Saturday, July 12, 2014

Cascaded SAW Filters (1575MHz)

The second SAW filter board was populated with 1575MHz filters. The EPCOS B3520 has a DCC6C footprint and can be used on the same PCB as the B3711  315MHz filter.  Evaluating the response turned out to be more difficult than anticipated due to leakage from the source into the measurement setup above 1GHz.  There was a lobing baseline present even with no input or no filter.  This behavior was present on both the 7L12, A-B boards, and SA0314.  To avoid it, rather than using a B board as a source, two B boards are mixed using 4Ghz and (4GHz - Fo) where Fo is the frequency being evaluated.  This way no synthesizer is at the frequency of interest.  The following are response scans from an A-B-R stack using Si compared to the datasheet.  The Si units are in absolute dBm while the datasheet is relative to source (dBc).  The limiting factor is the dynamic range of the A-B-R stack along with the limited output level of the source.
Narrow B3520 2X cascade response
Wide B3520 2X cascade response
The passband insertion loss was measured with the 7L12 and SA0314 and is as expected using 2X filter insertion and a 3dB coupling pad.  At roughly 1538 MHz there is a small spike in the response visible in the datasheet wideband response.  This appears to be -33dB below the passband.  In both the narrow and wide A-B-R scans this peak is only -40dB below the passband (in a 2X cascade).  If you eyeball a couple of points midway in the transition you do get a 2X value.  This tells me the rejection is not as much as expected particularly far outside of the passband.  All of the measurements were taken without shielding between stages. 

This has turned out to be more nuanced than the 315MHz measurement.  Given the test equipment at hand (sensitivity, source level control) very low signal levels are difficult to measure and without shields not valid.  The filter appears to work as expected even if I cannot measure every aspect of it I had hoped to. 

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