[time-nuts] Sneaky Errors

Bill Hawkins bill at iaxs.net
Wed Oct 19 17:53:30 EDT 2011


This is the famous "man who has two watches does not know what time it is"
problem.

Lucent solved it for telecom with the RFTGm (Reference Frequency and Timing
Generator) equipment, consisting of GPS disciplined OXO and Rubidium
oscillator modules that continuously checked each other via 1 PPS and 10 MHz
links. The frame housing the oscillator units selected one or the other for
distribution to six connectors. Sadly, the output is 15 MHz.

Otherwise, the only solution is more watches, preferably by different
manufacturers.

In some ways, this reminds me of the ancient parity check for memory
locations. Parity is not used anymore for commercial computers, because a
memory error is usually not alone, and errors soon make the computer lose
its way and halt. In this case, plan on the TB either running correctly or
failing due to some alarm. Alarms must be monitored, of course.

You could set up a time interval counter to show the phase between the two
outputs. The Racal-Dana 1992 does that at 10 MHz.

Out of curiosity, what would be the consequences of a steadily increasing
phase error? Would it offend your sense of perfection or would it have real
consequences?

Bill Hawkins


-----Original Message-----
From: David VanHorn
Sent: Wednesday, October 19, 2011 2:47 PM

I have two thunderbolts, set up so that I can switch over to the backup unit
if the primary fails.
All is well with that, but what could I do to detect a less obvious failure,
like 9.999999 MHz output?

If they disagree, I don't know how to resolve which is correct.






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