Miniaturized Hairpin Tunable Filter with the Single Control Voltage

This paper presents a varactor-tuned miniaturized hairpin tunable filter employing single control voltage. The proposed tunable filter is designed and fabricated with the constant ratio rule in which the capacitances of the each stage maintains a constant ratio without any dependency on the miniaturized electrical length. As a result, the filter can be tuned with the reserved equivalent condition using a single control voltage. To show the validity of the proposed method, a third-order 0.5 dB ripple Chebyshev filter with a center frequency of 900 MHz and a fractional bandwidth(FBW) of 10% was designed and fabricated. The measured center frequency of the fabricated filter is changed from 606 MHz to 944 MHz, 338 MHz with the single control voltage from 0.5 V to 4 V. Expect of slight change of the bandwidth, the measured filter characteristics are well reserved with the various control voltage.