Thursday, June 25, 2015

ADC without Amplifier

In an effort to evaluate the ADC harmonic distortion and spur clusters I decided to create a variant of the ADC board which did not have the amplifier (earlier version).  This allows me to revisit the ground plane, add resistors to the digital lines and re-layout the sample clock signal.  It ends up being as close to the ADC reference design as possible.
BeagleBone ADC board with transformer input.
The first unit anti aliasing filter was not populated to allow evaluation at multiple frequencies.  Two different regulators were tried - the original 150mA and a 300mA version.

Constructed ADC board with transformer input and no amplifier. ADC anti-aliasing filter not populated.  TCXO in upper right, input transformer at lower right, ADC right of upper center, regulator and reset on left.
As in previous measurements, a DDS at 10.640MHz was used as the input.  This was filtered through a 2x 10.7MHz ceramic filter.  Without the amplifer this setup was not able to drive full scale, rather -3dBFS.  That spectrum is shown below.
-3dBFS 10.640MHz input
HD2 is still -79dBc and the spur clusters are present at the same levels and locations as the same version of the board with amplifier ( HD2 compare and spur cluster compare).  The grounded input spectrum looked similar to the previous versions with the amplifier (flat at -107dBc with small clusters at 2MHz and 4MHz in the -100dBFS range).

The continued presence of the spur clusters is disappointing but I am out of ideas on the root cause and decided to put it away for a while and move on to other topics.

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.