[time-nuts] Z3805A cooling requirements?
saidjack at aol.com
Sun Dec 16 19:22:35 UTC 2012
You are correct on that.
Bob is right in that thermal sensitivity is measured in a thermal chamber with constant airflow at a very constant rate and temperature. While this works, it is also somewhat unrealistic because who is going to set up a thermal chamber for their ocxo in the field?
Because of that many companies put airflow shields on top of the ocxo in the test chamber to simulate more real life conditions where massive airflow is not happening, and self heating needs to be taken into consideration.
Moving-air skews the operating power of the ocxo as it removes heat from the ocxo. Thus a manufacturer could claim +75C max operating temp inside a test chamber, but if you try to operate in still air at 75C in a small enclosure your ocxo would overheat quickly due to the internal power consumption adding to the ambient temperature. Thus the test chamber actually works to "cool" the ocxo by removing excess heat and keeping its case at 75C no matter how much power is consumed inside the ocxo.
As an example of this consider that a typical DOCXO runs at 55C case temp in still air with 22C ambient. Now say that Docxo has 1E-010 per degree C thermal sensitivity (not a great docxo.. But thats actually better than the spec of the ocxo used on the Mini-T: that one is 10ppb from 0C to 60 C as far as I know)
What happens when I turn on a strong fan right next to the Docxo? The fan will throw ambient air at the same 22C temperature at the unit, and immediately start cooling off the 55C case of the ocxo due to the temp difference between the ocxo case and ambient air.
The result? If the fan can cool off the Docxo to say 30C, we have had a massive 25C temperature change shock on the docxo without a single C temp change in ambient air!
Now 25C * 1E-010/C = 2.5E-09 change in frequency which gives a 2.5ns/s drift rate just because the fan switched on!
Typical single oven ocxos will have about 1ppb per C sensitivity, so the above example would result in 25ns/s drift just because the fan came on. Thats really bad for Gpsdo type performance expectations, and will certainly ruin your ADEV performance which us time-nuts expect to be around 1E-012 not 2E-08 for a good gpsdo :)
Hope this shows why a fan on an ocxo is not a good idea.
On Dec 16, 2012, at 9:47, Volker Esper <ailer2 at t-online.de> wrote:
> Indeed? I didn't expect that. There are people who say, that the control loop of OCXOs is optimized for still air and no additional cooling at all.
> Said told us, that...
> >...a fan is about the worst thing you can do for your Z3805 it will
> significantly worsen the stability of the output frequency...
> Since the main task of the OCXO-oven is to stabilize the internal temperature, I can't imagine, that it get's into trouble when not externally cooled!?
> If I'd ventilate the air around the OCXO case the heater had to work more and the power dissipation would be greater.
> Am I wrong with taht?
> Am 16.12.2012 15:35, schrieb Bob Camp:
>> By far the most common way to test and certify OCXO's is in moving air. It's rare to see one get in trouble from to much ventilation. The more common problem is thermal runaway due to inadequate ventilation.
>> On Dec 16, 2012, at 7:57 AM, Volker Esper<ailer2 at t-online.de> wrote:
>>> What is the intended and what is the actual supply voltage? Which current is the unit consuming?
>>> When we know that, we can compare the power consumption with our units. If it is in the same range, it should - with a little luck - be working properly.
>>> My two units are intended to be supplied by 19.5 to 30 V. I use 24 V and the Z3805s draw 0.9 A each. If I increase voltage the current decreases (typical for the switching supplies inside the Z3805).
>>> I don't cool the units, they just lie on an old electronics magazine (for not to scratch the case of my signal generator, lying below the magazine), so they can freely convect their heat.
>>> Am 12.12.2012 01:21, schrieb Stewart Cobb:
>>>> This may be a newbie question, but I'm a newbie, so:
>>>> Do the HP telecom GPSDOs (Z38xx) require external airflow for cooling?
>>>> They don't have built-in fans, but they sorta look like they depend on a
>>>> rack-level cooling fan, which a telecom rack would almost certainly have.
>>>> I ask because I bought a Z3816 awhile back which worked for about a week
>>>> and then failed. I traced the failure to an internal power supply brick,
>>>> which had a big finned heat-sink attached but nevertheless smelled
>>>> overheated and was shorted internally.
>>>> I never found a replacement power brick, and I don't have time to mess with
>>>> it right now, so I recently bought a Z3805A. It, too, looks like it's
>>>> working, but it started to feel awfully warm after a few hours, so I
>>>> unplugged it for now.
>>>> It probably wouldn't take much of a fan to bring the internal temperature
>>>> down close to ambient, and the fan could be powered easily enough from the
>>>> supply rails. But that might create a temperature gradient where the
>>>> designers didn't intend one. Or it might cause problems I don't even know
>>>> about yet.
>>>> At the moment, the Z3805A is in a fan-less 19-inch rack with a bunch of
>>>> other equipment, in a lab environment. Should it have its own fan?
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