[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
Thu Oct 2 03:21:14 EDT 2014

On 2 Oct 2014 07:10, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
> David,
> The character of starting high/low and then stabilize some 5-30 min later
is typical of oven oscillators. Underdamped ovens have been seen before, I
have even seen one on the brink of oscillation.

Thank you. Do you know the likely cause of the somewhat odd behaviour
between about 150 and 220 s?

This eBay auction,  which someone posted


says it's the high stability oscillator for an HP 8753D or 8753ES. I
checked the part number at parts.keysight.comand it would appear it is the
same as used in my VNA and several other microwave VNAs.

That said,  I have noticed a few errors on parts.agilent.com, one of which
resulted in me buying the wrong part. I was later warned by Agilent not to
trust the accuracy too much, especially on older equipment. It is better to
check with them before making purchases.

But they have a very helpful parts service that does make every effort to
sort out what parts are. They spent quite some time finding out what
connectors were on am obsolete cable for me.

> TCXO will not have the same wide range, as it compensate for the

This answers my original curiosity now - did I have an OCXO  or TCXO.

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.

> Cheers,
> Magnus

Thank you.


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