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I am using this crystal (the 7.3728 MHz variant).

The datasheet specifies a 5pF to 55pF load capacitance; is this really so? I was under the impression that most quartz crystals had an optimal capacitance which was fairly critical for oscillation.

In my design, I have specified 18pF based on another crystal (of the same frequency and nearly the same specifications), which is not available at the moment. In that case, the datasheet clearly specified 18pF.

If it matters, I am using a dsPIC33FJ128GP804 microcontroller/DSP.

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The crystal manufacturer (ACT) can make crystals with any load capacitance in that range.

This page states that it is 30 pF for the crystals supplied to Rapid Electronics:

http://www.rapidonline.com/Electronic-Components/7-3728mhz-S-mount-Crystal-90-1914/?sid=6e129b4b-b0e4-4437-a520-c8e5cf20e996

That isn't the actual value you use for the feedback capacitors, of course. They are effectively in series.

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  • \$\begingroup\$ So, for a 30pF rated load capacitance, I would use 15pF? (Or ~60pF?) \$\endgroup\$ – Thomas O Feb 1 '12 at 22:56
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    \$\begingroup\$ Something like 50 pF, allowing for stray capacitance. \$\endgroup\$ – Leon Heller Feb 1 '12 at 23:02
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The datasheet linked to is for the whole series from 3 - 100 MHz, so the load capacitance stated is the range of load capacitance over the series.

For that particular frequency, the load capacitance is given on the product page (under technical specification - relevant part shown below):

Crystal specs

As you can see the recommended value is 30pF for 7.3728 MHz.

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  • \$\begingroup\$ +1 for spotting that - an unclear datasheet. I'm guessing Rapid get the loading data from somewhere else than that datasheet. \$\endgroup\$ – Thomas O Feb 1 '12 at 22:55

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