High-Q tuned active bandpass filter for wireless application

The paper presents a novel high-Q microwave tuned active bandpass filter using varactor diodes for tuning and a BJT to compensate for losses. The experimental filter with stepped resonator circuits and special coupling elements have been fabricated and tested demonstrating a good agreement with the circuit model predictions. Tuning range of 400 MHz for the active filter is achieved with the centre frequency of 1.6 GHz. A 3 dB bandwidth of 50 MHz is obtained and passband gain is typically 0.5-1.0 dB in the whole tuning range.