A CMOS Active-RC Low-Pass Filter With Simultaneously Tunable High- and Low-Cutoff Frequencies for IEEE 802.22 Applications

An active resistor-capacitor low-pass filter with simultaneously programmable high-end and low-end cutoff frequencies is presented for IEEE 802.22 cognitive radio transmitter applications. Transfer function analysis shows that the integrator frequency of a dc offset cancellation block should be tuned inversely proportional to the open-loop gain to maintain the low-end cutoff frequency at a constant value. Realized in a 0.18- ??m complementary metal-oxide-semiconductor, the low-end cutoff frequency is successfully tuned between 740 Hz and 10 kHz over the 30-dB gain variation. Measured results show that the gain is tuned from -16.3 to +13.9 dB, and the high-end cutoff frequency is tuned from 2.1 to 6.04 MHz, while drawing 4.5 mA from a 1.8-V supply.