One more thing the Thunderbolt is essentially immune from since there is no separate XO for the GPS receiver which runs from the OCXO.
Didier Sent from my Droid Razr 4G LTE wireless tracker. -----Original Message----- From: Said Jackson <saidj...@aol.com> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Cc: Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Sun, 16 Dec 2012 7:16 PM Subject: Re: [time-nuts] Z3805A cooling requirements? One item I forgot to mention: One of the most airflow sensitive parts inside a GPSDO is the tcxo used on the GPS receiver. Some GPS don't even use a tcxo, just an XO. They shall remain unnamed. Just lightly blowing on an M12+ GPS will make it lose lock immediatly. This is easy to try for time nuts, and to verify the sensitivity to airflow. Bye, Said Sent from my iPad On Dec 16, 2012, at 4:37 PM, Said Jackson <saidj...@aol.com> 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 <li...@rtty.us> 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 <saidj...@aol.com> 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 <ail...@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? >>>> >>>> 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<ail...@t-online.de> 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 -- time-nuts@febo.com >>>>>>> To unsubscribe, go to >>>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>>> and follow the instructions there. >>>>>> >>>>>> >>>>>> >>>>>> _______________________________________________ >>>>>> time-nuts mailing list -- time-nuts@febo.com >>>>>> To unsubscribe, go to >>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>> and follow the instructions there. >>>>> >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@febo.com >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>> >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
