Hi On a normal OCXO or TCXO design you would put the product in a package with a hermetic outer wall to keep out crud and moisture. Per the papers cited you would put a small vent hole in a “space grade” product. Every time I see an eBay listing with a nutty price on a “space OCXO” I wonder if it’s been sitting “vented” on a nice humid / dusty shelf for decades.
Indeed the only way to test / use product vented like that is to put it in a thermal / vacuum chamber (or fly it to outer space). While my basement has a reasonable selection of this and that, I don’t seem to have missed out on picking up a thermal vac chamber …. How different is the performance with air in the package? It turns out to be very much a “that depends” sort of thing. Usually not as big a deal on a TCXO as on an OCXO. Bob > On Dec 8, 2018, at 9:29 PM, jimlux <[email protected]> wrote: > > On 12/8/18 4:52 PM, Bob kb8tq wrote: >> Hi >> What, no 0.046 +/- -.002” vent hole? > > Not on the physics package of the CSAC. As I understand it, the vacuum > around the physics package is more for thermal isolation than anything else. > You can tell that the vacuum is fading because the heater current starts to > rise > > > Long digression, near rant, on venting requirements follows > > > We use a Volume/Area ratio <2000 inches (oddly, in US customary units, not > metric, unless you want to spend time analyzing the launch pressure profile > and your orifice flow rates). I have no idea where the specific value came > from, other than it's the lowest tick mark on figure 1 in this paper. > > https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980236692.pdf > > In particular that paper cites a reference from 1970. > https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710018690.pdf > which alludes to failures, and also talks a lot about how you can do venting > wrong (put your vent hole where there's a shock wave, let hot gases in, > etc.), but doesn't really address the venting of a box within a box scenario. > > > > > I find that there is often little actual detailed rationale for such > requirements, other than "it worked before, it's easy to meet, so why bother > arguing". > I'll bet that paper (from 1998?) is basically an attempt to provide an > analytical rationale for the "rule of thumb" that probably dates back to the > turn of the 19th century in some way. Maybe Lord Rayleigh wrote about it? > > > > . > > > > >> That used to be a requirement on this sort of thing. >> Bob >>> On Dec 8, 2018, at 6:35 PM, jimlux <[email protected]> wrote: >>> >>> On 12/8/18 11:30 AM, Bob kb8tq wrote: >>>> Hi >>>> You might be surprised by how well the CSAC does in orbit. There have been >>>> a lot of cases >>>> over the years where a device has done much better once it is away from >>>> “poking fingers” >>>> like pressure and other semi-random stuff …. >>>> Bob >>> >>> Oh, I'm pretty sure it will do well in a very benign environment - I joked >>> with the reps that if we could figure out how to vent the enclosure after >>> on orbit, the whole "getter filling up" issue would go away. >>> >>> I don't know that I'll be able to measure the performance. We sort of >>> added it at the last minute, to be able to demonstrate the ability measure >>> & calibrate an OCXO without a GPS 1pps, and didn't give a huge amount of >>> thought to how to do real performance measurement. >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
