A comparison between AT and SC cuts' temperature stability can be found here:
<http://www.conwin.com/pdfs/at_or_sc_for_ocxo.pdf> On Mon, Nov 21, 2016 at 4:21 AM, Bob Camp <kb...@n1k.org> wrote: > Hi > > If the frequency drops as the oven warms up, you have an AT cut crystal. If > the frequency goes up as the oven warms up, you have a BT cut crystal. With an > AT or a BT, the frequency change between room and “hot” will depend a lot on > the details of the proper oven temperature. A frequency shift of 20 to 40 ppm > is not > at all unusual as the oven warms up. The oscillator will only tune on > frequency > once the oven is hot. > > http://www.aextal.com/tutorial-frequency-stability.htm > > Regardless of which cut you have (an AT is the best guess). The oven > temperature > would be adjusted to put the crystal at it’s minimum sensitivity point. For > an AT that > is the lowest frequency. If the oven is not heating to the correct > temperature, you will > probably be unable to get the oscillator on frequency. > > If there is a circuit problem, the most likely culprit are the inductors. > They form tuned > traps that put the circuit onto the correct overtone. This increases the > circuit’s sensitivity > to changes in inductance. 50 year old inductors may have been made with core > materials > that aged more than just a little bit. I have empirical data on this :) > > As others have suggested, check the oven heater along with the oscillator > circuit. The > issues you see might come from either one. > > Bob > >> On Nov 20, 2016, at 6:49 PM, Michael Ulbrich <m...@rentapacs.de> wrote: >> >> Hi there, >> >> I'm new to this list and have some interest in quartz crystal and >> rubidium oscillators - GPS, NTP, PPS and "clock watching" in general ;-) >> >> I snatched an R&S XSD frequency standard accompanied by an XKE frequency >> controller. Found the manuals on KO4BB's site (Thanks a lot for that! >> BTW there's a slight mix-up of pages in that some of the XKE pages have >> found their way into the XSD manual). >> >> My initial hope of a quick and easy restoration project faded a bit when >> I looked at the XSD output frequency after heating up the oven. It was >> off by about -1 * 10-5 and after cranking the fine tuning for some time >> and having another look at the specs I realized that the frequency was >> too far off to be dialed in by the fine tuning which only covers about >> +/- 2 * 10-7. >> >> Next step was to take apart the oven and check series capacitor, >> oscillator and the crystal itself. I found that even the coarse >> adjustment range of the cylindrical series capacitor (40 - 110 pF) would >> not allow to pull the crystal to it's specified frequency. When >> replacing the series cap by a ceramic and lowering the value to just >> before the oscillation breaks down (about 10 pF) I managed to set the >> oscillator frequency offset to +2.5 * 10-6 at room temperature. But >> even this will not suffice when taking into account that the frequency >> will drop by a few parts in 10-5 (cf. XSD manual) when the oven heats up >> to its operating temperature. I also checked some of the components on >> the oscillator PCB which might have an influence on frequency but could >> not find any fault. >> >> The crystal itself is a disk of about 30 mm diameter mounted in a sealed >> glass cylinder of about 38 mm dia. and 43 mm height. There is no socket >> just 2 bare wires.. It does not show any visual signs of damage. >> According to a reference given in the R&S XSD manual the crystal's >> construction follows a publication from A.W. Warner "Design and >> Performance of Ultraprecise 2.5-mc Quartz Crystal Units" in Vol 39, >> Issue 5 of Bell Labs Technical Journal (Sept. 1960). According to that >> it is an AT-cut 5th overtone design. >> >> Now my questions: >> >> a) Considering that this gear is about 50 years old, a "crystal gone >> bad" situation shouldn't be that much of a surprise, right? But could >> there be any other cause of the "huge" frequency offset besides a bad >> crystal? I would very much appreciate any idea that I could try to get >> this baby back on spec. >> >> b) if nothing else helps: Could any of you give me a hint about who >> might be able to supply a spare crystal? I tried my directly reachable >> contacts but unfortunately to no avail so far. Please consider that >> similar crystals might also have found their way into other >> manufacturer's constructions from that era - Sulzer, Racal, HP ... you >> name it ... >> >> Many thanks in advance! >> >> Best regards ... Michael U. >> _______________________________________________ >> 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.