A Zero TCF Band 13 SAW Duplexer

RF spectrum is becoming a very sparse resource and several systems have to coexist while separated by small frequency gaps. For example, public safety band NS07 is 2.5 MHz below the LTE band 13 TX band. Standard SAW and typical Temperature Compensated SAW (TC-SAW) filters do not provide enough rejection in this band, especially within the required operating temperature range. The only way, when not using an NS07 compliant duplexer, to avoid noise radiation into the public safety band is to reduce their transmitted power when the public safety network is activated. To be compliant with NS07, the band 13 TX filter needs to have a very steep transition. Therefore, high quality factors as well as low temperature coefficient of frequency (TCF) are required. To reduce the TCF, the current standard approach is to bury the electrodes inside a silicon oxide overlay. Since silicon oxide has a positive TCF while piezoelectric substrates have negative TCFs, compensation is possible. Usually, TC-SAW filters on lithium niobate substrates have a TCF in the range of -20 to -25 ppm/°C. This corresponds to a frequency shift of 2 MHz at 800 MHz for a temperature range of 100°C. This is too large for applications requiring steep transitions bands. A band 13 duplexer meeting the NS07 rejection requirement is presented. The substrate utilized is lithium niobate with an orientation close to Y+128°. The oxide thickness is increased enough to obtain full temperature compensation. Piston mode transducers are used in order to suppress the transverse modes and to reduce the losses. By increasing the silicon oxide thickness, it is possible to reduce TCF toward some targeted ppm/degC value. The drawback is a reduction by about 40% of the coupling factor and thus, of the possible relative bandwidth. However, band 13 is narrow enough to be compatible with this approach. The resulting temperature compensation produces a frequency shift smaller than 200 kHz p.p. within the 100°C temperature range. Quality factors in the 1500 range are obtained. Typical results are a rejection of 25dB in the NS07 band within the temperature range with an insertion loss better than 3dB. It is understood that this duplexer is the first band 13 SAW duplexer meeting the NS07 rejection specification.