Page 418 - Complete Wireless Design

Wireless Issues

Wireless Issues 417

10.3 Wireless Board Design
10.3.1 Introduction
In the world of microwaves and RF—unlike most other disciplines of electron-
ics—the PCB’s layout, construction, and materials are almost as important as
the circuit design itself. In fact, a perfect wireless design of even the most basic
oscillator or filter can be ruined by improper board layout.

10.3.2 Board materials
At microwave frequencies insertion losses become a concern in choosing a PCB
board material. A majority of these losses comprise both conductor and dielec-
tric losses. When dielectric materials with low loss tangent (also called dissi-
pation factor, tan delta, and TAND) are used at lower frequencies (under 2
GHz), the losses in the copper conductors can overwhelm any losses in the
board material itself. At higher frequencies, however, this may not be the case:
A board material with a dielectric constant (also called E , K′, and relative per-
r
mittivity) of 3.5 will slowly begin to display an almost equal loss in the dielec-
tric and in the conductor as 20 GHz is reached. Higher-dielectric-constant
materials will also increase the loss in the dielectric, and an increase in the
conductor loss slightly more, over that of lower dielectric constant materials.
Thus, it can be demonstrated that both the dielectric and the conductor losses
(overall insertion losses) increase with increased loss tangent and dielectric
constant—especially at higher frequencies. In general, it is recommended that
a maximum E of 10 must be chosen for up to about 4 to 5 GHz, an E of 6 for
r r
up to 6 to 7 GHz, an E of 4 up to 13 to 14 GHz, and an E of 3 up to 30 GHz.
r r
These values will be maximum recommended dielectric constants in order to
minimize insertion losses. However, dielectric constants of smaller values can
be employed to control the size of the distributed circuit elements.
At microwave frequencies, high-dielectric-constant materials can force the
microstrip circuit topologies to become too small to be realized, both economi-
cally and physically. Increasing the substrate’s thickness, to a certain extent,
will ameliorate some of this effect, but at the cost of possibly confronting dif-
ferent undesired modes of signal propagation, along with increased board via
inductance. Nonetheless, thinner substrates will increase insertion losses,
while increased thickness will decrease these losses. (Major compromises in
RF must always be considered in every design.)
Since the ubiquitous FR-4 circuit board material, which is made up of epoxy
resin/glass laminates, exhibits excessive loss and dielectric constant variations
at approximately 1.5 GHz and above, other board materials must be chosen for
operation in excess of this frequency. FR-4 has a loss tangent of 0.008 at VHF,
and approximately 0.02 at higher frequencies, making the loss tangent too high
for most microwave work. The majority of the higher-frequency board materi-
als are not only far more expensive than FR-4 type materials, but are also dis-
turbingly soft structures, and may bend quite easily; an example is Teflon™

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