Adaptive dual-loop algorithm for cancellation of time-varying offsets in direct conversion mixers

This paper presents a new algorithm and structure that is to be used in conjunction with a specially modified CMOS Gilbert cell mixer to remove the time-varying DC offsets in direct conversion mixers. In contrast to other work published, in our approach the DC offset is detected at the output of the mixer using a dedicated crude ADC and then the offset is tracked using a low gate count digital structure. The estimated offset is then used to cancel the offset in the mixer by dynamically adjusting the bias of a specially modified Gilbert cell mixer. This prevents saturation and desensitization of the analog downstream stages. The presented approach is suitable for TDMA as well as continuous reception techniques such as W-CDMA and FH-SS while being fully integrable on-chip.