Design of frequency agile filters in RF frontend circuits

Frequency agile filter are key components for future cognitive radio applications. Bandpass filters based on the principle of lowpass upconversion allow simple and CMOS-friendly solutions to cognitive radio filter problems. Because the center frequency depends on the LO-frequency only, the band selection is adjusted with the receive mixers LO simultaneously. A frequency agile filter has been incorporated into a 600 MHz to 800 MHz wideband LNA with a gain of 18 dB at 700 MHz. It consumes 8.7 mA from a 1.4 V supply. With a 3 dB-bandwidth of 14 MHz, the filter achieves a selectivity of 9.5 dB at 40 MHz frequency offset. Measurement results of a prototype chip in 90 nm CMOS prove the presented theory.