Page 293 - Complete Wireless Design

Filter Design

292 Chapter Six

considered. Unhappily—unlike LC filters—the calculations to design such dis-
tributed filters are far too laborious. Most of these types of calculations, as well
as the necessary optimization of such designs, are best left to a high-end soft-
ware program such as Eagleware’s M/Filters. However, a few easy-to-imple-
ment designs are presented for undemanding applications.

6.2.2 Types of distributed filters
Because of various requirements and specifications for microwave filters—
such as stopband attenuation, narrow versus wideband, return loss require-
ments, and microstrip element lengths—a multitude of distributed bandpass
filter structures have been created for microwave frequencies. Only the most
common will be discussed here. Nearly all of the following structures can be
designed with the various general passband shapes, such as Butterworth,
Chebyshev, Bessel, as well as the less common singly equalized transitional
gaussian (6 and 12 dB) and the elliptic types of the Cauer-Cheby:

1. Edge-coupled bandpass filters (Fig. 6.37) are effective in narrowband appli-
cations. But they can radiate—which necessitates shielding. They can also
be an unusually long structure at the lower microwave frequencies: At 1
GHz, edge-coupled bandpass filters are up to 32 inches long in a seventh-
order configuration (depending on board dielectric constant and thickness).
2. Combline bandpass filters (Fig. 6.38) employ both distributed and lumped
(capacitor) elements, and are used in narrowband applications where size
is at a premium: a seventh-order combline is only a little over 1 inch long
at 1 GHz.
3. Folded edge-coupled bandpass filters (Fig. 6.39) are utilized in narrowband
filtering applications, and are very similar to the edge-coupled bandpass fil-
ters above. However, they are considerably shorter: 8 inches at 1 GHz for
the seventh-order type.
4. Interdigital bandpass filters (Fig. 6.40) are adopted in narrowband applica-
tions, and are quite compact: 2 inches long at 1 GHz for a seventh order.

It is especially important at high frequencies for the measurement of stub
lengths to be accurate in distributed design, since the higher the frequency the
shorter will be the stub, and the less room for error there will be. An effective

Figure 6.37 Sixth-order edge-coupled distributed BP filter.

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