[time-nuts] HP 5372A TIA Question

[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, 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 calibrated, 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