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Old 10-26-2019, 04:22 PM   #11
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Join Date: Nov 2011
Posts: 2,302
Default Re: Improving a slow/fast digital clock

Originally Posted by redwire View Post
I still find mains clocks beating cheap quartz crystals for drift.
That depends on the environment and application. E.g., an XTAL deployed in an environment where there are few temperature variations is relatively stable (like a wristwatch that is ALWAYS worn).

I have used an LFC in virtually all of my designs (medical instruments, process control systems, etc.) going back to the early 80's as an accurate (stability != accuracy) and (long term) stable time/frequency base. There's very little cost/space to implementing it. It also gives you an early warning indicator for power failure and, for battery-backed devices, lets you know when you're operating off that battery (without requiring a separate signal from the charger)!

I knew that I could accurately measure time intervals with it (using the local oscillator as described above for fine granularity and to eliminate short term drift) and base measurements/controls on those observations -- without the cost/space that a TCXO would require.

[TCXOs primarily find use in my maritime products -- huge temperature extremes, poor power quality, absence of local technical support]

Knowing the utilities were required to provide this level of service made it a safe design bet.

Now, the LFC only has value in frequency locking to the instantaneous mains frequency (to improve SNR, reduce display beating, etc.) -- i.e., I want to SEE the actual variations in line frequency and exploit them!

Thankfully, there are other inexpensive sources of time (inverse frequency) information that are now available for many applications (e.g., NTP and PTP for network-connected devices) so you can still avoid the need for a LOCAL precision timebase. (I use a bastardized PTP here to ensure the clocks in my devices track to within a fraction of a microsecond across scores of nodes)
Curious.George is offline   Reply With Quote