EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: magic on August 08, 2021, 09:03:17 am
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I recently found that high frequency (MHz) quartz resonators are available in 3x8mm cylindrical cans similar to common 32kHz crystals and in smaller yet 2x6mm cans. The latter has to be really small inside; narrower than even the SMD crystals I have seen.
Unfortunately, the 2x6mm cans are only found on auction sites without datasheets, so I'm not entirely sure what to expect. Are those things likely to be drop-in substitutes for common HC49 crystals? Is there any "bigger is better" when it comes to quartz?
My need right now: free up a bit of space in a commercial USB device by replacing a 12MHz HC49 crystal with something smaller. Accuracy requirement is a modest 500ppm.
I generally always believed that quartz is quarts, nothing to see here, but then one day I discovered overtone crystals ::)
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I'm not sure if it only applies to 32kHz tuning fork crystals in that can size, but I suspect that crystals of that size might have a lower maximum drive capability compared to HC49 packaged ones.
Comparing the specs of the smallest SMD crystals ought to give you some sort of indication.
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OK, I did my homework.
It appears that the smallest SMD crystals out there are actually 1.2×1mm, much smaller than I thought. So 2mm diameter doesn't seem that suspicious anymore. Specs for load capacitance and ESR are similar to the big ones, but maximum drive level is lower indeed - typically 50~100µW instead of 500~1000µW.
Drive level is ESR times RMS crystal current squared and some good people helpfully advise that crystal current can be estimated by measuring RMS voltage swing at the input side of the oscillator amp (with a low capacitance probe) and dividing by reactance of the load capacitor plus any stray capacitance.
Assuming 19pF total capacitance we get 700Ω at 12MHz. With a pretty high (IME) input voltage swing of 0.5Vrms (1.4Vpp) we get 0.7mArms and it takes 100Ω ESR to get 50µW, while crystals in that range are specified 30Ω max.
If my math is right, it seems that even small crystals in the 10~20MHz range are unlikely to be overdriven by a typical MCU oscillator.
If those round can crystals don't operate on some completely different principle I suppose they are gonna be good.