Page 419 - Complete Wireless Design

Wireless Issues

418 Chapter Ten

(called Duroid). Still, these softer board materials can be backed by a thick metal
ground plane, called a carrier, that adds the required rigidity. The carrier is typ-
ically aluminum plate, and can be up to a quarter of an inch thick.
All of these microwave circuit board materials can be chosen to exhibit dif-
ferent dielectric constants (2 to 11), loss tangents, temperature effects, and
dielectric and dimensional tolerances. Unfortunately, most demand special
processing to fabricate a complete printed circuit, so companies that process
FR-4 may be unable to work with the more specialized materials. These pro-
cessing steps will also increase the price of making the printed circuits and
vias on the microwave PCB substrate. Nonetheless, a relatively new board
material from Rogers Corporation (the largest creator of high-frequency PCB
substrates in the world) gives any board house the ability to manufacture a
complete microwave PCB. This rigid, high-frequency laminate material is the
Rogers RO-4000 series. While the cost of processing a layout with this sub-
strate is comparable in cost to FR-4, the board material itself is, of course,
slightly more costly, so FR-4 should be used whenever possible for large pro-
duction runs. Operation of the RO-4000 material is viable up to 20 GHz, with
good dielectric constant variations over temperature, as well as very even
thickness tolerances across the PC board.
Rogers Corporation conducted a test (utilizing Agilent’s microwave simula-
tion software EEsof) to demonstrate the interactions that can occur when the
dielectric constant, loss tangent, or dielectric thickness of a PCB is varied
(while the other two properties are left unchanged). This is an important exam-
ple of how board material can affect the outcome of a design, sometimes quite
drastically. Figure 10.1 displays a distributed edge-coupled bandpass filter cen-
tered at 1 GHz with a bandwidth of 10 percent, exploiting a 50-mil Duroid
material. Figure 10.2 is a frequency domain graph showing all substrate param-
eters nominal for this bandpass filter. Figure 10.3 shows what occurs to the fil-
ter’s center frequency and passband when the dielectric constant is varied both
upward and downward. A slight increase in E shifts the passband lower in fre-
r
quency, while a slight decrease in E shifts the passband higher. Variations in
r
the E of the board material will affect the passband frequency of a filter
r
because any change in E changes the electrical length of each of its elements
r
by altering the velocity of propagation through the dielectric.
Both manufacturing and temperature variations will influence the dielectric
constant, necessitating a board material with a tight initial E tolerance and a
r

Figure 10.1 Sixth-order edge-coupled distributed BP filter.

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