John Miles wrote:
It's worth noting that the 5370's accuracy is supposedly never better than 1
ns (specifically, +/- resolution +/- timebase*interval +/- trigger jitter
+/- 1 ns systematic). We tend to rely on it to do much better than that --
or at least we all count on the "1 ns systematic" to look like random noise
in the long run -- but it still bears remembering, because that systematic
error swamps any other effects in the short term.
Some time ago John A. plotted his 5370's residual floor, about halfway down
the page at http://www.febo.com/pages/hp5370b/ . He fed a 1-pps signal into
a T-connector at the START jack, followed by 35 ns of coax to the STOP jack.
I ran a similar test recently, but with a TADD-2 1-pps divider driving the
START jack and a similar amount of coax driving the STOP jack from the 10
MHz input side of the TADD-2, trying to get an idea of the composite floor
for a typical measurement using the TADD-2, the 5370B, and a bunch of coax.
The 5370B was driven by a Thunderbolt for this test.
My results (attached) were a bit worse than John's, but with a nearly
identical slope for the duration of his test. I left it running for a few
days, and saw some significant flattening on timescales of 3-4 hours that
matched my air conditioning cycle time (as reported by the shop thermometer,
aka Lady Heather.) Based on this graph I don't consider temperature
variations to be a grave concern above floors of 1E-14, or conversely at
timescales less than an hour or two, but they should definitely be minimized
or eliminated for HP 5370A/B measurements below that.
I still need to try variations of this test with the internal reference,
with and without the TADD-2 between the START and STOP jacks, and with
10-second and 100-second means on the 5370B. Clearly it would be good to
achieve better than 1E-10 accuracy at t=1s, since most good crystal
oscillators are better than that.
It would be interesting if you could plot the tau*ADEV(tau) curve, then
the long-term deviations would be more apparent. This is a form of
TDEV-similar plot, but if you only have TDEV that will also help.
tau*ADEV(tau) is about 7E-11 up to tau 1000 and then rises as
environmental effects kick in.
If I had the time I would ask for the raw-data and fool around with that.
Cheers,
Magnus
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