[time-nuts] Brooks Shera

David McGaw n1hac at alum.dartmouth.org
Mon Mar 25 15:37:11 EDT 2013

Minimum clock width is not the window for metastability.  That is 
usually 10s to 100s of picoseconds.

On 3/25/13 3:20 PM, Attila Kinali wrote:
> On Mon, 25 Mar 2013 11:18:06 -0700
> Chris Albertson <albertson.chris at gmail.com> wrote:
>>> Ie the output of the counter becomes
>>> (more or less) random. Which in turn means the lower 4 bit of the
>>> input to the PI control loop are wrong[1]. Or in terms of time, we
>>> might be off by +/-2^4*42ns=672ns, which is a major hit against the
>>> PI loop (like knocking it with a sledge hammer).
>> But these numbers don't go directly to the PI loop, there is a
>> software layer between that can inspect for outliers.
> The problem here is that these outliers need not be a seldom occurence.
> If you take a LEA6-T as PPS source, you get the PPS pulse within +/-10ns
> in 90% of the cases. The 24MHz clock is, even though it's a cheap
> crystal, still a crystal. So you can assume a stability of better than
> 10^-7 short term (couple of seconds to a few minutes), probably in the
> range of 10^-8 to 10^-9.
> The window for metastability is 15ns (minimum clock pulse width).
> Now we have two things to consider:
> 1) Both the clock and the PPS are relatively stable compared to
> the "size" of the 15ns. Ie if we ever get into region of metastability,
> the probability that one of the next PPS will trigger again a metastable
> condition is relatively high.
> 2) We have a 24MHz clock, which means the clock pulse width is
> 20ns (T=1/24MHz, T/2 = 20ns). Ie the probability that our clock pulse
> is too short is 75% (15ns/20ns).
> I dont know why this isnt a big issue with the Shera GPSDO.
> I guess that the specs of the 74HC4520 are very conservative
> an that the minimum clock pulse length is rather in the range
> of 5ns than 15ns for most devices and operation temperatures.
> It might also be that the probability of bits being affected
> is less for the upper bits than for the lower bits.
> Or the PI loop is stable enough to deal with the additional noise.
> 			Attila Kinali

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