[time-nuts] Frequency Stability of Trimble Mini-T

[SAIDJACK at aol.com]

Hi Hal,
 
>First, we understand metastability.  We can measure it and  predict it.  It 
it 
>mattered, we could test each  individual part.
 
That was exactly my original point. The discussion got off topic, I  just 
wanted to point out that it is a matter of probability, and the math to  
calculate this probability of failure is well known and understood.
 
The math says that you cannot prevent a metastable event, even if  there are 
1000 flip flops. The probability is never zero. And since the  processes are 
stochastic, the first failure could happen within 2 seconds of  power on, even 
with 10000, or a million cascaded flip flops. You can never  prevent a 
metastable event going through all of the flops. You can just make  that probability 
approach zero. Actually, if you had a million cascaded flip  flops one of them 
would probably fail due to other reliability reasons  :)
 
>Second, the failure mode is exponential in a parameter we can  control.  So 
>given a particular set of parts to pick  from, it's reasonable to make a 
>design with a probability  of error small enough so that other things are 
much 
>more  important.
 
Right! My original point again. The question was: how often do  crystal jumps 
happen. How often do metastable events happen. Both may be just a  matter of 
statistics and probability.
 
If we don't see a crystal jump within say one month of operation, we  can 
probably say the probability of one happening is extremely low.
 
But I have seen jumps happen after 3 months of continuous, documented  
operation without any jumps, so again we may never have certainty that  
jumps/metastable events will not happen. We just have to make them happen so  seldomly 
that they won't matter to the application at hand. But again if it  jumps after 3 
months, no one can guarantee it won't jump after 10  minutes.
 
bye,
Said



  
____________________________________
 From: SAIDJACK
To: SAIDJACK
Sent: 10/16/2008 16:43:33 Pacific Daylight  Time
Subj: Fwd: [time-nuts] Frequency Stability of Trimble Mini-T




>From iPhone

Begin forwarded message:



From: Hal Murray <_hmurray at megapathdsl.net_ (mailto:hmurray at megapathdsl.net) >
Date:  October 16, 2008 15:40:12 PDT
To: Discussion of precise time and  frequency measurement 
<_time-nuts at febo.com_ (mailto:time-nuts at febo.com) >
Subject:  Re: [time-nuts] Frequency Stability of Trimble  Mini-T
Reply-To: Discussion of precise time and frequency  measurement 
<_time-nuts at febo.com_ (mailto:time-nuts at febo.com) >







We have done some extensive research trying  to correlate jumps to


external phenomena (radiation, vibration,  thermal effects, etc).


There is no straight forward  correlation.



It's similar to asynchronous switching  inside a digital computer.


You can add levels of flip flops to  synchronize across two


asynchronous time-domains,  ...


I think that's  misleading in two ways.

First, we  understand metastability.  We can measure it and predict it.  It  
it 
mattered, we could test each individual  part.

Second, the failure mode is  exponential in a parameter we can control.  So 
given a  particular set of parts to pick from, it's reasonable to make a  
design with a probability of error small enough so that  other things are 
much 
more  important.



If it absolutely, positively can't take any  hit, then some more work is  
involved


I would say that the  first step is to put a number on "absolutely,  
positively".

If you aren't  willing to take some risk, you won't get off the drawing  
board.



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




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