Electronic – How close do I really have to place a 25MHz crystal


I'm laying out a very space constrained PCB. Normally I would place a 25MHz crystal absolutely as close as possible to the chip using it. However, on this PCB, something else really needs the space that the crystal would be.

How bad is it really to move the crystal about 5-7mm from the chip?

Rough sketch of 25MHz crystal layout

The PCB is mostly digital electronics, but there will be some analog stuff about 20mm from the crystal.

Best Answer

7 mm between a 25 MHz crystal and the chip driving it is no big deal. What is far more important is that the ground side of the crystal caps connect back to a ground pin on the part, not just punched thru to the ground plane. You don't want those high frequency currents running accross the ground plane else it will become a center-fed patch antenna. All the ground pins and other immediate ground connections to the chip (like the crystal caps) should be connected in a net with lines as short as you can manage, then that net connected to the main ground in one place only. This keeps all the little high frequency currents local, with only the external currents flowing accross the ground plane.