An integrated LMS adaptive filter of TX leakage for CDMA receiver front ends

The theory and practical implementation of a continuous-time LMS adaptive filter of the TX leakage in CDMA receivers are described. The filter works by injecting a matched out-of-phase copy of the TX leakage into the LNA output. It requires a reference signal coupled from the TX chain, whose I and Q components are appropriately scaled to generate the matched copy. The scale factors are the results of the correlation between the filter output signal and the I/Q components of the reference signal. The filter was designed as part of a 0.25-μm CMOS cellular-band receiver. The effect of the DC offsets in the correlators on the TX leakage rejection ratio (TXRR) was minimized by using the sign-data variant of the LMS algorithm and by increasing the gain of the correlating multipliers. The loop stability margin was improved by swapping the I and Q reference inputs of the scaling multipliers. Without a significant group delay of the TX leakage relative to the reference signal, the filter achieved the maximum TXRR of 28 dB, which was limited by the reference signal coupling. The group delay introduced by the SAW duplexer reduced the minimum TXRR to 10.8 dB. The filter degraded the LNA noise factor and gain by 1.3 dB and 1.7 dB, respectively.