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

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Oct 16 23:57:11 UTC 2008


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






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