Does an oscillator module need bypass caps? - Page 1
Forum section: electronics / beginners

author: RogerRowland
author_rank: Regular Contributor
timestamp: October 04, 2015, 07:28:04 pm
content: Maybe a stupid question but, I normally use just an external crystal (with load caps) when running my PICs at 20MHz, but now I've got hold of some oscillator modules - these oneshttp://www.ebay.co.uk/itm/20Mhz-Crystal-oscillator-TTL-HCMOS-8-pin-DIL-format-Qty-1-NEW-/111777853837?- because it saves me a desperately needed pin. All works well on breadboard, but I'm wondering if I need bypass/decoupling caps on the oscillator's power pins?As you can see from the eBay listing, there's no datasheet in sight and Google hasn't helped.I guess it wouldn't hurt, but is it normal to add bypass caps to these modules?

author: Gyro
author_rank: Super Contributor
timestamp: October 04, 2015, 07:33:43 pm
content: Yes it does, treat it like a normal high speed logic chip. It might possibly have some limited internal supply decoupling but this will be severely restricted by package size so assume there isn't.

author: RogerRowland
author_rank: Regular Contributor
timestamp: October 04, 2015, 07:40:14 pm
content: Quote from: Gyro on October 04, 2015, 07:33:43 pmOk, thanks for the quick answer - I'll make sure I add this to the schematic before I get PCBs made.

author: bktemp
author_rank: Super Contributor
timestamp: October 04, 2015, 07:43:12 pm
content: All DIP8/DIP14 sized crystal oscillator modules I have opened had a decoupling cap inside, but you can't be sure unless you open each module, so add an external one.

author: Ian.M
author_rank: Super Contributor
timestamp: October 04, 2015, 08:38:11 pm
content: Datasheet:http://www.quartz1.com/price/PIC/405N3134800.pdfThe test jig circuit has a bypass cap, so you should use one. Also pin 1 is output enable and should be tied high.

author: RogerRowland
author_rank: Regular Contributor
timestamp: October 05, 2015, 04:44:18 am
content: Quote from: Ian.M on October 04, 2015, 08:38:11 pmExcellent find with the datasheetThat's really helpful, thanks. The eBay description didn't mention the enable pin, which I guess is either pulled up internally or I just got lucky, because it works floating. I'll make sure to tie it high now. Thanks again.

author: jwm_
author_rank: Frequent Contributor
timestamp: October 05, 2015, 04:59:42 am
content: Quote from: RogerRowland on October 05, 2015, 04:44:18 amI aquired about 200 20mhz oscillators just like that as part of an ebay score somewhere, after using about 4, having fun dremeling open some others and watching what happens when i take a sharpie to the crystal to tune it, I have roughly 190 left. If someone wants a couple I'll send em for the price of postage from southern california.For some reason, occasionally those oscillator modules come up in bulk for insanely cheap on ebay, usually loose with bent leads. maybe someone knocks over a bin at the factory and they sweep em up and put em online? I never had issues , appear within spec according to my standard so I don't think they are factory rejects. I always use them for self-made boards to just not worry about getting the capacitance/layout right for stable crystal starting up. that and i have a couple hundred to use up... When all you have is a bin of 20mhz oscillators, everything looks like something that should run at 20mhz...

author: Cliff Matthews
author_rank: Unknown
timestamp: October 06, 2015, 12:58:30 pm
content: Quote from: jwm_ on October 05, 2015, 04:59:42 amPM'ed Thanks!I promise I won't be cruel with a sharpie, but how much deviation were you able to get? Any photo's?

author: retrolefty
author_rank: Super Contributor
timestamp: October 06, 2015, 02:30:07 pm
content: QuoteLove it.I once scored like 25 50MHz crystal clock modules, just because I could.

author: Electro Fan
author_rank: Super Contributor
timestamp: October 06, 2015, 08:13:07 pm
content: Quote from: jwm_ on October 05, 2015, 04:59:42 am+1 more