MM0704, Lesson 2
Task. The skills and knowledge taught in this lesson are common to all missile repairer tasks.
Objectives. When you have completed this lesson, you should be able to explain the purpose, operation, and use of
Conditions. You will have this subcourse book and work without supervision.
Standard. You must score at least 70 on the end-of-subcourse examination that covers this lesson and lessons 1, 3,
and 4 (answer 23 of the 30 questions correctly.)
A filter is a circuit containing a number of impedances consisting of resistors, inductors, or capacitors. These
impedances are grouped together to produce a definite frequency characteristic. Filters separate AC from DC, low-
frequency current from high-frequency current, or signals within a band of frequencies from signals outside this band.
Although resistors provide a uniform attenuation factor to all frequencies and although the impedance of an inductor or
capacitor varies according to the frequency of operation, resistor-capacitor (RC) filters are quite often used. Filters
permit free passage over a certain range of frequencies and reject or transmit poorly over another range of frequencies.
The range where transmission occurs freely is called the pass band, and the range of poor transmission is called the
attenuation or stop band. The frequency at which attenuation starts to increase rapidly is known as the cut-off
There are three basic configurations into which filter elements can be assembled: The L (half section), consisting of one
series and one parallel (shunt) arm. The full T-section, consisting of two series arms and one shunt arm. The full or
section, consisting of one series arm and two shunt arms.
Several sections of the same configuration can be joined to improve the attenuation or transmission characteristic.
When filters are inserted in a circuit, they are usually terminated by resistances of the same value at both the input
the circuit with which the filter is used. The desired cutoff frequencies are also predetermined. You have to know the
values of capacitors and inductors, which give the desired frequency characteristics, when designing a filter.