield signal to noise+distortion ratio (SNR) from 92 dB (15 b) in narrowband ( 366 kHz) to 44 dB (7 b) in broadband (62.6 MHz) about a center frequency of 55.5 MHz. Modulator sampling rate is 4 GHz and it is implemented in AlInAs-InGaAs HBT technology. The performance represents an improvement of approximately a factor of 10 in bandwidth, resolution and center frequency over other reported bandpass modulators.