Band-Pass and Band-Elimination Filters

Author: J.B. Hoag

It is possible to transmit a certain range of frequencies through a band-pass filter, and more or less completely suppress all lower and all higher frequencies. A simple filter of this type, together with its attenuation curve, is given in Fig. 7 H.

Fig. 7 H. Simple band-pass and band-elimination filters

It is also possible to construct filters which will, more or less, transmit all frequencies except those which lie within a certain range. A simple form of band-elimination or band-suppression filter is also given in Fig. 7 H, together with its attenuation curve.

With multi-section filters of these types, the separation of adjacent frequencies can be made much sharper than with the simple L-sec-tion. On the other hand, in practice, the coils are not pure inductances but have some resistance. Now Q is our measure of the inductive-ness versus resistance of a coil, i.e., Q = 2πfL/R. In Fig. 7 H, the dotted lines show in what manner the attenuation is affected when low- and high-Q coils are used. Similar unsharpening effects occur in practice with all filters.

It is well known that a quartz slab cut from its crystal in proper fashion and mounted between two metal plates acts like a circuit of very high Q. Hence, in circuits where the frequency of the currents is comparable with the natural vibrational frequency of the quartz, it is possible to obtain very sharp cutoff filters of all types.¹

1	Details are given by W. P. Mason and R. A. Sykes, in The Bell System Technical Journal, Volume XIX, page 221, April 1940.