Discussion:
HP 5372A TIA Question
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Adam MacDonald
2018-05-29 22:26:42 UTC
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Time Nuts,
I have a nice new-to-me 5372A that I'd like to use as part of a DMTD stability analysis system.  After getting a few odd glitches from a very stable source spaced 2 ns above or below the main time interval reading, I decided to run the performance verification checkout from the service manual.  The checkout calls for a BNC-T from the rear 10 MHz timebase output split to both A and B channels.  The spec is 100 ns +/- 200 ps but I am consistently getting either 101.6 or 101.4 ns in Time Interval mode on either channel.  So it seems to be high/off by exactly 1.4 ns.  The standard dev is very low (typically 0-60 ps over several 100 point runs).  Curiously, in Continuous Time Interval mode, it reads dead on (100.0 ns).  I've run the sensitivity cal because I got the 160 error (dead battery), but not the interpolator cal (I don't have the appropriate riser card and the manual calls for a specific cable).  Has anyone seen this?  I suspect maybe the interpolator board needs to be ca
librated, but maybe something else?  Has anyone had experience trying to adjust the board without a riser?  Any other tests I might run to shed light on what might be causing this?

Thanks,Adam MacDonaldShalimar, FL
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Magnus Danielson
2018-05-30 21:15:01 UTC
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Hi Adam,
Post by Adam MacDonald
Time Nuts,
I have a nice new-to-me 5372A that I'd like to use as part of a DMTD stability analysis system.  After getting a few odd glitches from a very stable source spaced 2 ns above or below the main time interval reading,
OK, so the 5372A uses a 10811A as OCXO and then synthesize a 500 MHz
coarse clock which then runs in custom high-speed chips. The
interpolator is a delay-chain of DFFs.

So, if you jump +/- 2 ns it is that you most likely have an issue with
the start/stop and coarse-counting. Could also be that the interpolator
misses one way or another. Regardless I will ponder on it.
Post by Adam MacDonald
I decided to run the performance verification checkout from the service manual.  The checkout calls for a BNC-T from the rear 10 MHz timebase output split to both A and B channels.  The spec is 100 ns +/- 200 ps but I am consistently getting either 101.6 or 101.4 ns in Time Interval mode on either channel.  So it seems to be high/off by exactly 1.4 ns.
Consider that you need to swap the cables to verify that it is static
with the setup and does not follow the cables. Also, there can be
trigger voltage offsets which may be the cause, so that may be due to
other reasons.
Post by Adam MacDonald
  The standard dev is very low (typically 0-60 ps over several 100 point runs).  Curiously, in Continuous Time Interval mode, it reads dead on (100.0 ns).  I've run the sensitivity cal because I got the 160 error (dead battery), but not the interpolator cal (I don't have the appropriate riser card and the manual calls for a specific cable).  Has anyone seen this?  I suspect maybe the interpolator board needs to be ca
librated, but maybe something else?  Has anyone had experience trying to adjust the board without a riser?  Any other tests I might run to shed light on what might be causing this?
Let me ponder on this one. I don't think it is the interpolator boards,
sure they might need calibration, but that should not be the issue here,
you miss a cycle in the coarse counter on their the start or stop
channel. That should be the focus.

Cheers,
Magnus
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Poul-Henning Kamp
2018-05-30 23:03:20 UTC
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--------
Post by Magnus Danielson
Consider that you need to swap the cables to verify that it is static
with the setup and does not follow the cables.
Run long series of measurements with the HP5372A OCXO free-running
and with it slaved to the DUT clock.

Any imperfection in the interpolator will show up as different noise
characteristics in the two results.

There is a test in the service manual which will give you a good
indication how well tuned the interpolator is, run that before
touching anything, and don't attempt to tune the interpolator
unless you can follow the procedure in the manual *exactly*.

PS: I do not recall it being mentioned, but the 1 MOhm input pods
are horribly unstable compared to the 50 Ohm input pods, which are
basically just a stripline and a BNC connector.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
***@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Ed Palmer
2018-05-31 16:15:06 UTC
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On 5/31/2018 10:00 AM,
PS: I do not recall it being mentioned, but the 1 MOhm input pods are
horribly unstable compared to the 50 Ohm input pods, which are
basically just a stripline and a BNC connector.
One thing to note about all the 5372A pods: They should all be checked
for bad solder joints between the rear connector and the circuit
boards. In my collection of pods of all three types, almost all had one
or more broken solder joints.

Ed

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Magnus Danielson
2018-06-03 19:10:59 UTC
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Hi Adam,

OK, good to hear about the progress. Let us know about what you learn
and if you need additional input. I'm curious what is the cause of this
error.

Cheers,
Magnus
Thanks Gents,
I ran the additional tests recommended by Poul and Magnus and I concur that the interpolator board is not the likely culprit.  I reversed the input cables as well as the pods in separate experiments to rule out those contributors with no different results.  I thought it may be a trigger circuit error so I fed the inputs with a locked TTL signal rather than a sine wave using Manual Trigger with no difference.  It seems to be a coarse count error so I'll investigate the start/stop logic chain. 
Adam
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