Longitudinal mode behaviour in surface transverse wave two-port resonators

This paper presents results from a systematic experimental study on the behavior of longitudinal modes in 1.26 GHz extended cavity surface transverse wave (STW) two-port resonators which are well known for their low loss and high Q. The experiment was performed with 8 devices using the same transducer, reflector and waveguide geometry and deposited on the same photomask. The difference between them was the length of the spacer in the cavity between input and output transducer which was altered by a step of lambda/16 from device to device. Groups of 8 devices were fabricated on rotated Y-cut quartz substrates with different film thicknesses and a nearly constant mark-to-period ratio. The longitudinal mode behaviour was evaluated by measurements of the device frequency and group delay responses. The same photomask was then used to fabricate 780 MHz SAW devices. The data from these devices were carefully evaluated and compared with the data from the STW ones. In the second part of the experiment the spacer was kept constant and STW devices with different film thicknesses were fabricated and tested. The results of this study provide a comprehensive guide on how to select appropriate film thicknesses and perform spacer corrections in practical device designs.