A 2-Pole Tunable Bandpass Filter Using Commercial Digitally Tunable Capacitor

This paper presents a novel type of tunable band-pass filters with constant bandwidth and wide tuning range for the 520 - 860 MHz wireless band. A constant absolute pass-band has been obtained by adding a compensating passive circuit at the input and the output of the filter. The filter fabricated on duroid substrate can be tuned from 520 MHz up to 860 MHz, by using commercial Digitally Tunable Capacitor (DTC) controlled through widely supported 3-wire SPI interface. Matching is better than -15dB and the 3dB pass-band is 110 MHz+/-5MHz. Measured insertion loss is less than 3 dB at the lower end of the band and 4.8 dB at the upper end. The filter shows no compression up to 25 dBm of input power, and no measurable degradation of ACPR at the same level of power.