[snip] %> %>It is common to see high frequency circuits on PCBs separated with %>ferrite beads and having isolated, DC-coupled 0V planes. Often, the %>oscillator is designed with its own Vcc/0V island. % [snip]
This is a little bit off topic from making islands, but based on my memory of some material I've read and my own opinion, I think the following might (i.e., I'm guessing) be a good idea. With all oscillators, you could run the power (not the ground) through a ferrite. Then you would place a bypass cap to ground, between the ferrite and VCC. The ground plane would not be filtered or separated in any way--it remains in one solid piece. The net effect should be a cleaner power and ground system. What this ferrite/cap configuration would do is to force the oscillator to draw all transient current from the capacitor. Or, in otherwords, the rest of the caps on the board would be unable to help provide fast-transient current because of the ferrite. This would keep the current loop (power AND GROUND) small and prevent it from infecting the rest of the board. The path for the transient current surges would be from the capacitor to the IC's power pin, out the IC's ground pin and back to the negative side of the capacitor (a relatively small loop). Come to think of it, this EMC method would probably work with any type of IC, with the only limitation being the storage capacity and ESR and ESL of the capacitor and the impedance of the leads and via's, etc. If memory serves and I interpreted the article correctly, I think this method of keeping VCC and ground clean was described in the EDN article that I mentioned previously. Max mkel...@es.com
