The circuit of the frequency conversion series resonance test set is composed of capacitors and inductors connected in series and parallel. Resonant circuits are widely used in radio technology and broadcasting and television technology; In addition, various wireless devices, equipment, measuring instruments, etc. cannot do without resonant circuits. The significant feature of this circuit is its frequency selection ability. It can retain useful frequency components, filter out useless frequency components, and perform many other useful tasks.

The common feature of capacitors and coils is the ability to store electricity. When a voltage is applied to the plates of capacitor C, static charges are formed in the thin dielectric layer separating the two plates; If the load is connected to the leads of the capacitor, the rate of discharge through the load through the circuit time constant depends on the resistance and capacitance.

Inductance L can also store electrical energy and dissipate it through a load. When voltage is applied to the coil (or any variant, always with a certain degree of inductance), a magnetic field is formed in the surrounding space; The function of establishing a magnetic field requires energy, which is stored in the magnetic field rather than dissipated in the form of heat as 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 series resonant circuit, the impedance is maximum at the resonant frequency, and the current is minimum at this time; In a series circuit composed of resistors, inductors, and capacitors, when the capacitance reactance XC is equal to the inductance reactance XL, i.e. XC=XL, the phase of the voltage U and current I in the circuit are the same, and the circuit exhibits resistance. This phenomenon is called series resonance. When a series resonance occurs in a circuit, the impedance of the circuit is Z=√ R ^ 2 (XC-XL) ^ 2=R, the total impedance is minimized, and the current reaches its maximum value. Any of these LC circuits can be connected in parallel or series to produce through or band resistance.

In an ideal situation, a resonant circuit will resonate indefinitely, but in the real world, a small amount of resistance will gradually reduce the output to zero, and the waveform displayed on the oscilloscope is called a damping wave. This situation usually occurs in nature and electronic circuits; If electrical energy is continuously fed into the circuit, there exists a situation called oscillation, and the oscillator is a fundamental component of superheterodyne used in almost all modern broadcasting and television equipment.