| I/O lines
usually are signals that enter and exit whatever electronic box we may be designing. The
I/O lines consist of signal lines that need to be connected to another external box or
piece of equipment located somewhere else. This box or equipment can be anywhere from a
few centimeters away to as far as 10's of meters away. The potential long lengths the
wires have to be in order to carry the I/O signals, means these wires can be potential
antennas. For example, at 300 MHz, the wavelength is one meter. Any I/O wire length that
approaches half a meter long makes a good unintentional antenna. To keep the I/O signal wires from potentially radiating
unwanted signals, we need to be very careful how we route the I/O signals inside the box
we are designing. We need to keep the I/O lines or traces away from any high frequency
signals on the printed circuit board. It is good practice to position I/O devices close to
the I/O connector away from other high speed traces. If we route the I/O traces far into
the board away from the I/O connector, there is an increased chance the I/O signals can
have unwanted high frequency harmonics capacitively or inductively coupled to them. Since
the I/O wires exit the box and may be sufficiently long to be effective antennas, by
keeping the I/O traces near the entry/exit point of the box away from any high frequency
lines, we will minimize the likelihood of unwanted emissions.
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EMC Tip 14 |