Microstrip Switchable and Fully Tunable Bandpass Filter With Continuous Frequency Tuning Range

A compact switchable and fully tunable bandpass filter (BPF) with continuous frequency tuning range has been presented in this letter. Since the BPF is formed by two varactor-tuned switchable resonators, which can be switched to higher and lower resonant modes, a very wide frequency tuning range can be achieved. Moreover, the external quality factors and coupling coefficients can be well controlled over the entire tuning range. For demonstration, a second-order BPF with frequency tuning range from 1.1 to 2.1 GHz (fractional tuning range: 63%) and constant absolute bandwidth of 40 MHz has been designed and fabricated with microstrip technology. The prototype circuit size of the BPF is only about 0.06 λg × 0.27 λg.