Monitoring the output frequency whilst the oven "heats" up may give additional clues as will monitoring the heater current.
Bruce > > On 04 March 2018 at 16:12 Tom Holmes <thol...@woh.rr.com> wrote: > > Bob... > > Interesting point about the heater not working vs the XTAL having drifted > too far. Mine has the same symptoms as the others reported (EFC at the end of > its rope) but have not tackled it yet, figuring I'd have to dismantle the > whole thing. Certainly troubleshooting a non-operating heater would be much > more pleasant. > > Thanks for that insight. > > Tom Holmes, N8ZM > > -----Original Message----- > From: time-nuts <time-nuts-boun...@febo.com> On Behalf Of Bob kb8tq > Sent: Saturday, March 03, 2018 2:07 PM > To: Tom Curlee <tcur...@sbcglobal.net>; Discussion of precise time and > frequency measurement <time-nuts@febo.com> > Subject: Re: [time-nuts] Z3801A OCXO manual trimming > > Hi > > First off some basics about OCXO’s. > > In a single oven design, you have a heater that warms up the entire > crystal and the guts of the oscillator. It is on all the time and > it gets things up to a temperature that makes sense for a given crystal. > It can be adjusted based on manufacturing data or by > trial and error to match the characteristics of that crystal. > > In a double oven design, you have two ovens that are on all the time. One > heats up the other one. They both work together to > achieve the end result. The gain of one adds to the gain of the other to > give an improved result. On some double ovens, the > entire heat range of the inner oven is only 10’s of degrees …. > > In a boosted oven, you have a second heater to get things going when it > is very cold. This is an unusual approach and rarely > seen. Its normally easier to just design a bit more power into the main > oven circuit. In a boosted design, the boost heat goes > away in normal operation at typical temperatures. In normal operation, > the gain of the boost circuit does not count. > > SO …. > > The oscillator in the Z3801 is a boosted 10811. It is boosted to allow > them to hit a spec of -40C on the unit. At the time it was > designed, there was talk about mounting these things in un-heated boxes > outdoors. After they got a bit further into all the > details of the designs … that part went away. The spec still hung around > long enough to apply to very early designs. > > The net result is that you can pretty much destroy the outer heater stuff > and the oscillator will work fine. There is no need > for it in a typical lab. There are some alarm triggers that need to be > wired “ok” when you do so. The details are in the archives. > > But … > > Best guess if your unit is at max EFC = the “real” heater on the 10811 > has quit working. To get at that, you will need to dig > into the guts of the unit. Given the massive EFC on the Z3801 version of > the oscillator, it would take a crazy amount of aging > to hit limit. > > Bob > > > > > > On Mar 3, 2018, at 1:37 PM, Tom Curlee <tcur...@sbcglobal.net> > > wrote: > > > > Since the Z3801A is being discussed, I thought I'd ask about an > > issue I'm having with my unit. I use my Z3801 as my working lab standard > > for the usual pieces of RF test equipment. In the past year or so I've had > > the unit drop out of lock and go into standby mode. Resetting/cycling power > > would bring it back into lock for a while, but it generally got worse and > > now stays in hold over mode. LH (thanks Mark Sims!!) reports that > > everything is operating normally except that the it has a PLL unlock. The > > one highly suspicious item is that the DAC is at 99.996902% - full output. > > The unit shows it has over 94.5K hours run time, so I suspect that > > the OCXO has aged to the point that the EFC can't pull it into lock. I > > disassembled the OCXO to see if it had a trimmer capacitor like the > > standard 10811 units. After removing the outer case and foam insulation, I > > see that the outer heater is one of the thin printed circuit serpentine > > heaters on what I think is Kapton. That would need to be peeled off of the > > case to either get to the hole for the trimmer (if there is one) or to > > remove the cover for the inner case. > > What I don't understand is the purpose of what looks like another > > coil or heater wrapped around the Kapton printed circuit heater stuck to > > the inner case. This second coil/heater is 2 layers of 1/8" thick red foam > > wrapped completely around the inner case, with fine copper wires wrapped > > over each layer. At least I think there are wires on each layer. This whole > > second heater is taped down and I don't want to dig any further until I > > know a bit more about what I'm doing or find that there isn't a trimmer on > > the double oven 10811 oscillators. > > Any ideas on the purpose of the outer heater (or whatever it is)? > > Does the double oven 10811 have a manual trimmer that I can adjust to bring > > the oscillator back to the center of the EFC range? Assuming that I can > > manually adjust the OCXO back into adjustment range, will there be any > > issues with the Z3801 performance, things like phase noise, short term > > stability, etc? > > Any suggestions will be most appreciated. > > Thanks, > > Tom > > > > _______________________________________________ > > 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.
