Saturday, June 13, 2015

Prj137 ADC Spur Clusters

The spur clusters from the previous work proved to be an evasive problem.  Very little changes the location or the level. The structure seems to indicate there are many harmonics present based on the clustering.  To better understand this I walked the source up and down to see which ways the clusters moved (i.e. in even or odd nyquist intervals).  A capture of that is below.
Spur cluster walk from 10.640MHz up in steps of a few kHz.
Using the even/odd interval, I then stepped the fundamental by a few kHz for several steps and captured the peak of each cluster.  A fundamental delta of 10kHz moves the clusters 10kHz, not some multiple.  This told me I was looking at a modulation, not a set of harmonics of something.  If you put all of this into a spreadsheet and play around ruling out things based on even/odd nyquist and unrolling a given peak to all of the frequencies it could be you get the following table.
Spur cluster peak unrolling across Nyquist intervals
The cluster peaks are under "ADC (MHz)" and the color codings of cells match up the expected frequency the cluster represents after folding.  The rows at the bottom capture the delta in MHz of the peak from the fundamental at 10.640MHz.  So for the highest peaks of the spur clusters they appear to be 2MHz from the fundamental and appear to me to be switching power supply noise, evaluated here.  The lower spurs within the cluster would then be the AM modulated signals of the higher harmonics of the 2MHz switching impulses folded around the Nyquist regions.

Indeed, you can do the same thing at a lower sampling rate.  This both shows the fine scale structure and the harmonic spurs and verifies the above unrolling at a different set of frequencies. The following is a 2.5MSPS capture of a 10.640MHz input signal.
10.640MHz input signal using 2.5MSPS showing the spur cluster internal structure.
Numerous attempts were made to ameliorate the clusters.  This included adding/changing the supply chokes and bypass coupling.  As previously, these made no impact.  Additional ground ties were added, and removed - again with no impact.  The next logical step (in my mind) is to understand if these are coming from the amplifier or the ADC and to remove as much of the circuitry as possible.  This means backing up to a just the ADC and a transformer input - basically mirroring the ADC evaluation unit circuit.

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