A Tunable Three-Pole 1.5–2.2-GHz Bandpass Filter With Bandwidth and Transmission Zero Control

This paper presents a three-pole tunable combline bandpass filter with center frequency, bandwidth, and zero control. The filter is designed on a Duroid substrate with ε
r
=10.2 and
h
=25 mil . A frequency range of 1.5-2.2 GHz with a 1-dB bandwidth tuning from 50 to 170 MHz (2.2%-11.2% fractional bandwidth) is achieved. The transmission zero can also be controlled, and a zero location of 1.37-1.64 GHz is demonstrated at center frequency
fo
of 2.05 GHz. The measured third-order intermodulation intercept point and 1-dB power compression point at midband (1.85 GHz) and a bandwidth of 110 MHz are >;15 and 8 dBm, respectively. To our knowledge, this is the first three-pole combline tunable bandpass filter with center frequency, bandwidth, and transmission zero control.