Fifth order low-pass transitional Gm-C filter with relaxation oscillator frequency tuning circuit

The design and implementation of gm-C filter with on-chip automatic tuning circuit is described. Transitional Bessel-Chebyshev filter is implemented to compromise between high attenuation and low group-delay variation. The circuit is fabricated with 0.18 μm CMOS process with 6-metal single poly. The filter operates at a cutoff frequency of 10 MHz with a single 1.3 V supply voltage. The measurement results shows an attenuation of 70 dB at stop-band can be achieved, while keeping a low group-delay variation of less than 7 nSec up-to 1.5 times of cutoff frequency. The cutoff frequency can be linearly tuned from 4 MHz to 10 MHz through the tuning loop. By employing relaxation oscillator, accurate and robust frequency tuning scheme is implemented. Measured with sufficient samples, the frequency tuning circuit shows an cutoff frequency tuning error of less than 7.0% over wide range of supply voltages (1.3 V to 2.1 V) and operating temperatures (-40°C to +85°C) in the whole tuning range.