[Dprglist] Stability of CPU oscillator

[John Swindle]

Kip,
Thanks for writing.
As you probably read, Murray and Dave also recommended GPSDO, which I had rejected for size, weight, and cost, but Murray pointed me to a tiny, light, affordable GPS receiver with PPS. And then I realized I could characterize the CPU's TSC and use TSC without an external reference.
Then I realized I was calibrating the wrong reference: TSC tells me when the data are processed, not when they are sampled. TSC can tell me precisely when the processing takes place, but that is irrelevant. TSC is great for benchmarking, but I need a timestamp. That timestamp already exists:
Playrec, which uses portaudio, counts every sample coming from the audio adapter, regardless of whether the sample is processed. That means that "lost" samples are also counted, so the sample count is correct. The sample count is my timestamp. (Playrec also has an extremely coarse getStreamStartTime command, but its resolution is whole integer seconds.)
I am not sure whether playrec/portaudio uses the audio adapter's clock or whether it resamples using the CPU's clock. In my application, that's irrelevant because each audio adapter is on a separate CPU, so whichever clock is being used is the clock that I need to characterize. So now I will characterize the sample rates of the three audio adapters in the localizer and I think I'll have what I need.
Thank you all for getting me to think about what's really needed. I was about to add unnecessary hardware and collect unnecessary timing information. The existing parts of the localizer already have what I need. I just didn't see it before.
In the future, if I need an accurate, cheap, small, lightweight timing reference, I will look at GPS PPS again.
Later,John Swindle

-----Original Message-----
From: Kipton Moravec via DPRGlist <dprglist at lists.dprg.org>
To: dprglist at lists.dprg.org
Sent: Mon, Jun 8, 2020 3:39 pm
Subject: Re: [Dprglist] Stability of CPU oscillator

 Look at a GPSDO GPS Disclipined Oscillator if you need more accuracy (and more cost) 
 
 https://en.wikipedia.org/wiki/GPS_disciplined_oscillator
 
 Some quick examples in the $50 to $80 range. 10 MHz
 
https://www.digikey.com/product-detail/en/mikroelektronika/MIKROE-1032/1471-1009-ND/4495374?utm_adgroup=Evaluation%20Boards%20-%20Expansion%20Boards%2C%20Daughter%20Cards&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Product_Development%20Boards%2C%20Kits%2C%20Programmers_NEW&utm_term=&utm_content=Evaluation%20Boards%20-%20Expansion%20Boards%2C%20Daughter%20Cards&gclid=Cj0KCQjww_f2BRC-ARIsAP3zarGwf8Rd71SgPkxbg_0s2pf5COe-2X_HmcmN8cce9yr8Wgjz1RYF7v0aAn3cEALw_wcB
 
https://www.digikey.com/product-detail/en/sparkfun-electronics/GPS-11858/1568-1151-ND/5673737?utm_adgroup=RF%20Evaluation%20and%20Development%20Kits%2C%20Boards&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Product_RF%2FIF%20and%20RFID_NEW&utm_term=&utm_content=RF%20Evaluation%20and%20Development%20Kits%2C%20Boards&gclid=Cj0KCQjww_f2BRC-ARIsAP3zarGq8wQ9V83W1qbV6_vM8a6ndNj-zvWG1dwi5Rn1boW5lpTu2B_BTpkaAohUEALw_wcB
 
 No prices but you can find them probably on Mouser or Digikey. 
https://www.microsemi.com/product-directory/clocks-frequency-references/3826-gps-disciplined-oscillators-gpsdo
 
 Finally you can see what is at EBAY when looking for a 10 MHz reference.
 
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR3.TRC2.A0.H0.X10+MHz+reference.TRS0&_nkw=10+MHz+reference&_sacat=0
 
 Kip
 
  On 6/4/20 9:55 PM, John Swindle via DPRGlist wrote:
  
 
Folks: 
  What is the typical stability of the oscillator in a consumer x86 platform? 
  I don't find that specified for systems, or I don't know where to look for the spec. The part numbers on the crystal often don't seem to be sufficient to find the stability in the XTAL datasheet. 
  I don't have access to test equipment to measure it. 
  I appreciate any help, but just to be clear: I'm asking about the stability, not the accuracy, so it would be ppm of drift and/or jitter over time, not ppm of accuracy. 
  I intend to use the invariant TSC in x86 processors, but if the oscillator is not stable, then the invariant TSC numbers will not be as useful to me. 
  Thanks. 
  John Swindle 
   
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