Wideband ladder filters fully covering digital TV band based on shear horizontal plate wave

A cognitive radio terminal using vacant frequency bands of digital TV channels, i.e. TV white space, strongly requires a compact tunable filter covering a wide frequency range of the digital TV band. The authors proposed a new bandwidth- and frequency-tunable filter combining an ultra-wideband ladder filter and tunable band rejection filters. T-type and ρ-type ladder filters composed of ultra-wideband 0th shear horizontal mode (SH₀) plate wave resonators were fabricated on a submicron thick LiNbO₃ plate supported by a Si substrate. The wavelength ratio of the parallel/series resonators in the ladder filters was changed as a design parameter. As a result, ultra-large 6 dB bandwidths of 41 to 51%, which fully cover the digital TV band, were measured. The lowest peak insertion loss was low as 0.8 dB. The ultra-wideband ladder filter is useful for the above-mentioned new tunable filter for digital TV cognitive radio communication.