An Active Self-Interference Cancellation Coupler with 60dB Isolation Applied in a 24GHz SFCW Radar

This work introduces a compact planar implementation of a self-interference cancellation coupler (SICC), which is designed for 24 GHz continuous wave (CW) radar frontends. The SICC utilizes a part of the TX signal which is modulated in amplitude and phase to generate a cancellation signal, which suppresses the most severe TX-RX-leakage components. Instead of using a closed control loop to minimize leakage, an approach with precise offline calibration is used. We showed the reproducibility of this approach with repeated isolation measurements applying the calibrated control values for the cancellation modulator. With this approach an isolation of about 60 dB in a range of 23.0 GHz to 24.25 GHz is achieved. A further novelty of this work is the implementation of the RF domain canceller in a monostatic radar setup to investigate the influence of TX leakage and the improvements that come with the SICC in the presence of close weak reflecting targets by analyzing the respective IF spectrum. The results of the stepped-frequency continuous wave (SFCW) radar measurements show a significant reduction of the leakage peaks in the IF spectrum as well as an increase of the target peak amplitude, allowing a more secure detection of close targets and reducing the effect of RX frontend saturation.