A 4-40MHz Continuous-Time LPF with on Chip Automatic Tuning for a Direct Conversion DBS Tuner

A continuous-time 8^th-order Butterworth Gm-C low pass filter (LPF) has been implemented for a direct conversion DBS tuner in a 0.35 um SiGe BiCMOS technology. The filter's -3 dB cutoff frequency f₀ can be programmed from 4MHz to 40MHz in fine discrete steps through a 7-bit DAC. A PLL-based on chip automatic tuning system is used to maintain the precise filtering characteristics against process variation, temperature drift and aging. The filter has OdB passband gain, 24nV / radicHz input referred noise, -3 dBVrms passband IIP3, 27 dBVrms stopband IIP3. The I/Q channel filters with the tuning system draw 21 mA (with f₀=20MHz) from 5 V supply, and occupy an area of 0.5 mm².