[time-nuts] Fwd: Re: Frequency Stability of Trimble Mini-T

[Hal Murray]

> You could always look at: United States Patent US4820939.
> But dont bet on it.

I'd go so far as to suggest betting against it.

Back in the 80s when word about metastability was reaching trade rags and 
real designers, I went to a trade show that had a panel on it.  One guy got 
it totally wrong.  1 or 2 mumbled and didn't contribute much.  The only guy 
who got it right was John Wakerly from Stanford.  (Of course, maybe I drunk 
his cool-aid and not theirs.)

Back in those days, there was a lot of activity in designing kludgey circuits 
to "fix" metastability.  I could usually find the flaw.  It got boring after 
a while.  The classic was a circuit to detect metastability and reset the FF. 
That reset signal would sometimes have runt pulses.

There are several problems with such kludges.  First is that they don't work. 
 Second is that they usually make things worse by adding logic in the 
critical path thus reducing the settling time.  On top of that, they are 
usually a pain to analyze.  (If the designer analyzed them correctly, he 
would have thrown it out.)

It's much better to keep the circuit simple.  Then you have a chance of 
correctly estimating the MTTF.

The key is settling time.


The best analog of metastability that I know of is rolling a ball over a 
speed bump.  If the ball is slow, it bounces back.  If the ball is fast, it 
goes up and over.  If the speed is just right, the ball gets to the top of 
the bump and stalls.  If you have a frictionless bump and such, the ball will 
eventually fall off one side or the other.  (You can't adjust the speed to 
the exact value.)

Johnson and Graham's book on Black Magic has a good section on metastability. 
 I think there are some good scope pictures, but my copy isn't handy.

I used to use metastability as a calibration on logic design books.  If I 
couldn't find metastability or synchronizer in the index I didn't expect 
much.  If I did find something, I would scan that section.  A lot of them 
weren't very good.



-- 
These are my opinions, not necessarily my employer's.  I hate spam.