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[WP34S] Crystal
Message #1 Posted by robertoataulfo on 27 Oct 2011, 4:22 p.m.

I need crystal to maintain the accuracy of the clock.
Anyone know where I can get?
      
Re: [WP34S] Crystal
Message #2 Posted by Marcus von Cube, Germany on 27 Oct 2011, 4:29 p.m.,
in response to message #1 by robertoataulfo

Any electronic parts shop should carry these. It's a 32768KHz watch crystal together with two 15pF capacitors in 0603 SMD format.

Edit: Correction to the capacitor specification (was nF instead of pF).

Edited: 28 Oct 2011, 3:45 a.m. after one or more responses were posted

            
Re: [WP34S] Crystal
Message #3 Posted by Walter B on 27 Oct 2011, 4:33 p.m.,
in response to message #2 by Marcus von Cube, Germany

Be warned the latter are damn small !

                  
Re: [WP34S] Crystal
Message #4 Posted by Tim Wessman on 27 Oct 2011, 4:57 p.m.,
in response to message #3 by Walter B

Not really. It is a whole 590% larger than a 0201! Seems pretty big to me.

TW

            
Re: [WP34S] Crystal
Message #5 Posted by Jim Horn on 27 Oct 2011, 5:18 p.m.,
in response to message #2 by Marcus von Cube, Germany

Note that the two capacitors are 15pF each, not 15nF.

                  
Re: [WP34S] Crystal
Message #6 Posted by Eric Rechlin on 27 Oct 2011, 5:41 p.m.,
in response to message #5 by Jim Horn

And you do have a little flexibility, too. Basically as long as it's the right order of magnitude it should be fine.

My kit (which I will bring back for sale soon) uses 18 pF capacitors.

                        
Re: [WP34S] Crystal
Message #7 Posted by Kerem Kapkin (Silicon Valley, CA) on 27 Oct 2011, 7:09 p.m.,
in response to message #6 by Eric Rechlin

I noticed one can get several different type capacitors ie ceramic, thin film, etc in 0603 SMD with different tolerances 5%, 3% or 2%. Would anything work, or one better than the other for this application? Any suggestions.

                              
Re: [WP34S] Crystal
Message #8 Posted by Katie Wasserman on 27 Oct 2011, 7:52 p.m.,
in response to message #7 by Kerem Kapkin (Silicon Valley, CA)

They should all work fine. The tolerance and temperature coefficient will be factors in determining the price but not the frequency of oscillation, the tolerance of the crystal will be much more of a factor (not to mention luck in picking a good one). A while back I suggested trying to fit a tiny trimmer cap in place of one of the fixed caps to allow you to adjust the frequency, but I haven't gotten around to experimenting with that.

                                    
Re: [WP34S] Crystal
Message #9 Posted by Paul Dale on 27 Oct 2011, 8:55 p.m.,
in response to message #8 by Katie Wasserman

When I had job of calibrating real time clocks half a dozen years back, I got the joy of determining crystal tolerances. It very much seemed that you got the tolerance you paid for. Buy a 30ppm crystal and it will be very close to that. Not once did we see a crystal performing a lot better than its specified tolerance.

Our conclusion was that crystals are likely measured at the time of manufacture and graded accordingly.

- Pauli

                                          
Re: [WP34S] Crystal
Message #10 Posted by Katie Wasserman on 28 Oct 2011, 4:00 a.m.,
in response to message #9 by Paul Dale

I make some small electronic timers and stuff to give as gifts and have done some similar crystal testing. I think that you're right about that in general, but I found that the 30ppm ones were typically off by 20ppm. But being off about the same amount and (within a batch) almost always the same direction is a good thing. You can compensate for that by choosing load capacitors to pull the crystal to its stated frequency.

                              
Re: [WP34S] Crystal
Message #11 Posted by uhmgawa on 27 Oct 2011, 8:50 p.m.,
in response to message #7 by Kerem Kapkin (Silicon Valley, CA)

Quote:
I noticed one can get several different type capacitors ie ceramic, thin film, etc in 0603 SMD with different tolerances 5%, 3% or 2%. Would anything work, or one better than the other for this application? Any suggestions.

From a temperature/environmental stability perspective: polystyrene dielectric if you can still find them. Outside of military/esoteric capacitors they were the choice for timing critical applications. Unfortunately they are only available in leaded packages, though quite minuscule and inexpensive at the values required here. Failing that an NPO/COG ceramic.

But yes, this will address drift only and you'll need to compensate the xtal to the intended frequency if that's needed. Another thought is to forego absolute accuracy of the xtal oscillator in favor of stability, and compensate for whatever off-target frequency exists numerically.

                                    
Re: [WP34S] Crystal
Message #12 Posted by Marcus von Cube, Germany on 28 Oct 2011, 3:49 a.m.,
in response to message #11 by uhmgawa

We had the latter idea earlier. It would imply keeping an adjustment factor and adding this whenever the RTC is read. If the correction warrants it, adjust the RTC and start over. I'm not very keen at implementing this.


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