Tunable 1.55–2.1 GHz 4-Pole Elliptic Bandpass Filter With Bandwidth Control and > 50~dB Rejection for Wireless Systems

This paper presents a four-pole elliptic tunable combline bandpass filter with center frequency and bandwidth control. The filter is built on a Duroid substrate withεᵣ=10.2andh=25~ mils, and the tuning is done using packaged Schottky diodes. A frequency range of 1.55–2.1 GHz with a 1-dB bandwidth tuning from 40–120 MHz (2.2–8% fractional bandwidth) is demonstrated. A pair of tunable transmission zeroes are synthesized at both passband edges and significantly improve the filter selectivity. The rejection level at both the lower and upper stopbands is> 50~dBand no spurious response exists close to the passband. The measured third-order intermodulation intercept point (TOI) and 1-dB power compression point at midband (1.85 GHz) and a bandwidth of 110 MHz are> 14~ dBmand 6 dBm, respectively, and are limited by the Schottky diodes. It is believed that this is the first four-pole combline tunable bandpass filter with an elliptic function response and center frequency and bandwidth control. The application areas are in tunable filters for wireless systems and cognitive radios.