A Spurious free VHF front end SAW Filter for Satellite AIS

Satellite based Automatic Identification System (AIS) is a potent tool for global maritime surveillance. AIS operates in the Very High Frequency (VHF) band of frequencies over a narrow band width of 25 KHz. The signals transmitted by ships carrying AIS transceivers are received by AIS satellites orbiting in Low Earth Orbit. The band selection in the satellite payload is typically achieved by the use of a precise narrow band Surface Acoustic Wave (SAW) filter. This paper highlights the challenges faced during the design and realization of such a narrow band SAW filter and presents ways to overcome them. Third harmonic technique is used for filter design and its advantages from a filter fabrication point of view are discussed. However, the use of this technique deteriorates the far out of band spurious performance of the filter. Techniques implemented to achieve a spurious suppressed filter response, in order to meet the overall spurious specifications of the payload, are presented.