Four-pole frequency agile bandpass filter with fully canonical response and constant ABW

This paper reports on the first four-order frequency-agile bandpass filter (FA-BPF) with four transmission zeros (TZs) and constant absolute bandwidth. The proposed filter employs four varactor-loaded resonators and can be controlled by only two bias. Therefore, very simple design and control procedures are achieved while the good insertion loss (IL) performance are obtained when comparing to other cross-coupling high-order tunable filter. The design starts with the element-variable coupling matrix. By synchronously tuning the element-variable coupling matrix, the center frequency (CF) of the resultant passband is tuned and the bandpass bandwidth is kept unchanged. According to the matrix, the 1.24-1.72 GHz FA-BPF with a constant ABW of 61.5±1.5 MHz is realized and measured. The measurement results validate the prediction.