Loss Reduction in Coupled Resonance Surface Acoustic Wave Filters Operating at Ghz Frequencies

s of Publications I–VI I The concept of coupled resonators is applied to synthesize surface-acoustic-wavefilters. Employing two parallel-connected filter tracks, with a frequency shiftimposed between them, a wide passband with low insertion loss together withwell-controlled rejections is achieved. The operation of the two-track device isbased on the mutual interaction of the individual transfer functions for the pairof tracks. Each track serves to contribute a part of the passband, enabling a wideband. Outside of the passband, the signals passing through the two channelsmay cancel each other, thus facilitating efficient control over the rejections. How-ever, obtaining rejection stopbands at just the predetermined frequencies requiresprecise values for the materials parameters and a reliable fabrication process.Prototype devices fabricated with this approach are demonstrated both on quartzand – for the first time – on42◦-LiTaO3. Results for two-track devices havingeither two or three transducers per track and operating either single-ended orwith a balanced output are presented. The devices are designed employing thecoupling-of-modes model and transmission-matrix approach, and the separatetracks are optimized simultaneously and independently. The center frequenciesare 868 MHz and 1960 MHz. On quartz, a minimum insertion loss of 4 dBand a passband width of 0.23% are achieved at 868 MHz. On42◦-LiTaO3, thecorresponding figures of merit are 1.3 dB for minimum insertion loss and 4.1%bandwidth at 1960 MHz. The filters on42◦-LiTaO3 also have remarkably flatpassbands. II Longitudinally coupled resonator filters provide unbalanced-balanced operationwith wide bandwidth, low loss and high suppression levels. However, reducingthe insertion loss in the 1.8-2.2 GHz range remains a challenging problem becauseat high frequencies the resistive losses arising from the relatively wide apertureof the filter may degrade the performance. A 5-interdigital transducer (IDT)filter has six gaps at which the periodicity of the grating is broken, resultingin additional loss due to scattering into the bulk. In this paper, we show thatreplacing the gaps between the transducers with short transducer sections havingtheir pitch different from that of the main transducers reduces the insertion lossof the device. We present devices with balun operation at 1842 MHz with widebandwidth of 4.5% and −40 dB suppression, with a minimum insertion loss lessthan 1 dB in the best devices, and a maximum insertion loss of −1.2 dB inthe passband. The passband is quite flat, with < 1 dB ripple. We also discussthe layout of the contact pads and the connections and its effect on the deviceperformance and balance characteristics.