Page 427 - Complete Wireless Design

Wireless Issues

426 Chapter Ten

6. An even number of stages in an amplifier strip should be avoided, since this
can cause in-phase feedback to the chain’s input—either through circuit
trace interaction or conductive dirt contamination.
7. Circuit input and output traces should be kept separated from each other
to avoid bypassing the circuit itself or creating feedback.
8. Employ only components made for RF service, as the parasitics and low Q
that most affect wireless circuit design are due to real-world component
inadequacies of inductors and, to a more limited extent, capacitors and
resistors. These problems involve:
a. The capacitance from turn-to-turn in an inductor—an effect lessened by
smaller diameter coil turns
b. The inductance inherent in all leads (a -inch lead of a through-hole
1
4
capacitor can reach 10 nH, while even leadless capacitors can still have
inductances of 1 nH) can be somewhat alleviated by running capacitors
in parallel
c. The capacitance to ground natural to all components—lessen by employ-
ing smaller components
d. The mutual coupling of inductors—decrease by not running inductors in
parallel, only at right angles
e. Limited inductor Q—helped by using only a manufacturer’s specific
“high-Q” coils, by winding your own coils in which the length is equal to
the diameter, or by utilizing a distributed inductor

Surface mount components are all that should be considered at frequencies
above a few 100 MHz. Nonetheless, not all SMDs are created equal when it
comes to high-frequency operation. The only resistors, inductors, and capac-
itors that should be adopted for microwave operation are those that have
been specified by the manufacturer to dependably operate above the design
frequency—without hitting any series or parallel resonances (except in cer-
tain coupling or bypass applications). Since many resistors are not specified
for their maximum frequency of operation, they must sometimes be selected
only on the basis of the type of high-frequency resistor design employed; usu-
ally thin- and thick-film types can be depended on to reach to very high fre-
quencies. Still, as discussed in a previous chapter, as a resistor’s resistance
values are increased, its ability to operate at microwave frequencies is
decreased (Fig. 10.11). In fact, for sensitive or very high frequency circuit
operation, candidate RF resistors should be tested for any resonances, as
well as a lack of major resistance changes versus frequency, to at least 20
percent above the desired frequency of operation.
As stated, all active and passive components that are subjected to RF must
be able to properly operate at the desired frequency, but components that are
out of the RF path may be low-cost, low-frequency parts. This will become
especially meaningful in DC bias circuits (Fig. 10.8b) where the RF choke
(RFC) must stop most of the RF from entering the bias supply, with any that
gets through being bypassed by the high-frequency capacitor, C , to ground,
B1

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