Page 424 - Complete Wireless Design

Wireless Issues

Wireless Issues 423

This formula infers that varying the diameter of a via will not do much to
change the via’s inductance, while altering its length has a profound effect,
which is why all vias (and conductors in general) to ground must be kept short,
or the inductive reactance can become significant at higher frequencies.
The printed copper board’s traces will have a certain amount of resistance,
and this resistance can adversely affect a circuit’s operation. Since a trace has
this resistance, it will mean that any two points along a trace will not have the
same voltage, which can cause problems as to what the actual ground refer-
ence level is (it should be zero volts). At RF, this resistance is further increased
by the skin effect.
Vias should be placed at regular intervals of a quarter-wavelength or less
through the top ground plane down to the true bottom ground plane in any
RF circuit, as the only real ground plane of a two-sided board (one substrate,
one upper and one lower copper sheet) is considered to be the continuous bot-
tom copper layer.
All components at RF frequencies will have some reactive and resistive par-
asitic effects, so only resistors, inductors, and capacitors that are rated at the
frequency of operation—or above—should be used in an RF circuit.
Depending on the application (coupling, decoupling, filtering, matching, etc.),
running into a component’s series or parallel resonance unexpectedly can
destroy proper functioning of the wireless circuit.
Mutual inductance (coupling) of traces, components, and wires must be
accounted for in any design. This undesired coupling of energy can be alle-
viated by:

1. Keeping traces that are carrying RF currents separated by distance
2. Employing shields
3. Reducing the area of the current-carrying loops
4. Using right angles between traces

A concept similar to tapering the microstrip into an active component to
lessen the impedance bump is shown in Fig. 10.9. To decrease impedance vari-
ations and lower VSWR when a RF signal encounters a passive component,
such as a coupling capacitor, the component should ideally be of the same
width as the microstrip and the solder fillet itself should be smooth so as not
to disturb the signal flow.
As the return currents of a microstrip ground plane are flowing directly
under the microstrip that is carrying the signal currents (Fig. 10.10), the
ground plane must never be broken, or an unexpected impedance discontinu-
ity will result within the microstrip.
Always terminate all microstrip transmission lines with their characteristic
impedance to avoid unpredictable reactive effects.

High-speed digital design. A digital signal’s rise and fall time, and not its fre-
quency, govern the signal’s speed as it relates to PCB design. The traces for a

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