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10.3.2.4
Vias (Feed-Through Connections)
Another impedance source that degrades high-frequency circuit performance is vias. Expert lay-
out techniques can eliminate vias to avoid reflection sites on PCBs.
10.3.3 Interference
We have discussed reflections in high-frequency design, their causes and techniques to minimize
them. The following sections discuss additional issues related to high-frequency design, includ-
ing interference. In general, interference occurs when electrical activity in one conductor causes
transient voltage to appear in another conductor. Two main factors increase the interference in
any circuit:
1.
Variation of current and voltage in the lines causes frequency interference. This
interference increases with the frequency.
2.
Coupling occurs when conductors are in close proximity.
Two types of interference are observed in high-frequency circuits:
1.
Electromagnetic Interference (EMI)
2.
Electrostatic Interference (ESI)
10.3.3.1
Electromagnetic Interference (EMI)
Electromagnetic Interference (EMI) is a problem at high operating frequencies: when operating
frequency increases, signal wavelength becomes comparable to the lengths of some of the inter-
connections on the printed circuit board. EMI is a phenomenon of a signal in one trace which in-
duces another similar signal in an adjacent trace. There are two types of coupling between parallel
traces which determine the amount of EMI in a circuit. These are called the inductive coupling
and the radiative coupling.
PHYSICAL DESIGN AND SYSTEM DEBUGGING
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Figure 10-21. Avoiding 90-Degree Angles
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10-25
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