Hi First order immune due to the OCXO. You still have the RF stuff hanging out there in what ever breeze comes along. The OCXO does indeed change a bit if you go from still air to a raging blast. Not as much as a TCXO, but there is a shift.
Bob On Dec 16, 2012, at 8:56 PM, [email protected] wrote: > 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 <[email protected]> > To: Discussion of precise time and frequency measurement <[email protected]> > Cc: Discussion of precise time and frequency measurement <[email protected]> > 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 <[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. > _______________________________________________ > 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.
