[time-nuts] Measurement of frequency of HP 8720D option 1D5 oscillator after switch on
Dr. David Kirkby (Kirkby Microwave Ltd)
drkirkby at kirkbymicrowave.co.uk
Sat Oct 4 14:18:33 UTC 2014
On 4 October 2014 13:24, Tom Van Baak <tvb at leapsecond.com> wrote:
> Thanks for the raw data. Attached is my plot. Your data looks ok to me.
> If you're interested here are some random comments about your 8720D-1D5-oscillator-frequency.csv file:
> - Thanks for including the description of the data as comments in the file. I can't tell you how many times I get data files from people and you have to play "20 questions" before you know what the columns are, or what the units are, or what the sampling interval is, etc.
Yes, I can imagine it. Some people have some odd ways of presenting data.
Slightly different, but I recall trying to help a student debug her
software. It became apparent that all variable names were Disney
characters! The number of photons might have Aladdin, the absorption
coefficient MickyMouse, the scattering coefficient SnowWhite ...
I believe she went onto work with missiles, which is worrying.
> - Thanks for including the elapsed time (column 6) as part of the data set. This is the preferred way to determine the sampling interval; 6.3 seconds in this case.
It seems the obvious one to me.
> - You have 355 data points. Notice that the last 319 measurements are all 9.99999400. This is partly due to the fact that the OCXO has warmed up by this point. But this is mostly due to the fact that your OCXO is now more stable than the counter has resolution.
Yes, I see that.
> - Out of habit my plot uses ppm units rather than Hz. I converted your frequency measurements to relative frequency error. For example, a frequency of 10.00013100 MHz the error is (10.00013100 - 10) / 10 or 1.31e-5. The five frequency lines above simply become:
Yes, I can see the logic of that. At least to a time-nut. Not so sure
I would present it like that to most people.
> and this shows that even though it's a 7 digit counter there are only 2 or 3 digits of resolution (the DUT being quite close to 10 MHz).
I should have a look to see if that can be improved. I hacked the the
software to collect that together in 15 minutes or so, but it was
heavily based on something I wrote in 2008. At that time, the job was
urgent and on some occasoins I was working to 2 AM to get the code
finished. I doubt it well written as it could be. Perhaps the
resolution could be improved.
I guess I should also get a better counter. I used to have a HP 5370B,
but sold it to someone on this list - a decision I later regretted. I
have thought of buying another microwave counter (mine is pretty
dead), but I doubt that would have the resolution of a lower frequency
model. And microwave counters tend to be quite expensive.
> - By the time you get to all the 9.99999400 MHz readings, the counter is down to 1 digit of resolution, e.g., -6e-7.
> - Most people plot points and draw interpolated lines between them. The Stable32 plot I attached shows a staircase. This is partly due to the fact that the raw data is quantized in both the x and y axis (sample quantization is 6.3 s, frequency quantization is 1 Hz). It is partly done to convey that frequency is inherently an average across some interval rather than a point measurement.
I see Stable32 is some specialist software for time-nut related items.
> Again, none of this has any impact on your original question about your VNA timebase, but I thought you might be interested in what's hidden in the data. Now imagine the fun you could have making 12 digit measurements...
I guess I will have to look for another 5370B!
If I recall correctly, when I did look some time ago at getting
another TI counter, the Agilent 53230A seemed to have some
specifications *worst* than the 5370B it replaced. It was not clear it
was an upgrade.
This 53230A on eBay does not seem very good value, at about 60% *more*
than the price of a new one from Keysight!!!
Perhaps I should ask what discount I could get if I buy 10!
I just stuck an offer on a SRS 620, which is sold as seen. I'll take a
chance it works if my offer is accepted.
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