Analog IQ impairments in Zero-IF radar receivers: Analysis, measurements and digital compensation

We address the Zero-IF or homodyne radio architecture as a pursuable way for small Radar receivers. While Zero-IF is beneficial for integration, several inherent analog impairments place a limit on the achievable dynamic range. The most dominant non-idealities are gain and phase imbalance in the IQ branches, mixer nonlinearity, and DC Offset. In the case of IQ Imbalance, careful receiver design can at best achieve an Image Rejection Ratio (IRR) of 30-40 dB. Also, IQ imbalance tends to be frequency-dependent with increasing bandwidth (BW). It has to be investigated, whether sophisticated digital post-processing is able to deliver a dynamic range sufficient for Pulse-Doppler Radar. After establishing some theoretical background and proposing digital correction methods, we will present hardware measurements of frequency-dependent IQ imbalance made on a Zero-IF receiver with large bandwidth. Despite significant improvements can be achieved using an offline calibration, time-varying drifts due to temperature changes will degrade the achievable IRR. Therefore adaptive circularity-based algorithms should be applied to track those changes. However, Radar Chirp signals at complex baseband (BB) cannot be used directly, as they are not circular. To restore the circularity for estimating the Complementary Autocorrelation Function (CACF), we propose applying a digital band-stop to the operational data beforehand. Highly increased IRR values are technically feasible: Digital Assistance acting jointly with state-of-the-art RF circuit design can pave the way for adequate performance in integrated receiver solutions.