EEVblog #1139 - TCXO Oven Oscillator Repair - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: October 25, 2018, 01:51:39 am
content: Well, repair-ish. Fixing the Systron Donner oven crystal oscillator.

author: johnlsenchak
author_rank: Frequent Contributor
timestamp: October 25, 2018, 03:44:49 am
content: Sell the whole test unit for gold scrape , make a few bucks and then buy a large amount of 3 cent micro-controllers

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 25, 2018, 08:29:00 am
content: Interesting for historical (engineering) reasons.As the oven control runs directly from 110VAC, I assume, that this glass part on top of the heater winding is sort of mechanically acting thermo-switch only, not an NTC. The probability of that is high after all these years. Please check this part by a continuity measurement, and also the winding resistance, if it's burnt.Maybe there's simply an open soldering junction.In the end, the whole assembly is an OCXO, not a TCXO.Frank

author: MT
author_rank: Super Contributor
timestamp: October 25, 2018, 03:54:22 pm
content: Oven crystals is usually aged before assembled. Dont ruin it!

author: Towger
author_rank: Super Contributor
timestamp: October 25, 2018, 05:08:35 pm
content: Cringed with your opening technique.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 25, 2018, 07:28:46 pm
content: Quote from: Dr. Frank on October 25, 2018, 08:29:00 amIsn't an OCXO a particular form of TCXO?

author: edpalmer42
author_rank: Super Contributor
timestamp: October 25, 2018, 07:57:08 pm
content: Quote from: SiliconWizard on October 25, 2018, 07:28:46 pmNo.TCXO = Temperature Compensated Crystal Oscillator (XO is an old ham radio abbreviation for crystal oscillator). It's designed with components that drift with temperature in such a way that everything balances out and the frequency stays approximately constant.OCXO = Oven Controlled Crystal Oscillator. An oven is used to maintain the circuit at a precisely controlled temperature. It's not a random temperature. Every crystal has a sweet spot where small changes in temperature cause almost no change in frequency. It's usually in the range of maybe 60C -80C, but can be outside that range. This temperature varies slightly from crystal to crystal even if the part numbers are the same. The shape of the temperature vs. frequency curve depends on how the piece of quartz is cut from the raw quartz. The two most common cuts are AT-cut and SC-cut. Other cuts were used in the past so the OCXO in this counter might be an older cut. Very old ovenized oscillators used the oven to just keep the crystal above ambient temperature. The 'sweet spot' idea might not have been known at that time.

author: thm_w
author_rank: Super Contributor
timestamp: October 25, 2018, 08:31:03 pm
content: Quote from: edpalmer42 on October 25, 2018, 07:57:08 pmIt looks like AT cut is most stable at 60-80C and SC cut most stable 50-70C or so.But as you said other cuts exist, which could have the optimum range within 25C, or slightly above that. Although there must be some reason these were not used much commercially? Because AT cut seems like a bad choice for a device operating near ambient office temperatures.http://www.quartzpro.com/crystalsfacts.htmlhttp://www.conwin.com/pdfs/at_or_sc_for_ocxo.pdfhttps://www.iqdfrequencyproducts.com/blog/2015/07/30/quartz-crystal-stability-how-myths-and-misconceptions-mask-good-value/

author: richnormand
author_rank: Unknown
timestamp: October 25, 2018, 08:51:17 pm
content: well... It looks like I was wrong in my previous post (first video) about your OCXO.It looked as the same form factor but it is different than the one in my Systron Donner 6054B.In my case the trim control is well buried in the foam and there is a plastic guide for a slim screw driver as you can see in the photo. There is also a coarse adjust and a temp adjust (inside).You can also see two heating coils around the inner casing. You have to wait about an hour for the temperature to stabilise for the fine trim to actually work properly.I'll put an edit on my previous post pointing to this one for accuracy's stake.Enjoy your videos.Cheers.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 25, 2018, 10:36:11 pm
content: Quote from: thm_w on October 25, 2018, 08:31:03 pmI don't know why some of the older cuts have fallen out of use. Maybe harder to manufacture or require special procedures. Maybe just that AT and SC do everything necessary.QuoteNo, the drawing you found is correct, but incomplete. If you look at the attached drawing, you'll see that you can get an AT-cut crystal that has just about any sweet spot you want. The blue curve looks like it would be quite flat from about 15C to 35C.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 26, 2018, 01:37:42 am
content: Quote from: edpalmer42 on October 25, 2018, 07:57:08 pmYes most of us know the definitions. Obviously those are different devices in the most common use of the terms.But behind a bit of wordplay, I was still trying to expose some food for thoughts.TCXO also stands for "temperature controlled crystal oscillator". An OCXO is also temperature controlled actually. Obviously in one case there is no action on temperature itself and the oscillator is compensated for temperature variations. In the other case, the temperature at which the oscillator runs is controlled (which allows much better stability). Still two sides of a similar story in a way.Anyway, as Dave's video bears the correct title, I'm sure this was a finger slip when typing this thread's title. Either that, or he was also trying to provoke some thoughts.