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
308
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