1-22
By now you know that we have just ignored our old friend resistance from previous discussions. In
an actual circuit, at resonance, the currents will not quite counteract each other because each component
will have different resistance. This resistance is kept extremely low, but it is still there. The result is that a
relatively small current flows from the source at resonance instead of zero current. Therefore, a basic
characteristic of a practical parallel-LC circuit is that, at resonance, the circuit has MAXIMUM
impedance which results in MINIMUM current from the source. This current is often called line current.
This is shown by the peak of the waveform for impedance and the valley for the line current, both
occurring at fr the frequency of resonance in figure 1-11.
There is little difference between the circuit pulsed by the battery in figure 1-8 that oscillated at its
resonant (or natural) frequency, and the circuit we have just discussed. The equal and opposite currents in
the two components are the same as the currents that charged and discharged the capacitor through the
coil.
For a given source voltage, the current oscillating between the reactive parts will be stronger at the
resonant frequency of the circuit than at any other frequency. At frequencies below resonance, capacitive
current will decrease; above the resonant frequency, inductive current will decrease. Therefore, the
oscillating current (or circulating current, as it is sometimes called), being the lesser of the two reactive
currents, will be maximum at resonance.
If you remember, the basic resonant circuit produced a "damped" wave. A steady amplitude wave
was produced by giving the circuit energy that would keep it going. To do this, the energy had to be at the
same frequency as the resonant frequency of the circuit.
So, if the resonant frequency is "timed" right, then all other frequencies are "out of time" and
produce waves that tend to buck each other. Such frequencies cannot produce strong oscillating currents.
In our typical parallel-resonant (LC) circuit, the line current is minimum (because the impedance is
maximum). At the same time, the internal oscillating current in the tank is maximum. Oscillating current
may be several hundred times as great as line current at resonance.
In any case, this circuit reacts differently to the resonant frequency than it does to all other
frequencies. This makes it an effective frequency selector.
Summary of Resonance
Both series- and parallel-LC circuits discriminate between the resonant frequency and all other
frequencies by balancing an inductive reactance against an equal capacitive reactance.
In series, these reactances create a very low impedance. In parallel, they create a very high
impedance. These characteristics govern how and where designers use resonant circuits. A low-
impedance requirement would require a series-resonant circuit. A high-impedance requirement would
require the designer to use a parallel-resonant circuit.
Tuning a Band of Frequencies
Our resonant circuits so far have been tuned to a single frequency - the resonant frequency. This is
fine if only one frequency is required. However, there are hundreds of stations on many different
frequencies.
Therefore, if we go back to our original application, that of tuning to different radio stations, our
resonant circuits are not practical. The reason is because a tuner for each frequency would be required and
this is not practical.
