Design of varactor tuned bandpass filter

The tunable bandpass filter is presented. The filter is realized in microstrip technology employing two stepped-impedance resonators tuned with varactor diodes. The filter has constant absolute bandwidth of 30 MHz in the frequency range from 380 MHz to 920 MHz. The insertion loss is lower than 5 dB and return loss higher than 17 dB in the full tuning range. The out of band attenuation is higher than 40 dB at frequencies below the passband and is higher than 50 dB at frequencies above the passband. The first spurious modes are observed at 3 GHz. Absolute bandwidth and the time delay in the full tuning range change by 33% only while center frequency changes 242%.