High attenuation SAW IF filter for digital terrestrial TV

In digital terrestrial broadcasting system, adjacent high-level signals of coexistence analog system act as a spurious signal, which causes the generation of intermodulation and a bit error after digital demodulation. Therefore, a band pass filter with a large out-of-band rejection is required in IF stage in digital TV receiver set. Generally, for a high-rejection transversal SAW filter with apodized transducer, overlapped length of comb finger tends to be very narrow in side-lobe portion of apodized transducer. In spite of compensating diffraction effect, the high suppression level cannot be realized due to second order effects in such narrow overlapped region. The developed SAW filter has dual-track structure. It consists of a main track of conventional apodized IDT and a sub truck of just narrow overlapped IDT. The total aperture length increases only about 5% to conventional single-track structure. The sub track set inversion phase with main track. The developed filter achieves 50dB attenuation level with 31-lambda total aperture. The improved attenuation level is successfully realized with the same chip size.