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

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.

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

************************************** See what's free at http://www.aol.com.

SAIDJACK@aol.com wrote:

Said

Actually the interpolator in the PRS10 has 200 picosec resolution.
However they have not provide a means of accurate autocalibration of the
interpolator offset and gain.

Not exactly the point, the entire counter complexity and PCB area was
reduced by using a single chip for the counters etc., and it was easy to
incorporate an interpolator with sufficient resolution for the target
market. One doesn't usually add a synchroniser with more than the
required resolution even if it only costs a few dollars, if a device
with adequate resolution can be obtained at very little added cost when
its incorporated within the counter chip itself. Having decided to
implement the counter in a CMOS chip, the  jitter due to internal cross
coupling and noise within the chip would have  made it difficult to
achieve a usable resolution much better than the 150ps actually achieved
even if a cheap high resolution external interpolator were used.

Bruce

SAIDJACK@aol.com wrote:

Said

The timestamp resolution may well be much greater than needed
particularly if they leveraged an existing well characterised and
debugged design from another product.
This can save time and money over the alternative of developing a time
stamp circuit with just the appropriate resolution.

You have to be more precise in your definition of accuracy.  An unknown
but very stable time offset is of little consequence when disciplining
an OCXO, however a scale error is another matter. Thus an interpolator
with an unkown offset of a few nanoseconds which is stable to within a
few tens of picoseconds allows an interpolator with a resolution of say
100ps sec to be useful if its gain is known sufficiently accurately and
the signal being timestamped has sufficient stability.

Bruce