Page 439 - Maxwell House

MORE COMPLICATED ELEMENTS OF FEED LINES 419

To satisfy such specification the Butterworth filter should have 16 stages, Chebyshev type I and
II seven while elliptic only 5. That demonstrates the fact that elliptic filters meet requirements
with the lowest order than any other minimum phase filter type. If so, they are your workhorses
when the weight and dimension are primary requirements like in communication satellites. But
nothing comes for free. Their electrical parameters are typically quite sensitive to the quality of
production and tuning procedure that makes them rather costly compared to other types of
filters. In general, there is a trade-off in the filter transfer functions and the group delay, which
means that more out-of-band signal suppression results in more phase distortion in-band and
vice-versa.

The summary advantages and disadvantages of standard filters are listed in Table 8.2. Keep in
mind that this chart is far from completion. Each filter has diverse and often unique application
range not simply being “better” or “worse.” Only the careful analysis of a system specification
allows reaching a compromise in suitable filter choice. However, such system issue is far
beyond the scope of this course. Note in conclusion that the transfer function and group delay
of non-minimum phase filters might be synthesized almost independently. The punishment for
such improvement is filter complexity and more difficult tunability. It is important to point out
that the filters are typically two-port devices and their synthesis might be based either on its
equivalent circuit like shown in Figure 8.4.1 or on one of their scattering matrices introduced
in Chapter 7. The second approach is preferable being more universal and effective when filter
is manufactured using the dielectric multilayer thin-film technology. Such devices belong to
the wide class of interference optical filters that we are going to introduce later in Section 8.4.12
of this chapter. The next paragraphs are devoted to the filters with assumption that the low-pass
filter synthesis is already accomplished, and all prototype components are known.