A widely tunable continuous-time LPF for a direct conversion DBS tuner

A continuous-time 7th-order Butterworth Gm-C low pass filter (LPF) with on-chip automatic tuning circuit has been implemented for a direct conversion DBS tuner in 0.35 μm SiGe BiCMOS technology. The filter's −3 dB cutoff frequency f₀ can be tuned from 4 to 40 MHz. A novel on-chip automatic tuning scheme has been successfully realized to tune and lock the filter's cutoff frequency. Measurement results show that the filter has −0.5 dB passband gain, ±5% bandwidth accuracy, 30 nV/Hz^1/2 input referred noise, −3 dBVrms passband IIP3, and 27 dBVrms stopband IIP3. The I/Q LPFs with the tuning circuit draw 13 mA (with f₀ = 20 MHz) from 5 V supply, and occupy 0.5 mm².