On 10/02/2014 09:21 AM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
On 2 Oct 2014 07:10, "Magnus Danielson" <mag...@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?
Could be that it overshoots in this non-linear way due to a little low
cooling, so that as the heater runs full current, it doesn't cool of
quick enough for the transition to be nice and linear. Rather it
overshoots, turn heater off and then cools off and the heater smoothly
goes on again and stabilizes. It's not a big problem.
You can have too little cooling, and then the heater chirps the heater
and then the crystal decays in temperature to get a heater chirp again.
This is really bad and leaves the crystal unstabilized for most of the
time and you get a modulation on frequency from the saw-tooth
temperature. You don't look to be close to that, but I've seen it happen.
This eBay auction, which someone posted
http://m.ebay.com/itm/151256172424
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
temperature.
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.
What is more relevant is will it greatly affect your VNA performance?
Precision on frequency is trimmable once the oven has been well heated.
Noise-wise I would say it suffice.
What is your VNA frequency resolution anyway? 1 Hz?
Cheers,
Magnus
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