[WP34S] Crystal



#2

I need crystal to maintain the accuracy of the clock.
Anyone know where I can get?

#3

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


#4

Be warned the latter are damn small !


#5

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

TW

#6

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


#7

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.


#8

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.


#9

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.


#10

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


#11

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.

#12

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.


#13

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.


Possibly Related Threads...
Thread Author Replies Views Last Post
  capacitors for wp34s crystal hack Andrew Nikitin 12 1,276 08-02-2013, 03:30 AM
Last Post: Bruce Larrabee
  I still have wp34s crystal and cap sets David Griffith 3 441 11-01-2012, 05:13 PM
Last Post: Dale Reed
  [WP34S] WP34S firmware on the AT91SAM7L-STK dev kit? jerome ibanes 1 402 10-04-2012, 04:59 PM
Last Post: Paul Dale
  [WP-34s] Updated: How to install crystal and IR diode Alexander Oestert 2 438 06-17-2012, 12:19 PM
Last Post: Geoff Quickfall
  [WP-34s] Crystal frequency jerome ibanes 6 608 05-25-2012, 07:36 PM
Last Post: Paul Dale
  [WP-34s] How to install crystal and IR diode.pdf Alexander Oestert 27 1,996 05-04-2012, 06:21 AM
Last Post: Dominic Richens
  I still have wp34s crystal and cap sets David Griffith 0 224 05-03-2012, 07:54 PM
Last Post: David Griffith
  WP-34s: Crystal and capacitor instructions? W. Bruce Maguire II 2 438 05-03-2012, 02:04 PM
Last Post: W. Bruce Maguire II
  wp34s crystal kits -- don't worry David Griffith 7 757 04-16-2012, 04:58 PM
Last Post: Marcus von Cube, Germany
  I still have wp34s crystal and cap sets David Griffith 0 209 04-03-2012, 01:07 AM
Last Post: David Griffith

Forum Jump: