Low-Pass Filters

Author: J.B. Hoag

The circuit of Fig. 7 D is called an L-type, low-pass filter. It contains a series inductance and a shunt capacitance. From the attenuation (db.) curve in this figure, it can be seen that the choking action of the coil, aided by the bypass action of the condenser, has materially sharpened the curve over that of a single series inductance (Fig. 7 C), especially in the neighborhood of the cutoff frequency, f_c,. In the equations given in Fig. 7 D, L is in henries, C is in farads, R is in ohms, and f_c is in cycles per second. Note that the generator and the load both have the same resistance.

Fig. 7 E. — T- and π-type, low-pass filters

Referring to Fig. 7 E, we see that several " T- "sections or " π- "-sections can be connected in series with each other to form two, three, or more section filters. The cutoff becomes increasingly sharp the more sections used. Then, of two frequencies near f_c, the lower one will get through and the higher one will not. The same design equations are used as for the L-section low-pass filter. Note that, in Fig. 7 E, only one-half the inductance or one-half the capacitance is used in certain places while the full value is used at others.

Fig. 7 F. An L-section, high-pass filter. L is in henries, C is in farads, R is in ohms and is the same at the input and output. The cutoff frequency, fc, is in cycles per second