Composite Filters

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.