Double-resonance SAW filters.
A novel surface acoustic wave filter on a leaky-wave 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/K2 and 1/kappa where K2 is the coupling coefficient of the substrate material and kappa 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 double-resonance filter. The substrate studied here is 42 degrees-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 degrees to 4.5 degrees. 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.
Meltaus
Johanna
J
Helsinki University of Technology, Espoo, Finland. Johanna.Meltaus@tkk.fi
Hong
Seong Su
SS
Holmgren
Olli
O
Kokkonen
Kimmo
K
Plessky
Victor P
VP
D000162:Acoustics / Q000295:instrumentation*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*; D013499:Surface Properties; D014159:Transducers