[time-nuts] Frequency Stability of Trimble Mini-T
John Miles
jmiles at pop.net
Thu Oct 16 21:27:20 UTC 2008
Couple of (somewhat naive) questions here:
> It's similar to asynchronous switching inside a digital computer.
> You can
> add levels of flip flops to synchronize across two asynchronous
> time-domains,
> but all you are doing is decreasing the possibility of a
> meta-stable failure to
> make it through the flops. Statistically you can never guarantee
> that there
> won't be a failure at all; even if the MTBF is 10 Million years
> by using five
> levels of synchronization etc, a failure could actually happen
> after 10s of
> operation.
Well, no, proper domain synchronization doesn't just give you an incremental
advantage. The use of flip-flops between clock domains is done to trade
latency for guaranteed stability. The idea is to isolate the effects of
metastability to a single clock edge that won't be used to clock anything
else. Unless a metastable event somehow lasts more than one clock period
(or half-period) it won't constitute a failure... and that never happens in
practice, in the absence of a hard failure. Correct? Or am I missing
something? (e.g., are we talking cosmic-ray hits, which are much more
likely to affect RAM elements than clock synchronizers?)
> The good news is: there are mitigating factors. Most jumps are in
> the E-011
> or E-010 range, and most applications won't even be affected by such an
> extremely small frequency change, and we have GPS to quickly correct the
> aberration.
Is it a good idea to tie a crystal to GPS with such a wide loop bandwidth?
GPS-locked loops are usually on the order of t=1 minute, right? Or do you
use 'speedup' tricks to temporarily widen the loop bandwidth when you see a
fast transition? That sounds reasonable as long as there aren't any GPS
propagation aberrations or timing-receiver artifacts that can't be
distinguished from a crystal jump.
-- john, KE5FX
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