Hi Eric,

On 2022-07-01 16:40, Erik Kaashoek via time-nuts wrote:

You should be careful to isolate it too much.

OK, let's get the basics. The oven aims to maintain a certain
temperature by running a heater continuously and balance the heating to
the cooling of the surrounding, as it dissapates heat. This is equally
true for double and tripple ovens, they just have different temperature
settings.

Now, if you over-isolate any oven, the heat transfer will be too low so
the heater will overshoot the heating. When this happens, the heater
turn fully off and the oven will coast down unregulated until low enough
temperature. What you then end up with is a bang-bang regulator causing
a saw-tooth like heating profile. This is then worse situation than before.

Naturally, this all depends on the design of the oven, and how it's
setpoints is done, but the ambient temperature specification gives the
clue of how far you can go. You need to remain the thermal loading to
maintain that minimum heat conduction out of the oven. For passive you
need to respect the highest ambient temperature (of the oven) for all
ambient temperature conditions of the device you build. Isolation needs
to be done carefully, and passive stability is hard. Active measures
naturally can help but you then need to handle cooling.

Rather than thinking isolation, you should rather avoid direct
variations of forces convection air path. Essentially, put the OCXO in a
draft-free corner. Essentially wind-shielding it but really not doing
any actual isolation to maintain heat conduction away from the OCXO
works really well. Either just a few metal walls or a plastic cap around
it will suffice to cause much of the effect without excess danger of
over-isolation.

For test-purposes, you find that I often put oscillators inside a
cardboard box with some antistatic bubble-wrap around it. Not enough
thermal mass for long-term things, but good enough to remove much of the
quicker fluctuations. There is usually an ADEV bump at 500-1500 s
traceable to the heating/AC. Similarly a beach towel have surved the
purpose for larger things.

Cheers,
Magnus

Hi

Modern OCXO’s are set up based on temperature run data.
They play with set point (and possibly electrical gain) to optimize
the TC contribution of the crystal and the rest of the parts in
the device. This is true of single and double ovens.

One not so obvious point is that these runs are done in a very
specific temperature environment. Forced air is normally part of
a test chamber. It also is often part of the OEM installation that
the OCXO goes into.

Change the “air pattern” to much and you change the gain of the
oven ( more insulation increases the gain). This can upset the careful
balance done when optimizing the TC of the device.

One can debate just how stable all of the “stuff” in an OCXO is over
the years. Is a set ( or screening ) done on a production line a decade
ago still relevant today? Random bits of evidence suggest that the TC
optimization holds pretty well, but there isn’t a lot of data.

A typical double oven should be < 5x10^-10 over 0 to 70C. Indeed
many manufacturers will sell you examples that are spec’d tighter
than that. Some offer single ovens with spec’s below 1 ppb over 0 to
70. 1x10^-8 is a very typical single oven spec.

How well does this or that example do? It is not uncommon to see
1x10^-8 level single ovens rolling off the production line at <2x10^-9.
On some designs > 80% of the units do this. Counting on any and
every OCXO to be 5X better than spec …. maybe not, but many designs
do.

How to “manage” an OCXO?

First step it to get a good one in the first place. If eBay is your source of
supply ( it is for me ….) what you get likely is not going to be 100% perfect.
Some level of testing and sorting will be involved. That needs to be done
before a lot of additional effort is put in.

Next up is to plan on keeping it on power all the time. OCXO’s don’t like
to be cycled. Sorry about that. If this bugs you, don’t head down this
road. There are good reasons for this to bug you so do think about it.

Drafts and abrupt temperature changes are to be avoided. Opening the
lab window next to your reference standard … not a great idea. Something
as simple as a towel or a cardboard box tossed over the device can do
wonders. Exotic enclosures are probably better, but simple gets you a
long way. Thermal mass might help as well.

Just as a note, things like Rb standards (and Masers) also are said to
benefit from fairly simple “draft protection” enclosures.

Most folks are pretty obsessive about regulated supplies. If anything they
go a bit overboard in terms of noise for an OCXO supply. What might get
overlooked is the need for a fairly substantial ( = low voltage drop) supply
wiring setup (along with good ground practices). If you plan some sort
of battery backup, consider the regulation impact as it cuts in and out.

Loading on the output of an OCXO does matter. How much is a “design
feature”. It is not uncommon to see a few minutes of disruption for a
significant load change. Simple answer here is not to play with moving
things around a lot :)

Many OCXO’s are tuned via an EFC. Feeding this input in a stable fashion
can get a bit crazy. Do try to run the EFC circuit ground straight back to
the OCXO. Oven current induced drops are not great for EFC stability ….

Fun !!!

Bob

On Fri, 1 Jul 2022 at 20:11, Erik Kaashoek via time-nuts <
time-nuts@lists.febo.com> wrote:

I removed an HP 10811A OCXO from a 5370B time interval counter the other
day and put it into a HP 5352B 40 GHz frequency counter. One thing that
really struck me is that in the 5370B there was a shroud around the OCXO,
which is around 5 mm away from the sides of the OCXO. It's made of
aluminium. But there's nothing like that in the frequency counter. The two
attached photographs show a significant difference. I took the photograph
from inside the 5352B frequency counter. The photo of the 5370B was one I
just found on the EEV blog site, as I did not want to have to mess around
taking another photograph.

I see Magnus respond to you.

My gut feeling is the designers of the 5370B were likely to have more
knowledge about the behaviour of oscillators than the frequency counter
designers, which makes me wonder if adding something around the oscillator
in the frequency counter, like in the 5370B time-interval counter, might be
a good idea.

Unfortunately I suspect it would be very time-consuming to evaluate the
difference a shield would make in the frequency counter, I have another HP
frequency counter where the fan blows over the oven, which does not seem a
very good idea.

Hi David,

We did a fairly simple measurement setup at work.

We had the oscillator sitting on a small test-board and measured the
frequency from start. Then a few seconds in we shifted the direction of
a fan at some distance onto the oscillator. We then did this with a
variation of simple shields, and concluded that a fairly simple wind
shield achieved most of the gains we where after. We then reapplied this
in various incarnations since, and it has not provided us with any
reasons to do things differently, but rather once the lesson was
learned, it was shown effective in many places, as forces convection is
an unfortunate needed thing in our products.

As most oscillators have a metal can, they conduct heat well and if
there is no direct forced air convection onto it, it allows the
radiation and still air conduction to be fairly well evened out and
those provide less temperature gradients to the oscillator.

I've also seen the 5370A/B shield. It works and solves the problem, but
often you can use simpler setups with good results too.

So, it comes down to not really shielding it from long-term temperature
variations, but just not make the situation much worse than it needs to be.

If one has a box with relatively low power consumption per unit volume,
forced air is not needed, and need for shielding can be relaxed. Just
putting the oscillator of from heat sources and in particular heat
sources that vary over time come far. The important part is that it is
in a thermally quiet corner, which include air and air-flows.

We had a pair of students doing work during summer vacation. They where
measuring the phase stability of one of our boxes. Three hours into the
measurements the variations seemed to go away. They where completely
puzzled. It was showing clear variations and then the systematic died
away. So I just asked them when they started the measurement. "Around
15:00", well that was all I needed. I informed them that the building AC
turned off at 18:00, and what they was measuring was variations in
ambient air condition. They where flabbergasted and wondered how it
could have such an effect. So I pulled the board out of the chassi and
showed them the oscillator location and showed them how the side-wise
blowing air hit the can. I then found them some foam tape and advised
them to apply it to the oscillator and redo the measurement. It was much
flatter naturally. However, for that product it was good enough without
the shielding, but we changed modus operandi for all our products since.
For this very good reason.

Cheers,
Magnus

On 2022-07-01 22:31, Dr. David Kirkby via time-nuts wrote:

Hi

If you tear into lots of HP devices with OCXO’s in them (not just the 10811 version),
the most typical place for the OCXO is right. next to the power supply. That puts it inline
with the output of the fan.

Why? The OCXO gets hot. Heat buildup in the instrument is not a good thing. They put
it there to get the heat out of the box as quickly as they can. Since the “draft” is a constant
(and not a puff puff puff) it’s not as big a deal as you might think. What it does do is to
move the changes in the outside environment over to the OCXO more quickly.  If there
is an impact, it is from your lab ….

Bob