[time-nuts] Timelab, two SR620s and losing samples

Attila Kinali attila at kinali.ch
Thu Jan 14 14:38:33 UTC 2016


I have here a setup with four (FPGA) nodes that produce synchronized pulses
with a 20kHz rate. I have two SR620s two measure those pulses.

Because the SR620s are not fast enought to capture all pulses, and because
i want them to be synchronized, I set up one of the nodes to generate an
additional pulse every 100ms (10Hz rate) 20us before the "main" pulse,
and feed that to the two EXT trigger inputs of the SR620s.

The two SR620s are both connected to an FS725 Rb frequency standard
(mostly because we have them around and nobody else uses them :-)

Now, I use Timelab (running on Windows XP, in a virtualbox on a linux system),
using two serial-to-USB converters (FT232), which are passed as raw USB
devices into windows. Capturing both SR620s together in timelab, I see
one of the SR620s "producing" less samples than the other.
Quite considerably less (it looks like 1-3% less or so).

I have already checked and rechecked the trigger settings, the trigger
voltages etc, but I cannot find why one produces less samples than the other.
I have changed cables, swapped nodes. But it's still the same SR620 that
loses samples.

Another curious thing I see that I couldn't make sense of is, that from
time to time, on both of the SR620s, I see time differences of 1s (yes,
one full second). Given that the FPGAs produce a pulse every 50us, this
shouldn't be possible, unless the crystal oscillator stops.

Any idea what I might have done wrong or what the cause is?

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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