[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.
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
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
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
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...
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