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

David Kirkby david.kirkby at onetel.net
Wed Jun 15 18:52:52 EDT 2005

Tom Van Baak wrote:
> David,
> Nice plots.

Thanks Tom. As you can see, I did not put any effort into making the 
graphs look nice, but they are functional.

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

Computers are good for that. I've still got it running and it just looks 
like it might be about to start another slowish change. It's actually 
quite cool now.

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

There is no averaging at all. Hence the data points always differ by an 
integer multiple of 20ps (the resolution of the 5370B), which is why you 
see the straight horizontal lines.

Actually the data points are more like 65ms apart, rather than the 100ms 
I stated, but the times on the axes are correct. They are set by the 
time for the 5370B to convert the data to ASCII and my GPIB board read 
it, rather than any intensional delay. (As was discussed before, reading 
in ASCII is not ideal, but for this, I don't need it any more speed)

> 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.

I will do that. I have a rubidium, but it not too convenient to use just 
now. I have another 10811-60111 I could put in should the need arrise, 
but I think I'll investigate the other (more likely) possibilities first.

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

I did wonder if drift in the trigger points would occur with 
temperature, which would have this same effect. That would seem quite 

Some look to be very sharp jumps though, whereas others do look to have 
an exponential characteristic, as if they might be temperature or drift 

There is probably more than one effect showing in those graphs.  I need 
to remove and/or reduce them all before I can make any meaningful 
comparisons of oscillators.

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

The problem is that that the room temperature is regulated not only by 
the Sun, but when I open windows etc.

> Unless you get better suggestions, the first few
> things to try are mains input voltage and ambient
> temperature and shock/vibration. 

I'd obviously thought about temperature, but not given mains or 
vibration a thought.

> 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).

You have given me some ideas there Tom.

I have to do some real work now, so will just leave it running until I 
have time to do some more experiments. Continuous monitoring temperature 
and mains voltage should not be hard, but vibration is not something I 
can easily measure, but is easy to induce as you say.

I'll let the group know what I find.

David Kirkby.

> /tvb
> http://www.LeapSecond.com
> ----- 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:
> http://www.g8wrb.org.uk/useful-stuff/time/data/abrupt-changes-in-TI/75hours.
> html
>>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.
>>should anyone want to look more closely, the original data (all 30MB of
>>its), can be found in the zip file in this directory
>>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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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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