A Switchable-Order Gm-C Baseband Filter With Wide Digital Tuning for Configurable Radio Receivers

A master/slave technique which allows digital tuning of a G_m-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 G_m-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.