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 εr=10.2 andh=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 frequencyfoof 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.
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Chiou et al. (2011) studied this question.