Mobile-coMMunications services require a filter's tuning range to be extended by at least 2:1. Yet such a wide tuning range often yields increased insertion loss at the lowest resonant frequency. To compensate for this insertion loss, designers can leverage an active filter with negative resistance characteristics. Unfortunately, such a filter will require additional area and power consumption compared to passive designs. To solve this dilemma, a varactortuned microstrip bandpass filter with a wide tuning range has been proposed by Jeongpyo Kim and Jaehoon Choi from Hanyang University's Department of Electrical and Computer Engineering in Seoul, Korea.

The proposed filter was designed to have a tuning range of more than 2:1 (500 to 1000 MHz). It has variations in insertion loss from lowest and highest frequencies of less than 3 dB. At the heart of this tunable bandpass filter is a hairpin line resonator and loading capacitors that use varactor diodes. This simplified structure boasts a wide tuning range of 87.9 percent from 430 to 1105 MHz. The passband insertion loss is less than 4.42 dB.

As the resonant frequency is lowered by the direct-current (DC) bias, the diodes' capacitances are simultaneously increased. Yet the coupling energy at the coupled portion is not decreased. An insertion-loss characteristic can therefore be developed. The lowest resonant frequency is 430 MHz with a 4.42-dB insertion loss. In contrast, the highest resonant frequency is 1105 MHz with an insertion loss of 1.524 dB. See "Varactor-Tuned Microstrip Bandpass Filter with Wide Tuning Range," Microwave and Optical Technology Letters, October 2008, p. 2574.