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
possible.
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
related.
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" <[EMAIL PROTECTED]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
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.
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,
G8WRB
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,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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