On 12/21/2010 10:26 PM, John Green wrote:
I tried substituting the 1 PPS output from the Z3801 and comparing it
to its own 10 MHz output and find the same jumpy behavior as I get
with the UBLOX boards. Well, not exactly the same but pretty much. Now
I am confused. I expected the 1 PPS to be in lock step with the 10
MHz.
Sometimes counters has a problem when the start and stop signals occur
too tight in time after each other. Running the Stop signal through some
extra cable avoid the trigger issue and out pops stable readings. Since
each meter of coax adds 5 ns of delay, it is a pretty handy method of
getting out of trouble.
Do measure the PPS to Clock property of your GPS. If your counter needs
it, use the cable trick above to get stable measurements.
A month ago Björn and I did some experiments together, found in my
posting of 20 Nov titled "PPS and 5/10 MHz GPSDO time relationship":
8<---
Fellow time-nuts,
Björn and I have been having some fun during our get-together in his
basement time-lab. I pulled with me some gear (CNT-90 and SR620) for him
to play with, so a good warmup exercise was to measure the offset
between the PPS and the clock output (5 MHz or 10 MHz).
The results was uhm... spread out... so we felt like sending you guys a
report.
First out was a RAPCO 1804M which has a HCD 66 SC 5 MHz oven diciplined
by an old Trimble SV6+ (?) GPS receiver. We popped the lid for other
purposes... :) It had the 5 MHz rising edge 32,17 ns behind the PPS
rising edge, with 100 ps RMS jitter. Quite noticeable offset but fair
jitter.
The good old RAPCO was jumperable to be on "OSC" or "GPS" on the
mysterious jumper LK9 and it was stuck hard on the "OSC" setting, but
some physical exercise later we got it into GPS setting and it had a
about 200 ns peak to peak sawtooth... nice and pedagogical exercise.
We then had a look at the Brandywine GPS-4 (mine on loan to Björn) and
found it had fairly nice numbers... until we discovered it has a
periodically reoccuring glitch of unknown system-source.
Natually we hooked in to Björns Thunderbolt and found the offset so
tight that we ran into trigger-problems, but offsetting the clock by
about 8 ns of coax cable we had a about 4 ns in average and 6 ns
peak-to-peak. The PPS thus jumped between two distinct offsets with
their respective gaussian distribution around them. Not all that neat,
and the RAPCO was the quietest in this shoot-out.
Over and out,
Magnus and Björn
--->8
Once you know how your favorite GPSDO behaves, you should know if you
can use the PPS directly as a quiet source or if you only should use it
as an external ARM to trigger measurements and then use the clock for
Start (Channel A) and DUT for Stop signal (Channel B).
Going for the PPS-arming directly avoids the issue somewhat.
When comparing a different frequency than 10 MHz I insert a generator
which locks to the 10 MHz and produces the nominal frequency onto which
I will do my measurements.
Don't forget to optimize trigger jitter and verify it separately for the
Start and Stop signals.
Don't forget to let your DUT warmup.
Cheers,
Magnus
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