Delta Modulation Technique for Improving the Sensitivity of Monobit Subsamplers in Radar and Coherent Receiver Applications

This paper introduces a technique for improving the sensitivity of RF subsamplers in radar and coherent receiver applications. The technique, referred to herein as “delta modulation” (DM), feeds the time-average output of a monobit analog-to-digital converter (ADC) back to the ADC input, but with opposite polarity. Assuming pseudostationary modulation statistics on the sampled RF waveform, the feedback signal corrects for aggregate dc offsets present in the ADC that otherwise degrade ADC sensitivity. Two RF integrated circuits (RFICs) are designed to demonstrate the approach. One uses analog DM to create the feedback signal; the other uses digital DM to achieve the same result. A series of tests validates the designs. The dynamic time-domain response confirms the feedback loop´s basic operation. Measured output quantization imbalance, under noise-only input drive, significantly improves with the use of the DM circuit, even for large, deliberately induced dc offsets and wide temperature variation from -55 °C to +85 °C. Examination of the corrected versus uncorrected baseband spectrum under swept input signal-to-noise ratio (SNR) conditions demonstrates the effectiveness of this approach for realistic radar and coherent receiver applications. Two-tone testing shows no impact of the DM technique on ADC linearity.