Hi It is not at all unusual for signals to be re-clocked when going into a micro. Often the documentation on this process is somewhere between vague and non-exsistant.
Bob > On Oct 29, 2014, at 4:15 PM, Simon Marsh <[email protected]> wrote: > > 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. > > Cheers > > > Simon > > <ADEV.png><Glitch Period Lengths.PNG><Phase_CloseUp.png><Transitions > Distribution.PNG><Zero-One Distribution (All Edges).PNG><Zero-One > Distribution (Rising > Edges).PNG>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
