[time-nuts] BeagleBone Black DDMTD Update

Simon Marsh subscriptions at burble.com
Wed Oct 29 16:30:24 EDT 2014

original post had a lot of attachments, these have been uploaded here 
for viewing:


This is a fairly long post, at the top is a bit of description of of
changes since my last posts and then around the middle is some
description of the data thats attached. The data raises a few questions,
and I'll put those in a separate post.


In terms of hardware setup, I now have two 74ac14 schmitt triggers, one
as a buffer for the reference/sampling clock and one as a buffer for the
two test signals. These are followed by two 74ac595 shift registers to
do the sampling and the whole thing is soldered on to a BBB proto cape.
Whilst the cape isn't perfect, it is better than pluggable breadboard.
The good news is that with all those changes I have glitches again, I've
never been so happy to see noise :)

Mr Postman also delivered a nice mv89a and 8663, so these should act as
better references. Along with the hardware, the software has been
overhauled somewhat, to simplify, make it more modular and speed up some
of the analysis.

The net result of these changes is shown in the attached ADEV plot,
which shows the setup measuring a PWM signal from a second BBB and a
Micro Crystal OCXO against the mv89a. Note that this isn't with the
setup working as a DMTD, but simply using the hardware as two channels
measured against the reference independently.

The ADEV is ok, but not great. In theory, the Micro Crystal OCXO should
be good to 5E-11 @ 1s according to the data sheet, so in the OCXO plot,
everything to the left of 10s is almost certainly measurement/setup
problems rather than the oscillator itself. This shows I still have some
work to do.

I've also included a closer look at the phase data, plotted with 3
simple edge detection algorithms (first edge, last edge and mean edge).
Note that you can see visually the difference between first and last
edge and this demonstrates the width of the period containing glitches;
in this case somewhere around 1.5 - 2ns. Also obvious is that there is
some periodicity to the phase data and that the 'last edge' algorithm
appears to be a pretty poor choice as it is way noisier than the first edge.


So, on to more data and and a closer look at whats happening during the
glitch periods.

Each of the graphs attached are histograms, covering approx 500k glitch
periods around rising and falling edges in an hour of data of the Micro
Crystal OCXO with mv89a reference. Both oscillators had their adjustment
pins grounded and the offset was about 66hz between them.

There are 4 graphs showing distributions of:
   - lengths of each glitch period
   - how far each transition is from the start of each glitch period
   - zeros and ones from the start of each glitch period (for all edges)
- red for zeros, green for ones
   - same as above but just for rising edges

The x axis is in units of reference clocks/samples (so ~100ns of real
time, or a vernier of 6.6E-13 of the DUT signal depending on how you
look at it) and 0 is the start of each glitch. The y axis is counting
the total number of glitch periods.

As an example, looking at the distribution of glitch period lengths,
shows the peak at around 2500 clocks/samples. 2500 * 6.6E-13 = 1.65ns
which corresponds nicely with the difference between first and last
edges seen in the phase data graph.



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