[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 06:38:32 EDT 2014

On 4 October 2014 08:01, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Tim,
> The shape does not look exactly what I would expect from a less than
> critical damped oven, it looks a little to peaky, but maybe it is the
> resolution of the graph that fools my eye.

I stuck the raw data as an attachment, so that should not fool your
eye.  It is only a few kB as compressed, and not that big when

But be careful in trying to do too much with the data after it has
"settled down", as I have no idea of the accuracy or stability of the
time base in the "HP 70310A precision frequency reference" which is
used to measure the VNA. I have no idea when it was calibrated. But it
had been powered on at least 24 hours. I don't even know the
specification of that - it might be better or worst than the VNAs
oven, but at least the VNA has been regularly calibrated.

What is a bit odd is one of the tests of the VNA performed by
Agilent/Keysight is the frequency accuracy at 20 GHz. Although when
calibrated by Agilent a year ago there was some error, this time it
was spot on 20.0000 .. GHz. But when the 10 MHz oscillator was
measured, there was some error. Of course, there will be some errors
in the VCO / PLL and the timebase could have drifted between the two
measurements, so such differences are not entirely unexpected.

The 3 cal certificates I have for this are here, in order of date
(most recent first)

1) Keysight a few weeks ago

2) Agilent just over a year ago

3) Techmaster (lacks any useful information), about 2 years ago

Although the Techmaster one tells you nothing useful, the Agilent and
Keysight ones do give frequency information, including the
uncertainty, which has actually increased since it was calibrated by

Anyway, at least I know what I have in this VNA- not that it makes a
huge different to know, but sometimes I am just inquisitive.

Other than invest in a GPS controlled TCXO, I am not going to do
anything to improve this accuracy, and if I am honest, I don't really
need any extra accuracy.

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