A Very Linear Low-Pass Filter with Automatic Frequency Tuning

A Gm-C third-order Chebyshev low-pass filter with a novel switched capacitor frequency tuning technique for a zero-IF Bluetooth receiver has been designed. The frequency tuning scheme is simpler and has more relaxed specifications than conventional ones. Furthermore, a highly linear pseudo-differential transconductor with a compact feedback loop able to operate with low supply voltage has been used. This control loop holds the input transistors in triode region and provides high output resistance, keeping high linearity in a wide range of transconductance. The filter bandwidth is 0.5 MHz and the overall scheme consumes 1.1 mA from a 1.8-V supply. The measured third-order intermodulation (IM3) distortion of the filter for a 1 Vpp two-tone signal centered at 300 kHz is -65 dB.