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Topic: Crystal uses (compared to ceramic resonators) (Read 20543 times) previous topic - next topic



One thing to consider with using a crystal versus a resonator for timing is how long you are going to be keeping time.  There are 10080 minutes in a week.   If your resonator is off by 0.5% (worst case) your clock will be off by 50 minutes.  where as a crystal at worse case (0.05%) will only be off by 5 minutes.   So if I need a timer for cooking an egg, the resonator is fine, but when I come back from a week's vacation, i could tolerate my arduino based alarm clock going off 5 minutes late, but not 50 minutes late.

Look at what you are wanting to do figure out what will work.  

Surely if you were worried out this sort of time scale you'd use a watch crycstal anyways? They have in turn a better accuracy than a typical microcontroller crystal


Ben, I agree with you 100% (which is why I make kits with resonators).

To be fair, the following illustrates the difference between resonators and crystals:

Price$0.18$0.27 (plus caps)
Tolerance0.5%0.05% (50ppm)
Temp Stability0.3%0.05% (50ppm)
10yr Stability0.3%0.05% (50ppm)

The worst case scenario (end of the temperature scale, 10+ years old, and worst sample) is about 1.1% (15.82MHz - 16.17MHz) while the crystal is about %0.15 (15.976MHz - 16.024MHz). Obviously you're unlikely to see this kind of error in either component. You'd have to look it up, but I believe NASA testing showed their resonators to be within 100ppm of the target frequency from the factory. It's also important to realize these numbers are based on a perfect board layout (and capacitor matching for the crystal, badly matched load caps can severely throw off the native frequency).

To be fair, here's a correction to your numbers there spiffed: 50ppm != .05%.  50ppm = .005%.  The ABL-16.000MHZ-B2 that I found on digikey is rated at +/- 20ppm (.002%).  According to the spec sheet the 2 at the end of the part number = 20ppm tolerance.  Worse stability for that part series is 35ppm.  Assuming 10yr stability (which isn't in the datasheet) might be 50ppm, that puts worse case on this partnumber at 105ppm, or 0.0105%.
That means the frequency range would be (worse case not accounting for caps) 15.99832MHz - 16.00168MHz, which to me is a good bit more accurate than the specs you quoted for the ABL-16.000MHZ-B2.

Current price difference between the afore mentioned parts on digikey is $0.12 for the resonator and $0.20+caps for the cyrstal.

I'm not trying to sway anyone one way or the other, I just noticed a misrepresentation of ppm (which stands for parts per million, IE 20ppm = 20/1,000,000).  My field of work deals with parts that require tolerance as low as <0.1ppm accuracy so every decimal makes a big difference, 10 fold in fact.


Quijonsith is correct, I dropped a 0 on all of the crystal measurements, and the "ordering options" does indeed indicate a native tolerance of +/-20ppm. I was however unable to find the stability option for either Mouser's or Digikey's offering. In volume these come as -X's with +/-20ppm. If you're saving a few pennies, -4H models are out there though, which is still only +/-0.003.5%.
Long term stability is given for 25yr lifespan at +/-5ppm, specs for the ABL's surface mount cousin (who's model escapes me) show a nice smooth graph ramping rapidly to about 4.5ppm within 5 years and then a slow rise to 5ppm over the next 20 years. It is not clear what ageing effects occur at temperatures other than 25c.

I apologize for lousy math on the crystal; most of my intention was to show resonator accuracy was within acceptable rates for many tasks. The crystal specs were shown for comparison of what could be achieved - lesson, don't do math at night, without using a calculator, then don't depend on it later to do more math.

With regard to pricing, those are /1000 prices. While commercial orders should be in the 10-1000K range, I'm fairly sure most thread readers are not budgeting their Arduino projects in that range.

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