[SI-LIST] : PCB design techniques for EMC control

Lund, Steve (SLund@tuthill.com)
Wed, 18 Feb 1998 16:11:12 -0600

Does anyone out there have any good first hand experience of PCB design
techniques for controlling radiated emissions? I have looked at a lot of
the available literature and find it does not directly relate to PCB
design. At this point I am mainly interested in the effects of isolated
power and ground plane islands around the offending circuitry.

Here is my situation. We have an embedded system in a sheet metal
enclosure. The PCB occupies an area of about 1 square foot in the bottom
of the box. The PCB is currently constructed using a continuous power
and ground plane for the whole board. This board is also utilizing
through-hole technology (DIP ICs, etc.)

A small part of the circuitry consists of a 110 MHz can oscillator
feeding a divider chain of 74AC161s. This circuitry occupies an area of
only 3"x4". Needless to say the harmonics of the 110 MHz oscillator are
causing our radiated emissions problem. I have looked at the signal
fidelity in this circuit and it is surprisingly good no doubt to the
relatively short traces and ground plane.

I would like to try to eliminate as much of this emissions problem at
the source if at all possible by manipulating the board layout in this
area. Here are some possible changes that might help the emissions
problem. Please let me know if you have any experience with these
techniques:

1. Put a metal can over the offending circuit area.

2. A separate isolated power plane coupled with a ferrite bead.

3. Bead isolated supply for the crystal oscillator only.

4. Separate isolated power and ground plane both isolated with beads.

5. Separate direct-coupled power and ground plane on the outside layers
with
the signals sandwiched between them.

Thanks in advance.

Steve Lund
Emco Electronics
Slund@tuthill.com