A BICMOS LOW-DISTORTION 8-MHZ LOW-PASS FILTER

An 8-MHz seventh-degree elliptic-function low-pass filter is described , demonstrating an approach to low-distortion antialias filtering for high-definition video applications. The filter's performance goals are achieved through the use of circuit design principles that capitalize on the strengths of BiCMOS technology. The integrator circuits compos ing the filter consist of a new wideband low-distortion transconductor circuit and a unique BiCMOS Miller-stage circuit. Integrator time con stants are determined by stable RC products, enabling a simplified fil ter calibration scheme that is insensitive to temperature-induced vari ations and requires no phaselock circuits. The prototype filter IC, co nsisting of seven integrators assembled in an active-ladder configurat ion, was fabricated in a 10-V, 2-mum 2.5-GHz BiCMOS technology that al so features thin-film resistors and polysilicon-plate capacitors. Meas ured results from the calibrated filter show passband flatness of 0.2 dB, with aberrations of less than +/-1 dB over a 100-degrees-C tempera ture range. Stopband attenuation meets its designed goal of 60 dB. Dri ven by 7-V(pp) differential input signals, the filter exhibits less th an -72-dBc third-order intermodulation distortion products at 1 MHz. F or 5-V(pp) inputs at 4 MHz, third-order intermodulation spurs remain b elow -65 dBc.