By hvhipot 
When capacitors and inductors (coils) are connected in parallel or series, a series resonant circuit or a parallel resonant circuit is formed. These two circuit components will prevent or pass through a single specific frequency in the diver’s mix. For this reason, resonant circuits enable the transmission and reception of radio and television, and perform many other useful tasks.
Capacitors and coils share a common characteristic of being able to store electrical energy. When voltage is applied to the plates of capacitor C, static charges are established in the thin dielectric layer separating the two plates. If the load is connected to the leads of the capacitor, the rate at which the capacitor discharges through the load is determined by the time constant of the circuit, which depends on the resistance and capacitance.
Inductor L can also store electrical energy and dissipate it through a load. The mechanisms are different. When voltage is applied to a coil (or any conductor that always has a certain degree of inductance), a magnetic field is formed in the surrounding space. The action of establishing a magnetic field requires energy, which is stored in the magnetic field rather than dissipating in the form of heat like a resistive load. If the inductor is now connected to the load, the magnetic field will collapse and its energy will be released into the circuit.
In a parallel resonant circuit, the impedance is maximum at the resonant frequency, so the current is minimum at this time. In a series resonant (also known as series variable frequency resonant) circuit, the impedance is minimized at the resonant frequency, so the current is maximized at this time. Any of these LC circuits can be placed in a parallel or series configuration to produce bandpass or bandstop.
Therefore, resonant circuits can be used to transmit or retrieve any desired signal. If you look at the graph of the resonant circuit output, you will see the peak or valley values of amplitude (Y-axis) relative to frequency (X-axis). This is the well-known frequency domain among FHV Hipot Electric Co., Ltd.ier transform students. The clarity of the curve corresponds to the Q (quality) of the circuit, but please remember that the appearance of the curve will also depend on the scaling ratio of the graph.
In order for a resonant circuit to operate, that is, to be in a resonant state, the inductance reactance and capacitance reactance must be equal. As long as they are the same, they can have any practical value. As the frequency increases, the capacitance reactance decreases and the inductance reactance increases. For this reason, these values will be equal at a specific frequency, which is the resonant frequency.
After injecting energy pulses into the resonant circuit, the energy is alternately stored in the form of static charges in the capacitor and stored in the magnetic field around the inductor. The oscilloscope connected to the capacitor plate will display a sine wave. Ideally, a resonant circuit would ring indefinitely, but in the real world, a small amount of resistance can cause the output to gradually decrease to zero. The waveform seen on the oscilloscope display is called a damping wave. This situation usually occurs in nature and electronic circuits.
If electrical energy is continuously fed into a circuit, there exists a situation called oscillation. Oscillators are the fundamental components of superheterodynes used in almost all modern broadcasting and television equipment.