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

Roland F. Portman (portman@bitmicro.com)
Thu, 19 Feb 1998 12:13:22 -0800

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Steve,

You state you are feeding a 110Mhz clock to a chain of 74AC161s. You
also
state you have 'relatively short traces'. The Fairchild spec for the
74AC161 states the outputs can drive +/- 24mA. I don't see a spec for
rise/fall times,
but 24mA is pretty healthy drive and the steep edge rates may contribute
to your EMI.
If the signals are point to point, may I suggest you
series terminate these outputs to reduce the rise/fall times and /or
consider a different device family
with lower drive current if your circuit allows.

Furthermore, you don't mention what voltage you supply to the 74AC161s,
PCB stackup, signal impedances,
nor on which layer the offending signals reside. These need to be
looked at.
Sandwiching the noisy signals inside the power/gnd planes, isolating the
pwr/gnd, or adding a metal
can are band-aid approaches. The problem needs to fixed at the source.

Just to note, Isolated power and/or ground planes open up a whole can of
worms which I am not
familiar enough to comment about. I know someone on the si-list will
address it thoroughly.

Good luck,

Roland Portman
portman@bitmicro.com

Lund, Steve wrote:

> 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

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