Page 157 - Anatomy of a Robot

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142 CHAPTER FOUR
Limit Openings in the Package
Interference can leak out of holes in the package, but given the fact that interference
waves have a definitive wavelength, there’s a trick we can use. Waves cannot easily get
through a hole that is too small for them. At most of the frequencies the FCC cares
1
about, holes of up to /8 of an inch (3 mm) in diameter are just fine. If the air holes, for
instance, are 3 mm in diameter, they can still provide cooling without letting interfer-
ence through. If the robot is to be used in environments that permit even less interfer-
ence, the holes may have to be made smaller again. The higher the regulations go in
frequency, the smaller the holes should be. This includes fan holes too; holes are holes!
One point must be made about limiting the size of holes in the package. When we
talk about limiting the size of holes to 3 mm, we are speaking about the longest dimen-
sion of the hole. If a hole is 3 mm wide and 9 mm long, it will not pass muster. The 9
mm dimension is 3 times too wide. The single worst types of holes in the package are
seams. Often, a cover is put on the package and screwed down. The cover may be 30
mm by 30 mm and be fastened down by several screws. Unfortunately, the 30 mm
seams will leak like sieves.
The interference that leaks out through such holes can be decreased in a couple of
ways. First, it’s possible to have a significant metal overlap at the seams. If the package
overlaps the cover by more than 1 mm, it’s possible to attenuate much of the interfer-
ence that may leak through. To be safe, have a large overlap. The alternative is to have
a spring-loaded metal barrier that acts to seal the seam. Companies sell strips of
stamped copper spring material that can be fastened down the length of the seams, much
like the weather stripping we use to seal storm doors against the cold wind.

Use Special Connectors

Connectors, and the external wires that will connect to them, are a prime place for inter-
ference to escape from the robot. Two characteristics of connectors must be considered:

First, make sure the connector has a shielded, grounded shell. This means the out-
side shell of the connector is connected to the chassis and is grounded. The cable
that connects to the connector can thus also have a shielded connector and outer
metal jacket.
Second, make sure that all the signals in the connector have attenuating filters in
series with them. Don’t forget; interference makes no distinction between input
signals, output signals, power, or ground. Interference can travel in and out of all
types of connector pins. Many connector manufacturers offer versions of connec-
tors with integral ferrite plates that will attenuate high-frequency interference on
every pin. The other option is to build filters into the PCB near the connector.