Investigation of novel microwave surface-acoustic-wave filter on different piezoelectric substrates

A novel GHz-band low-loss large-bandwidth microwave microstrip, surface-acoustic-wave (SAW) filter used in spread spectrum communication systems was designed and fabricated. The 16 μm input/output-interdigital-transducer (IDT) LiNbO₃ 61.00MHz SAW filter, with microwave square open-loop resonators, has an insertion loss of -3.987 dB. This device can also be used as a 1.064 GHz microwave microstrip SAW filter with four cross-coupled microstrip square open-loop resonators and two planar interdigital capacitors. It was found that this novel device has an insertion loss (S₂₁) of -2.962 dB, a reflection loss (S ₁₁) of -25.497 dB and a 3 dB bandwidth of 800 MHz. The characteristics of these low-loss large-bandwidth (BW = 80%) microwave microstrip SAW filters are influenced by the interaction between electromagnetic waves and piezoelectric SAWs. To confirm this claim, devices of identical design were fabricated on the three types of substrate: i) 128°-rotated YX-cut lithium niobate (LiNbO₃) with a high electromechanical coupling coefficient (K² = 5.3%), ii) GaAs S-I photo-piezoelectric material with a low electromechanical coupling coefficient (K² = 0.07%) and iii) non-piezoelectric SiO₂/Si material with a thickness of 9000 Å. The performances of these devices were significantly different. We will apply the principle of SAWs and the equivalent circuit of IDT to prove our experimental results.