Novel LSAW DMS filter structure for narrow duplex gap SE->Bal RX filter application

Design of the Single ended ->; balanced (SE->;Bal) receive (RX) filter for ultra narrow duplex gap duplexer application (PCS, EGSM, UMTS Band 2 and Band 8) is one of the critical tasks in the LSAW (leaky surface acoustic wave) industry. Extreme difficulty of this task is caused by the duplex gap width (<;1.05% for PCS band) and poor temperature properties of LSAW piezoelectric substrates suitable to the necessary passband width and insertion loss (IL). The TCF (temperature coefficient of frequency) of BAW (Bulk Acoustic Wave) filters, comparing to LSAW devices, is twice better, so LSAW filters on 36-48 LiTaO3 have to have much steeper left transition band for the same specification requirements. At the same time application of BAW and Film Bulk Acoustic Resonators (FBAR) RX filters have their own issues. Duplex gap of 20 MHz between signals of transmit (TX) and RX bands over the full temperature range (from -30°C to +85°C) requires design of the LSAW B2 RX filter with transition band Δf(3.5/50)<;13.0MHz to have minimal margin for frequency shift over working temperature range. Currently, there is no known SAW filter structure suitable for such a tough specification. The typical approach is to use some known structure along with expensive temperature compensating (TC) technology or (and) to reduce working temperature range and use a filter for UMTS or W-CDMA application only. In this work we use well known, having good rejection and simple layout "2 track 3->;4 IDT DMS (double mode SAW)" structure, with two critical modifications: 1. we added long (12-30λ) grating between output inter digital transducers (IDT) with period slightly smaller (by 0.5 1.5%) than period of edge gratings to improve left passband edge IL and left transition band steepness and 2. we use capacitive matching of reradiating (RR) passes to improve loading conditions between acoustical tracks and filter's input and output impedances (return loss (RL) performance). Such an extra capacitance can be implemented into a filter's layout in several different ways: 1. by using overlapped stripes (straight or curved) between RR passes; 2. by using overlapped stripes (straight or curved) between RR passes and grounded metal areas in the filter layout; 3. by using of 2-port resonator in RR passes between input and output acoustical tracks provides necessary cap and improves right transition band steepness; 4. by using specific width crossover connection. Results: A B2 RX filter based on proposed approach was designed. It has significantly improved left transition band steepness Δf(3.5/50)= 12.0MHz for most critical temperature t = 30°C, improved by 0.4 dB left passband edge IL, near band rejection of more than 50 dB and passband RL over temperature of 11.0 dB. Such a filter is fully compatible with standard LSAW technology and even for PCS (UMTS B2) RX applications does not need any expensive TC techniques. Size (0.8X0.9 mm2 before dicing) and cost of such a filter are extremely competitive. Such a SE->;Bal SAW structure can be used in the full temperature range for ultra narrow duplex gap duplexer along with BAW (FBAR) TX filter to provide enough power handling in the TX band.