On closer inspection, the TBO output isn't resynchronised to an internal
clock however a single chip custom timebase divider is used that uses
both +5V and +15V supplies.
Perhaps this is a high voltage CMOS (or even MOS) part that employs a
asynchronous cascading of decade divider outputs?
The output jitter of such a divider could easily be several nanoseconds.
Bruce
Bruce Griffiths wrote:
The problem could be due to the effect of synchronisation of the
divider output to an internal 100MHz clock.
A close look at the circuit would be required to see if indeed this
occurs.
A quick glance indicates that the TB out signal appears to be always
synchronised to an internal clock.
Bruce
John Miles wrote:
I would tend to say that the divider is pretty lousy for short
term, but it is all fine for longer runs, right?
Is this what I should expect from a TTL/ECL divider chain
designed in the '70s-'80s? How would this compare to a modern
divider chain, like the PIC divider or David Partridge's divider board?
The 5370 residual plot I mentioned in the other message was taken under
similar conditions as you describe, but with a TADD-2 divider, and is
almost
100x better. IMHO the 5328A solution doesn't look like a good one,
since
any inexpensive digital divider should be able to beat it.
It's true that factors like trigger levels and attenuator settings
matter,
but this will be observed at the tens-of-picoseconds level in my
experience,
not at 1ns+ timescales. Triggering on the correct edge is likely to
be more
important. Try it both ways and see if you notice a difference.
Since I get a straight line pretty much up to 500s or so, do I
conclude that the divider dominates the system noise up to there?
That's very safe to say (again, load that residual .tim file and
display it
next to your result and you'll understand instantly).
Of course, the 5370A timebase drift has to be taken into account
but is not subtracted on that plot.
Slow drift in the 5370 timebase isn't an issue for two-channel ADEV
measurements. If you had a problem involving excessive noise from
either
the timebase or trigger circuits at timescales close to your 1-second
tau
period, it could degrade the results, but that's extremely unlikely.
-- john, KE5FX
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