Saturday, April 12, 2014

Filter response with separate B/C boards.

The Si software was updated to program the C board aux output via the network while using a single stage down conversion with a B board, and an A board to scan the power (block diagram in this post is still valid, but with A and B on one BBB and C on a separate BBB).  The last cavity filter constructed was then scanned to produce the figure below.  The vertical scale is in dB (but to convert to an absolute dBm takes some calibration and calculations that I haven't incorporated into the software yet).
The gold line is the filter response.  The filter is removed to leave an open input and output producing the lower green line.  The C aux output is fed directly into the B RF input to produce the upper blue and green lines.  Here the baseline response (with nothing connected to the B RF input port) is nice and flat as opposed to the lobing behavior when the B and C boards are co-mounted (see previous posts on RF leakage and first measurement of this filter).  The waviness in the full scale response with the C board directly connected to the B board is new to me.  I haven't observed this previously but may not have been able to notice this fine structure when the boards were mounted together.  The piecewise linear segment nature and rough frequency span seem to indicate these are VCO bands within the synthesizer.

This response was then compared to the filter response taken manually using a spectrum analyzer.  That comparison is below.
This is encouraging (especially relative to the original response using a co-mounted B/C board as in the previous post).

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