Automatically tuned, variable bandwidth, Q-enhanced if bandpass filters

Abstract

Modern wireless products including software-defined and cognitive radios (SDR/CR) require variable bandwidth technologies to address multiple modulation standards. This paper shows that Q-enhanced filtering at the intermediate frequency of a superheterodyne receiver can support this need. We present an automatically tuned, coupled-resonator QE filter operating at 450 MHz, with variable bandwidth capability, a 14 dB noise figure, and up to 140 dB/Hz dynamic range. The problem of tuning the filter in the face of environmental changes is solved with a low-complexity algorithm and minimal additional circuitry. The associated problem of correcting passband asymmetry is also addressed and a 2-pole design tuned for 10 and 20 MHz bandwidths is shown to maintain flatness to within 0.5 dB from 25 to 55 degrees C.