Voltage tunable bandpass filter

A voltage tunable bandpass filter is presented. The filter is realized in microstrip technology using two stepped-impedance resonators. The voltage tuning is provided by means of varactor diodes. The filter has fixed absolute bandwidth of 30 MHz in the frequency range from 375 MHz to 915 MHz. The insertion loss is lower than 5 dB and return loss better than 17 dB in the full tuning range. The out of band attenuation is better than 40 dB at frequencies below the passband and better than 50 dB at frequencies above the passband. The first spurious modes are observed at 6 GHz. Moreover the time delay in the full tuning range changes by 30% only. The filter is thermally stable. It has 1 dB compression point of 10 dBm and third-order intercept point of 21 dBm.