Page 272 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
6

Filter Design

A filter is used to selectively pass or attenuate a particular band of frequen-
cies, and can be constructed of LC, RC, LCR, LR, or distributed components,
and can be either active or passive in nature. Active filters will contain some
sort of amplifier combined with any of the above lumped passive components,
while passive filters will simply employ lumped or distributed components,
with ceramic and crystal filters found in many passband and stopband appli-
cations. Any of these will dramatically improve a filter’s shape factor (steep-
ness of its skirts), as well as provide a variety of bandwidths all the way from
ultranarrowband to wideband. Surface acoustic wave (SAW) passive filters are
also becoming more common, and are available with superb shape factors from
narrowband to ultrawideband.
The RF spectrum contains quite a broad range of frequencies. Obviously, a
method had to be found that would allow us to segregate a small chunk of this
spectrum for transmission and reception, without interfering with other com-
munications channels. This can be done with the use of untuned and tuned fil-
ters. The basic passive untuned LC filter can function as a low-pass or
high-pass filter because of the inductor’s ability to increase its reactance, and
a capacitor’s ability to decrease its reactance, as the frequency is increased.
This makes the untuned LC filter frequency-selective. To act as a low-pass fil-
ter and attenuate higher frequencies (Fig. 6.1), an inductor will be arranged
in series (blocking the high frequencies), while a capacitor is located in shunt
(shorting out the higher frequencies). A high-pass filter, which attenuates the
lower frequencies (Fig. 6.2), has a capacitor that is in series (blocking the low
frequencies), and a shunt inductor (shorting out the lower frequencies). These
primitive filters may be cascaded to increase the sharpness of their skirts, as
shown in Fig. 6.3. A low-pass filter can be as large, or larger, than the 8-pole
structure shown.
Constant K refers to a filter that not only rejects or passes specific frequen-
cies, but will also match impedances between the generator and its load

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