[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 06:00:36 EDT 2014

On 4 October 2014 07:57, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>> This answers my original curiosity now - did I have an OCXO  or TCXO.
> With that curve it is definitely an OCXO, and my guess is that it is an
> SC-cut, which matches starting +40 ppm high (if I recall things correctly).
> It is also typical for 2" OCXO oscillators and the expected Q and thus noise
> will come at handy in a VNA.
>> Although I am not going to bother, as it will be easier and more accurate
>> to feed the VNA from a rubidium or GPS locked TCXO/OCXO, it would probably
>> be possible to buy one of those off of eBay and replace the OCXO with a
>> better one. Then stick it in my VNA - I would not want to modify the
>> original one.
>> There have some rather small double oven OCXOs on eBay recently for very
>> little money.  From the earlier comments about this oscillator, it would
>> appear its specification is quite poor for an OCXO.
> On time-nuts, things can get a little exaggerated in the "poor" aspect.

I would never have guessed that!!!!

> What
> is more relevant is will it greatly affect your VNA performance?

No, not at all. About the only application I can think of where very
high frequency accuracy would be needed would be to look at crystals.
And for that, this analyzer, covering 50 MHz to 20 GHz is not likely
to be the ideal too. I have just in fact bought one which covers 300
kHz to 9 GHz.

> Precision on frequency is trimmable once the oven has been well heated.
> Noise-wise I would say it suffice.

I don't think there's much point me trimming it, even if I cesium
here. I think it is "good enough" for my application.

I have a couple of rubidium oscillators here, and bought them to make
a "lab standard", but then I wondered whether the phase noise might
have been worst than what I already have, and that was likely to be
more important to me than absolute frequency accuracy.

> What is your VNA frequency resolution anyway? 1 Hz?

Yes, 1 Hz.

HP took the **** a bit with the earlier models of the 8720 series. The
8720A, 8720B and I think the 8720C too, all had a default resolution
of 100 kHz !!! To get one Hz, you needed a software upgrade - enter a
keyword. I would have found 100 kHz too limiting, but what I have is
good enough.

> Cheers,
> Magnus


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