[time-nuts] Fury Realhamradio listing

SAIDJACK at aol.com SAIDJACK at aol.com
Sun Apr 29 20:05:17 EDT 2007


 
In a message dated 4/29/2007 16:40:44 Pacific Daylight Time,  
bruce.griffiths at xtra.co.nz writes:

>Try  studying a little history, its been possible to achieve 25 
>picosecond  accuracy and resolution for over 30 years.
>Such resolution is routine  in Nuclear instrumentation. State of the art 
>nuclear instrumentation  strives for subpicosecond resolution and accuracy.



Hi Bruce,
 
never doubted that it was technically possible to get this type of  
resolution/accuracy. I myself mentioned the 15 year old Wavecrest units achieve  800 
femtoseconds resolution, single shot.
 
The point was
 
A) that type of resolution is not needed in a TI unit where the intrinsic  pk 
to pk noise on the TI intervall is >100ns (more than three orders of  
magnitude above 100ps).
 
B) that implementing it with that kind of resolution and getting a  
meaningful accuracy (say 250ps 6-sigma accuracy) is not easy while at the same  time 
keeping the cost to "Three-four transistors and a handfull of caps and  
resistors." In mass production a handfull of caps and transistors/resistors cost  less 
than $0.20.
 
Again if it was that easy and cheap, HP would have done it in their 5334A's  
or even the 5335A for example which have 1 or 2ns resolution I  believe
 
SRS would have given us 100ps resolution on their PRS10 time-stamping input  
- what better place to do it than in a highly-accurate frequency  reference.
 
>The reason that the 53132A doesn't have resolution and accuracy better  
>resolution than 150ps, is that a design choice was made to implement it  
>all (counters plus interpolators) in a CMOS chip using the delay of a  
>CMOS inverter to set the resolution. This reduces the cost  and 
>complexity significantly and allows faster cycling of  the interpolator 

Bingo. QED.
 
People choose not to do 100ps resolution in their  products because of cost 
and complexity, even in >$3K products such as the  53132A - let alone in $750 
products.
 
C) I don't believe the Z3801A has 100ps single shot resolution and accuracy  
(for resolution doesn't do anything without accuracy) until someone will  
prove it to me. And even then it would be wasted resolution since the GPS 1PPS  
source noise will totally swamp out any benefit a 100ps resolution would  give.
 
On top of that, all GPSDO's do heavy averaging of this time intervall, with  
a PRS10 typically doing 7 hours or more of averaging. 100ps per-second  
resolution in that kind of averaging window is meaningless, since the OCXO  cannot 
perform that well - it would require 4E-015 stability in a 7 hour window.  Not 
possible without a high-end Cs/Rb/H source. Certainly not possible with  the 
10811 that's inside a Z3801A.
 
Still hoping someone knows the TI hardware used in the Z3801A's...
 
bye,
Said






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