Electrical Communication is a free textbook on the basics of communication technology. See the editorial for more information....

Composite Filters

The constant-k filters discussed in the preceding pages are satisfactory for some purposes, but the rigid requirements of modern communication have made necessary filters with better characteristics.

From Figs. 29 and 34, it is seen that the attenuation offered by constant-k filters increases gradually beyond the cutoff frequency. Filters are needed that offer very high attenuation close to the cutoff point. Thus, the attenuation characteristic of the constant-k filter is one factor limiting its use.

As shown by Figs. 25, 28, and 33, the iterative impedance of constant-k filters varies widely with frequency. It follows that a constant-k filter would offer a poor termination to transmission lines and telephone equipment. Thus, the impedance characteristic is another factor limiting its use.

Figure 42. Mid-series constant-k, or prototype, filter sections, and corresponding m-derived sections.

For these reasons, composite filters, rather than constant-k filters, are often used. These composite filters usually consist of con-stant-k sections terminated with m-derived half sections. Composite filters can be made to have very high attenuation close to the cutoff point, and to offer a uniform iterative impedance over the band of frequencies transmitted.

Last Update: 2011-06-05