A novel surface acoustic wave filter on a leakywave substrate is studied. It features a hiccup-type resonance occurring around a distributed gap between two long interdigital transducers. Compared to a classical coupled resonator filter, it enables a relatively narrow passband (1% to 2% of center frequency) with low insertion loss, steep skirts, and improved suppression levels. The structure consists of long transducers having the number of fingers greater than 1=K 2 and 1= , where K 2 is the coupling coefficient of the substrate material and is the reflectivity per wavelength, separated with short transducer sections constituting a distributed gap. A strong, localized resonance is formed in the gap region, in addition to the resonance arising in the long structures-hence, the name doubleresonance filter. The substrate studied here is 42 -rotated lithium tantalite. We show experimental results for both single-ended and unbalanced-to-balanced filters at 1.6 GHz, having a minimum insertion loss of 1.07 dB, suppressions of 30 dB, and absolute ;3-dB bandwidth of 29 MHz (1.9% of the center frequency). For the balanced device, the amplitude imbalance over the passband ranges from ;0.6 dB to 2 dB and the phase imbalance from 1 to 4.5 . Furthermore, we have measured the acoustical power distributions using a scanning laser interferometer, and we compare these results with the profiles simulated using a coupling-of-modes model.
Experimental data for both device types are presented, and the possibility of using them as impedance elements in filter applications is briefly discussed.
A novel type of surface acoustic wave bandpass filter is presented. The operation of the device is based on the doubleresonance filter structure in which relatively long interdigital transducers (IDTs) are coupled acoustically. The filter consists of 3 main IDTs on a leaky-wave 42 • -LiTaO3 substrate, separated by short IDT sections with a reduced period (distributed gaps). Electrical input signal is connected to one of the side IDTs and electrical output to the other. The middle IDT is either left open (floating) or grounded. If the middle IDT is floating, acoustic energy is transferred from input to output by the resonances created in the gaps, corresponding to two 2-IDT filters cascaded. When the middle IDT is grounded, it effectively blocks the acoustic wave propagation from input to output. Therefore, the filter can be switched off by grounding the middle IDT. The device features a 3-dB relative bandwidth of ∼2% at 1575 MHz, with relatively low insertion loss (IL) in the passband, steep transitions and high rejection levels. Experimental results show minimum IL of 3.1 dB in the filter passband and suppressions on the order of -50 dB (after removal of electromagnetic feedthrough), with a relative -3-dB bandwidth of 27 MHz (1.7% of center frequency).
Resonant characteristics of longitudinal leaky surface acoustic waves (LLSAW) in infinite periodic interdigital transducers (IDT) with layered electrodes are calculated. An electrode is assumed to be consisted of two parts -under-layer of crystal material and top layer of aluminum. The simplest structure is a periodic array of grooves of rectangular shape etched in the substrate with aluminum (AL) electrodes positioned on tops between adjacent grooves. Such a structure on YZ-cut oflithium niobate (YZ-LN) shows calculated quality factors of two times larger as compared to AL electrodes on smooth surfacewith dissipation in electrodes taken into account. As an additional combination the under-layer of titanium-dioxide (TiO 2 )-AL is considered. This combination shows quality factors approximately three times larger than the structure with AL electrodes on smooth surface.
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