Multisection Filters

Multisection Filters in Tuned Circuits

All of the various types of filters we have discussed so far have had only one section. In many cases, the use of such simple filter circuits does not provide sufficiently sharp cutoff points. But by adding a capacitor, an inductor, or a resonant circuit in series or in parallel (depending upon the type of filter action required), the ideal effect is more nearly approached with multisection filters .

When such additional units are added to multisection filters circuitry, the form of the resulting circuit will resemble the letter T, or the Greek letter p (pi). They are, therefore, called T- or pi-type filters, depending upon which symbol they resemble. Two or more T- or pi- type filters may be connected together to produce a still sharper cutoff point.

The figure below, (view A) (view B) and (view C), and the next figure, (view A) (view B) and (view C) depict some of the common configurations of the T- and pi -type filters. Further dis-cussion about the theory of operation of these circuits is beyond the intended scope of this module. If you are interested in learning more about filters, a good source of information to study is the Electronics Installation and Maintenance Handbook (EIMB), section 4 (Electronics Circuits), NAVSEA 0967-LP-000-0120.

Formation of a T type filter.

Formation of a T type filter.

Formation of a T type filter.

Formation of a pi type filter.

Formation of a pi type filter.

Formation of a pi type filter.


When working with resonant circuits, or electrical circuits, you must be aware of the potentially high voltages. Look at the figure below. With the series circuit at resonance, the total impedance of the circuit is 5 ohms.

Series RLC circuit at resonance.

Remember, the impedance of a series-RLC circuit at resonance depends on the resistive element. At resonance, the impedance (Z) equals the resistance (R). Resistance is minimum and current is maximum. Therefore, the current at resonance is:

The voltage drops around the circuit with 2 amperes of current flow are:

EC = IT x XC

EC = 2 x 20

EC = 40 volts a.c.

EL = IT x XL

EL = 2 x 20

EL = 40 volts a.c.

ER = IT x R

ER = 2 x 5

ER = 10 volts a.c.

You can see that there is a voltage gain across the reactive components at resonance.

If the frequency was such that XL and X C were equal to 1000 ohms at the resonant frequency, the reactance voltage across the inductor or capacitor would increase to 2000 volts a.c. with 10 volts a.c. applied. Be aware that potentially high voltage can exist in series-resonant circuits.

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