Page 417 - Complete Wireless Design

Wireless Issues

416 Chapter Ten

noise-producing conducted emissions that can take place through the com-
mon power supply.
Digital EMI suppression. EMI created in digital circuits within the radio system
is quite similar to that in the above analog circuits. However, because of certain
design variances, and the huge amount of harmonics generated by the square-
like waveforms of a digital circuit, there are some added considerations.
Traces on a PCB can transmit EMI at high frequencies—almost as if they
were small monopole or loop antennas—if the traces are unterminated, or ter-
minated into a high impedance. If the trace is not microstrip, but of some
unknown or uncontrolled impedance, then the electromagnetic fields will leak
from the trace and couple into other neighboring ones, causing cross talk
between circuits. In fact, the EMI radiation itself may not even be at the fun-
damental of the actual frequency that is running through the trace, but may
be at one of its many harmonics. These harmonics are more easily radiated
because of their shorter wavelength. Indeed, as the length of the trace gets
closer to one-eighth the wavelength of the signal of interest, then transmission
line effects must be considered to preserve signal integrity, such as confirming
proper line impedances, using shorter line lengths, and employing proper ter-
minations. (Transmission line effects in general infer that after a trace
becomes longer than one-eighth the wavelength, the signal’s response to the
trace must be considered.) And as the frequency of operation increases, reflec-
tions will become more of a problem on a simple trace, causing extreme signal
degradation of the original digital waveform.
Just as in analog RF design, 50 ohms is accepted as the characteristic
impedance of transmission lines and terminations of most high-speed digital
designs, as inordinate I/ T cross talk and EMI (and increased power con-
sumption) are created with reduced impedance values.
For increased EMI suppression in digital circuits within wireless systems,
these considerations must be kept in mind during high-speed design and layout:
1. Keep traces away from the board edge. This can decrease PCB edge emis-
sions by up to 20 dB.
2. Traces should be routed at 90 degree angles to the next layer to lessen
cross talk.
3. Most modern boards have extremely high component and trace density, so
the coupling of EM energy (cross talk) from one trace to another is a large
concern. This can be partially mitigated by slowing pulse rise times,
increasing the distance between traces, and utilizing shorter traces.
4. Heavy decoupling in the high I/ T atmosphere of digital logic should always
be employed to prevent noise and signals running into undesired areas.
5. Splitting a common analog/digital ground plane into two partitions pre-
vents, or at least reduces, undesirable EMI signals in one section from
interacting with the other.

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