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Old 28th September 2009, 11:23 AM   #1
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Default GPS Master Clock for Digital Sources

This is way out of my comfort zone, but it seemed intriguing idea to me. Crossover of technology between fields of endeavour is always interesting.

I was at the NAS near Silverstone at the weekend and I got talking to a telecoms engineer. He wanted to know if anyone had used a GPS ( Don't laugh!) derived master clock in a disc player.

After all, quartz is by no means perfect and the GPS has incredible timing stability and accuracy. He said that all telecom networks are timed this way. I found AD do something that could be within reach of the DIY-er, but I'm too ill educated to do something with it or assess its worth. Over to you really clever guys....


http://www.analog.com/en/press-relea...GPS/press.html
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Old 28th September 2009, 06:03 PM   #2
oshifis is offline oshifis  Hungary
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That's right, GPS master clock is an alternative to Rubidium clock in telecom (namely SONET/SDH) world. Accuracy is referred to as Stratum 1 (best) to Stratum 16 (free running).
However, absolute accuracy is not an issue in audio, and I don't know what are the jitter specs of a GPS clock. It is PLL-based I think, so it could be nothing very special.
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Old 30th September 2009, 03:03 PM   #3
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The AD press release implies that GPS is good for 1 pps and the AD9548's PLL will improve that to 0.3 pps. Thats ten times better than the best crystals isn't it, or am I missing something?
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Old 1st October 2009, 12:34 AM   #4
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What matters is the phase noise spectrum: the jitter is obtained by integrating over the frequency range of interest. For digital audio reproduction, we care about jitter down to 10Hz or so, whereas most jitter specs are integrated from 12kHz-20MHz. Interestingly the AD9548 datasheet has data down to 100Hz.

No matter how clean the GPS clock is, it still needs to be recovered at the receiver. This is done by a PLL, which by its nature sets a limit on the phase noise level achievable.

Look at the spectrum charts on pages 18 and 19. The performance @ 100Hz is somewhere from -90 to -100 dBc/Hz. Now look at Figure 13 on page 20. The gray line is the phase noise of an OCXO, which is basically a crystal in a fancy climate-controlled oven. That's pretty much the best a crystal can do. OCXOs are pricey though, so compare to -110 dBc/Hz for an off-the-shelf VCXO:
http://www.crystekcrystals.com/cryst...o/CVHD-930.pdf
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Old 1st October 2009, 07:15 AM   #5
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Thanks Mako, I still think it would be a marketing man's dream though!
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