[time-nuts] Abrupt changes in TI with HP 5370B's own timebase

Tom Van Baak tvb at leapsecond.com
Wed Jun 15 13:44:29 EDT 2005


Nice plots. Although you got the "right answer"
in your initial test it was good of you to let it run
as long as you did.

What averaging mode were you using? Or were
you taking one 100 ns sample every 100 ms?

I agree it is unlikely a problem is with the OCXO
as in this self-test mode the absolute frequency
is irrelevant. You could double check by using
an external osc ref input.

However, that's not to say the OCXO is the only
temperature sensitive element inside a 5370,
especially when you're measuring picoseconds.

I say this because your events are roughly a day
apart -- (12, 18) + 24 sort of equals 33 - 43.

Unless you get better suggestions, the first few
things to try are mains input voltage and ambient
temperature and shock/vibration. Since you aren't
continuously monitoring these, you can simulate
each of them in turn to see if you get the 5370 to
show the abrupt change. Wiggling BNC cables is
easy as is jumping around your room. Varying
mains voltage is not that hard. And a hair dryer
works well to cause a temperature delta (as well
as pull nearby mains voltage down).


----- Original Message -----
From: "David Kirkby" <david.kirkby at onetel.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Tuesday, June 14, 2005 19:20
Subject: [time-nuts] Abrupt changes in TI with HP 5370B's own timebase

> I've set up my HP 5370B using its own internal 10811-60111 oscillator, a
> BNC-BNC lead to connect the oscillator output to the start input and the
> COM, SEP switch to the START COM position. This is used as a performance
> test (see page 4-2 of the manual).
> The readings should be 100ns +/-1 1ns, which mine are, but some rapid
> changes can be observed when data is collected over a long time (75
> hours). See:
> Data is read down the GPIB bus about once every 100ms, so there are a
> lot of data points.
> I'm surprised by the changes seen around 12, 18, and 33-43 hours. Can
> anyone explain what they might be? The data is not as "clean" as I would
> have liked.
> The jitter seen on multiple reads have an SD of about 30ps over short
> (let instrument computer SD of 100k readings), but clearly on very long
> times the SD is higher (101.258ps to be exact), due to these jumps.
> The instrument has its covers on, but is not in a temperature controlled
> room. Quite the opposite, the temperature does vary quite a bit, as and
> when a window is opened, which is not too far from the HP 5370B.
> But I would not think even any change in oscillator frequency should
> cause this.
> PS,
> should anyone want to look more closely, the original data (all 30MB of
> its), can be found in the zip file in this directory
> http://www.g8wrb.org.uk/useful-stuff/time/data/abrupt-changes-in-TI/
> -----
> David Kirkby,
> Please check out http://www.g8wrb.org/
> of if you live in Essex http://www.southminster-branch-line.org.uk/

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