Friday, June 20, 2014

Cascaded SAW Filters (315MHz)

As an experiment, a PCB with two cascaded SAW filters with fence option and 3dB pad separating them was built.  The following is a picture of unit #1 populated but without a fence.
Cascaded SAW board.  3dB pad coupling two 315MHz SAWs with shield mounting outline.
The filters used are EPCOS B3711 centered at 315MHz.  These come in a DCC6C package.  The advantage is that there are multiple frequency ranges available in this package allowing the PCB to be re used.  In addition it is large (on a relative basis) providing easier handling and assembly.

The wide band response of a single filter from the datasheet is shown below.

A quick simple setup was used with both the 7L12 and the Si application with single A-B stack to scan the filter response.  Neither device has enough dynamic range to scan the desired response range, however, it is a good sanity check and quick to do.  Those results are shown below.

Measurements show a passband insertion loss of 7dBm.  This aligns nicely with the datasheet insertion loss of 1.7dB for a single filter (2x for the cascade) with a 3dB coupling pad. The lower end peak at ~309MHz is 50dB down for a 2x cascade while the datasheet indicates -25dB for a single device.  All of this tells me the devices are not interacting badly and pulling each other.  In short, this is working quite well.

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