Tuesday, January 14, 2014

C Board First Tests

The C board is finally constructed and ready for evaluation.  The first BC board (unit #1 used in previous tests in a B configuration) was re-worked as a C board, the second BC board (unit #2 recently populated) configured as a B board.  In addition, both boards were mounted on A board unit #2.  This A board is configured with lower gain (and noise) than the A board used in previous tests.

After a cursory check of the synthesizer, mixer and amplifier I decided to make sure the SAW filter is acting as expected.   Since the mixers are single ended input and output the first LO will appear on the output of the first mixer.  This is used to estimate the filter response and act as a built in test.  The figure below summarizes the measurement setup. LO0 is swept across the frequency range of interest with LO1 set at 10.75MHz above LO0.  The C board IF output is connected to a 7L12 spectrum analyzer for manual measurements and absolute level evaluation or an A board for multistep scans. ( yes this would all be better done with automated test equipment, none of which I have or can afford ;-)
 Below is the filter response from the data sheet.
The following are two excel graphs from the Beagle Bone Black application software (it steps the frequencies, grabs the power estimate at the step and puts out a CSV file which is readily graphed in Excel).  The first matches the data sheet frequency span and center, while the second doubles the frequency span and catches the outer skirts.

In addition to the above, similar measurements were taken manually with the 7L12 for comparison and validation (albeit with many fewer points).  Those results match the above.

In comparing these results to the data sheet a couple of issues are apparent.  First the response on the upper side of the pass band is attenuated and not flat.  This matches well with the 7L12 measurements using different final IFs.  I believe this is an interaction between the amplifier, filter, and mixers.  The mixers are configured as single ended input and outputs with resistive termination (i.e. resistors for current source/sink).  This was done on purpose for simplicity and broad band application.  The down side is that the impedance presented at the inputs and outputs at higher frequencies is not constant or purely resistive may be skewing the filter response.

The second artifact present is that the stop band attenuation level is less than specified by the data sheet.  Again, these results match with manual 7L12 measurements.  I believe there is a certain amount of bleed through of LO0 into the second stage mixer around the filter.  This provides a lower power level floor at a given step which the filter cannot attenuate below.  Further measurements are required (this would not be a surprise as the RF isolation between the boards is not what it would be in a shielded environment).

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