18.

RESONANCE

a. As stated previously, inductive reactance increases with an increase of

frequency, whereas capacitive reactance decreases with an increase of frequency.

At some frequency, then, they are equal and the net reactance (difference between

the two) is zero (XL - XC = 0). The frequency at which this occurs is called the

resonant frequency.

b. Any circuit containing a combination of inductance and capacitance is

resonant to some particular frequency.

By making one or both variable, it is

possible to resonate or "tune" the circuit to any desired frequency within the

limits of their variation.

c. The condition of resonance depends on three factors: inductance,

capacitance, and frequency. Resonance can be obtained by varying any one of these

three factors while the other two are held constant.

And for a given set of

inductance and capacitance values, there will be only one frequency at which the

circuit will be resonant.

d. All tuned circuits

possess some resistance; usually resistance is

concentrated in the inductance

and distributed throughout the wiring that connects

the tuned-circuit components.

This resistance causes the tuned circuits to react

in particular fashions, as will

be discussed in paragraphs 19 and 20.

e. When you tune in your favorite radio station on your home radio receiver,

you are, in effect, changing the capacitance, so that the combination of this

capacitance and inductance forms a tuned circuit that resonates to the frequency of

the desired station. This tuned circuit, to be effective, must suppress all other

signals that are picked up by the antenna. We are speaking here of only one tuned

circuit, but in a practical radio receiver more than one is used since each tuned

circuit will assist the other to do a better job.

(1) To receive a higher frequency station, the plates of the

tuning

capacitor are unmeshed, giving a smaller value of capacitance.

(2) To receive a lower frequency station, the plates of the capacitor are

partially meshed, resulting in a larger value of capacitance.

(3) The tuned circuit could also be resonated by keeping the capacitance at

a fixed value and varying the inductance.

However, it is more

feasible, mechanically, to vary the capacitance.

19.

SERIES-TUNED CIRCUIT

The series-tuned circuit shown in A of figure 25 consists of a combination of

resistance, inductance, capacitance, and a source of ac voltage connected in

series.

a. At resonance the inductive and capacitive reactances cancel because they

are equal and opposite to each other.

The circulating current in the circuit is

thus limited only by the resistance. The line current for a series-tuned circuit

resistance of the circuit.

If the total resistance of the circuit is small, the

line current can be very large, relatively speaking, up to

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