Quote:
Originally Posted by redwire
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)