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Stability of a conventional LMS adaptive filter at RF is analyzed using the Laplace transform. It is shown that the RF signal delays along the filter feedback loop can make it unstable. A simple modification to the filter architecture is proposed to improve the loop phase margin. A practical implementation of the proposed architecture in 0.25μm CMOS technology is described. The designed filter is used as part of a cellular-band CDMA receiver to reject a transmitter signal leakage at the mixer input. It achieved the maximum rejection of 28dB.