mercury0x000d wrote: Oh, I see. So it's basically just a solid layer of copper designed to let the stray interference return to ground instead of jumping to other neighboring traces.
You might have been reading about what happens in high-impedance, low-frequency (like audio) work, where capacitive coupling is the major thing.
High-speed digital is low-impedance, and the inductive aspect is the greater concern. A signal line conducting current sets up a magnetic field around it. If the current level rises quickly (as is needed in high-speed digital's fast edges), the quickly changing electromagnetic wave surrounding it goes out, and, as it cuts through other lines, it induces a voltage in them. This magnetic field is kind of like inertia too though, and it makes it harder to quickly raise or lower the current in a conductor. There is an exception. If the ground plane is right there under the line, the magnetic field makes an equal current flow in the opposite direction, as close as possible to the signal line, and it kind of "shorts out" the magnetic field, making it easier for the signal line because a magnetic field does not have to be built up. This is the effect of mutual inductance. It makes it easier to get the fast rise times and also prevent overshoot; and since every signal current has its return current super close to it, the area around is quiet. By the same token, the pair becomes more immune to electromagnetic fields from outside too, because it is acting as a balanced pair.
Quote: I did a little reading on this it seems there should be layers as follows: the component (silkscreen) side, then the insulated board, then a middle ground plane (one big piece of copper connected only to ground), then another insulated board, then the circuit layer where the component leads stick out and the soldering is done. Is that about right?
If you can make all the connections on a single layer, you can get away with just two layers, and the ground plane can be the solder side if desired, and then thru-hole parts still get soldered on the back. Thru-plated holes will make the connections between layers where appropriate and will wick up the solder in the soldering process. Actually, with SMT, the component side is forced to be a signal layer.
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