Tunable rf bandpass pass filter with variable resonator coupling

A new tunable filter technique is presented in which the coupling between adjacent resonators of the filter decreases as the tuning frequency is increased thereby maintaining almost constant bandwidth across the tuning range. Varactor diodes can be used for continuous tuning or switched capacitor banks can be used for discrete tuning. Electromagnetic (EM) simulations using low temperature co-fired ceramic technology (LTCC) type resonators have shown that a 50 percent tuning range is achievable with bandwidth variations of +/− 5 percent using this technique. A three pole Chebyshev filter with varactor tuning was bread boarded exhibiting a 17 percent tuning range from 481 to 565 MHz, a 3 dB bandwidth of 106 MHz and a pass-band insertion loss of less than 1 dB.