[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

More information about the time-nuts mailing list