MM0704, Lesson 2
Purpose
Filters are sometimes used in home cooling or heating systems to remove dirt or moisture from the air. Similarly, in
electronics, filters are used to remove unwanted, undesirable frequencies from a circuit before the output is distributed
to other parts of a system. Two examples are the removal of the ripple frequency from a power supply and all but the
desired frequency at a receiving antenna. Whether low pass, high pass, or a combination of both, each circuit is
designed to obtain something: hum-free operation, better tone quality, or separation of the many signals received by a
radio antenna.
Operation
The operation of filter circuits may best be understood by the use of two formulas: inductive reactance
XL = 2fL,
and capacitive reactance
These formulas are explained in detail in the previous subcourse, MM0703, Basic Electricity, Part I.
Inductive Reactance. Inductive reactance is the product of the constant 2 and the variables frequency and
inductance. A change in reactance is, therefore, directly proportional to a change in either of the variables. When
frequency decreases, inductive reactance, or opposition offered by the inductor, also decreases. An increase in
frequency, however, encounters more opposition from the inductor, so an inductor offers less reactance to low
frequencies than to high frequencies. (For this discussion of filters, only frequency changes and the resulting change in
opposition are considered.)
Capacitive Reactance. The formula for capacitive reactance shows that frequency and capacitance are the reciprocal
of the reactance and that there is an inversely proportional relationship between them. When frequency increases, the
reactance decreases, and the opposition of the capacitor to higher frequencies becomes smaller. Just as an inductor
offers less opposition to low frequencies, a capacitor offers less opposition to high frequencies. Consequently, the
choice of a capacitor or an inductor in filtering will be determined, to a large extent, by the frequency range you want to
pass or reject.
FILTER DESIGN
Process
The design of filter circuits is a complicated mathematical process, with many advantages and disadvantages.
Although a filter will pass or reject the desired frequency or band of frequencies, filtering of an input signal is not the
only result that may be needed. The attenuation of the signal by the filter circuit and limited circuit flow in the load
circuit, must also be considered. Formulas that provide the desired result in one particular case must often be revised to
achieve the correct result in another.
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