[time-nuts] PPS and 5/10 MHz GPSDO time relationship

[Magnus Danielson]

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