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I am laying out a (2-layer) board, with a standard SMD Crystal as well as a Real-time clock placed on the top-side of the board.

What are guidelines regarding ground copper pour under the Crystal and under the RTC, for their optimal performance? In other words, should I maintain pour-keepout areas on either side of the board, or does it not matter?

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I generally try to keep ground pours away from crystal traces, especially if they're more than a few cm long. One reason is to keep stray capacitances to a minimum — even a few pF can be significant. Another reason is to avoid coupling currents that might be flowing in the ground plane into the oscillator, which could increase clock jitter (which may or may not be significant in a particular application).

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  • \$\begingroup\$ Dave's is a better answer for the top of the board, I only considered the bottom when I answered. \$\endgroup\$ – Some Hardware Guy Oct 25 '12 at 16:29
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No need to use keepouts, unless there's something specific in your manufacturers datasheet that calls for it. For what you describe there probably won't be.

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