Hi Well here's a few examples:
HP10811 is a single oven. It's spec'd at 4 ppb over -55 to +71C. It's certainly a common eBay item. It's also what's in a 3805A. http://www.hparchive.com/Manuals/HP-10811AB-Manual.pdf on page 3 The Morion DOCXO's on eBay are available at 0.2 ppb p-p -20 to +70C. (not quite -30 but close) http://www.ebay.com/itm/Morion-Double-oven-ultra-precision-OCXO-MV89A-/180791401266?pt=LH_DefaultDomain_0&hash=item2a1801db32 http://morion.com.ru/catalog_pdf/MV89-OCXO.pdf there are other examples. Bob On Dec 16, 2012, at 7:37 PM, Said Jackson <[email protected]> wrote: > Bob, > > We agree that Ocxos get real bad real fast if overheated. Not a problem in > the Z3805A box when run on a desk without clogging the vent holes, which is > how this thread started (do I need a fan for my Z3805A...) > > Your stability numbers are very optimistic. There are very few single oven > Ocxos with 10ppb peak over 100C. Typical numbers are +/-2.5E-08 -see for > example the MTI 230 series, or +/-5E-08 for the Morion MV103 series. Is it > possible? Yes. Our Mini-JLT single oven has +/-5E-09 from -40C to +85C > stability. But that uses an exceptional and quite expensive single oven Ocxo > with SC cut crystal. > > How about 0.3ppb over -30C to +70C? That's not standard by any means, very > few vendors can do that. Heck even the CSAC doesn't even get close to that > (its rated at +/-1ppb over that temp range). The best eurocan Docxo we offer > has +/-0.2ppb over that range for a total of 0.4ppb. and it costs a huge > amount of $$$. So which parts are you referring to here? > > We have done extensive tests here on exactly this issue: how do real life > Ocxos react to varying airflow. Any airflow causes turbulence, so even > constant airflow varies in this sense. All I can say is "not well at all". > Tom Van Back has done some testing on our FireFly-1A GPSDO some years ago > including tests with a fan, he may be willing to share the results of that > test. The FF-1A has a spec of +/-25ppb over temp, so it's a "typical" SOCXO > in our opinion. > > Lastly, don't forget that it's not just the ocxo that's airflow sensitive, > the DAC, filters, amp, and DAC reference all are thermally sensitive and will > be affected by airflow as well. > > The answer to this discussion would be easy to get: put a fan inside the > Z3805A and check its adev with and without the fan. I am willing to bet a > GPSTCXO eval kit that all else being the same the fan version will have > higher noise (ADEV an PN). If the 58503A or Z3815A desktop versions needed a > fan, I am sure HP would have designed one into it.. > > Bye, > Said > > > > Sent from my iPad > > On Dec 16, 2012, at 3:38 PM, Bob Camp <[email protected]> wrote: > >> Hi >> >> Ok where to start. >> >> Some typical numbers: >> >> A DOCXO likely will be specified at around 0.3 ppb peak to peak over -30 to >> +70C. That comes out to 0.003 ppb per degree. A single OCOX likely will be >> specified at around 10 ppb peak to peak over -30 to +70C. That's 0.1 ppb per >> degree. Yes, there are a number of variations, but those numbers are not at >> all crazy. eBay will happy cough up a lot of parts with those sort of >> numbers on them. >> >> --------------------------- >> >> Both devices will heat the crystal to a constant temperature when the >> ambient (however it's measured) is in the range of -30 to +70C. If it's >> below -30, there may not be enough oven power to heat the crystal. More >> likely there's not enough power to warm it up in the specified warmup time. >> If the ambient is above +70, even by a few degrees, the crystal can >> overheat. Exactly what temperature it starts to overheat is a function of >> several things, mainly the tolerances on cutting process of the crystal. >> When it does overheat, it's likely to move 10 to 20 ppb per degree for that >> first degree. That's *way* more than you would ever want to see. >> >> It would be nice if control loops had no noise, and if the crystal heated >> and cooled at the same rate. In fact, you have enough heater power to run >> things up at a very high rate (you want to warm the thing up very quickly). >> The cooling rate at the hot end is "not so fast" (small delta T = low heat >> flow) . A noise spike that gets the crystal too hot, will hang around for a >> while. You can have short term stability trouble before the oven is fully >> shut off. >> >> Simply put - OCXO's don't play nice beyond their upper ambient spec. >> >> ------------------------- >> >> In most gear, there's a fan, and it runs all the time. It may speed up a bit >> when things get real hot, but that's about it. With a fan like that, when >> the ambient moves a degree or two, the inside of the box moves a degree or >> two. There is no catastrophe when the fan suddenly comes on. If anything >> most variable speed fan setups result in the innards of the box seeing less >> temperature change rather than more. Thermally, fans are a good idea. >> >> ------------------------ >> >> Now, taking those numbers (and a few assumptions, and the numbers from the >> previous post): >> >> Let's say your gizmo does have a case that's 22C above ambient. Let's also >> say there is no fan. Let's also assume that there's something else in the >> box. It's pretty common for the "stuff" in a box to heat it up by 20C or >> more with no fan (The "ambient" inside the box is 20C above the outside >> air). If the gizmo is in the box, it's case is now 44C above the outside >> air. >> >> deep breath…. >> >> If the outside air is at 30C, your gizmo is at 74C. If it's an OCXO with a >> 70C upper end rating, it could be in trouble. The rest of the box is >> perfectly happy. It's designed with 70C rated parts and the spec on the box >> is 50C max. >> >> Yes, that's a lot of this and that. Yes there are some assumptions sprinkled >> here and there. Without assuming a few things there's not much way to get to >> a rational conclusion. Are OCXO's a bit strange thermally - yes of course >> they are. An eBay surplus rubidium is even stranger. Power transistors have >> issues as well. Stuff that has power associated with it needs some attention >> as part of the thermal design…. >> >> --------------------------- >> >> Are big / noisy / shaky / power hungry fans a good thing - of course not. >> They are simply the most common way to solve thermal problems in equipment. >> These days you can get fans that are less big/noisy/shaky than they once >> were. It's still better to not use them *if* you have all your thermal ducks >> in a row. That means you have the full specification on the OCXO (and likely >> a few other things). >> >> If we're talking about surplus gear or parts - you are not going to have a >> lot of information. The only reasonable assumption you can make is that the >> eBay OCXO is rated for moving air. Assuming you know the upper ambient of >> the OCXO, check it's actual case temperature. Keep it below the specified >> max ambient. Ideally, keep it 10 C or more below the rated upper ambient. >> The short term stability likely will be better …. >> >> Bob >> >> >> On Dec 16, 2012, at 2:22 PM, Said Jackson <[email protected]> wrote: >> >>> Hi Volker, >>> >>> 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. >>> >>> Bye, >>> Said >>> >>> >>> >>> >>> >>> >>> >>> On Dec 16, 2012, at 9:47, Volker Esper <[email protected]> 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? >>>> >>>> Volker >>>> >>>> >>>> >>>> Am 16.12.2012 15:35, schrieb Bob Camp: >>>>> Hi >>>>> >>>>> 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. >>>>> >>>>> Bob >>>>> >>>>> On Dec 16, 2012, at 7:57 AM, Volker Esper<[email protected]> wrote: >>>>> >>>>>> >>>>>> Steward, >>>>>> >>>>>> 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. >>>>>> >>>>>> Cheers >>>>>> >>>>>> Volker >>>>>> >>>>>> >>>>>> >>>>>> 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? >>>>>>> >>>>>>> Cheers! >>>>>>> --Stu >>>>>>> _______________________________________________ >>>>>>> time-nuts mailing list -- [email protected] >>>>>>> To unsubscribe, go to >>>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>>> and follow the instructions there. >>>>>> >>>>>> >>>>>> >>>>>> _______________________________________________ >>>>>> time-nuts mailing list -- [email protected] >>>>>> To unsubscribe, go to >>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>> and follow the instructions there. >>>>> >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- [email protected] >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>> >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
