Re: SI and EMI training

rickman wrote:
Paul Keinanen wrote:
On 25 Aug 2006 14:55:00 -0700, "rickman" <gnuarm@xxxxxxxxx> wrote:

I just took a course in SI and EMI and I was very impressed with the
instructor. He made the entire subject very simple to grasp. I feel
like I can do my next board level design with virtually no doubt about
meeting the SI and EMI requirements.

Doing RF design in the UHF or microwave bands for a few years is a
good background for digital equipment PCB design. When you intuitively
look at every PCB track as a microstrip or a stripline transmission
line should help a lot in designing trouble free PCBs.

Unfortunately adding one or two extra ground plane layers into the PCB
and/or using extra board area for transmission lines with proper
impedance levels, might not be acceptable for high volume product due
to cost reasons. For low volume products, I don't see why proper
design is not done, since avoiding a few iterations in the EMC tests
will save the extra cost of proper PCBs.

He also showed some examples of high volume products which have to at
least meet FCC part 15 EMI requirements. His method is to use a 4
layer board with power and ground closely spaced to the surface layers
to get the right impedance. Since this puts the power and ground
planes far apart the surface layers are flooded and used as the other
half of the plane capacitors. The combination of close spacing to the
traces and well decoupled power supplies reduces EMI from the board to
acceptable levels.

The example he showed us was the 4 layer board designed for the X-box.

Putting the power and ground close to the surface layers is a well
known decoupling trick. I have put power and ground *adjacent* and near
the surface a number of times, and it certainly helps (distributed
capacitance, minimum inductance to the planes). A 4 layer board can
have more control of layer distance than a much denser board, giving
more options for decoupling.

It is part of an overall strategy; it's not a panacea.

Cheers

PeteS

.

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