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A master/slave technique which allows digital tuning of a Gm-C filter's cut-off frequency is proposed. It relies on an accurate transconductance division technique, fixing the reference frequency independently from the filter cut-off frequency. The design of a digitally-tunable Gm-C switchable third/fifth- order Butterworth low-pass filter in a 0.13-mum 1.2-V CMOS process is also proposed. An extended tuning range (above 25:1) is obtained through the design of a linear very widely tunable transconductor. Measured results prove this chip to be compatible with the baseband part of direct-conversion cellular receivers, from GSM to W-CDMA, with a tuning accuracy better than 5% over the entire tuning range.