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Resonant circuit
A circuit that vibrates at a certain frequency. Commonly used ones include LC, RC, transformer coupling, and crystal oscillators. The principle of an oscillator is very simple, which is the positive feedback principle. LC determines the frequency of oscillation, and the crystal of a regular crystal oscillator can be equivalent to an inductor with a high Q value, using the charging and discharging of a capacitor to generate oscillation. Multi harmonic oscillators composed of RC are commonly used in inverter circuits. There are also self-excited oscillation circuits with transformer feedback.
The series resonant circuit has the minimum impedance at the resonant point and exhibits pure resistance. As the frequency offset increases, its impedance increases. When voltages of the same amplitude but different frequencies are applied to the series resonant circuit, the current flowing through the resonant point is the maximum and there is no phase shift. When the frequency offset is negative, the current phase leads the voltage, and when the frequency offset is positive, the current phase lags behind the voltage. When the frequency offset is constant, the larger the quality factor, the higher the impedance of the series resonant circuit, the lower the voltage, and the sharper the resonance curve.
Function of resonant circuit
The voltage waveform when resonance occurs in a series circuit. When a voltage square wave is applied to an LC series circuit, both the front and rear edges of the square wave will excite the LC series circuit (i.e. receive energy), and after each excitation, damping oscillation (i.e. energy loss) will occur. When the rise rate dv/dt value of the input voltage waveform is greater than the rise rate of the resonant circuit waveform (sine wave), the circuit will generate excitation; When the rise rate dv/dt value of the input voltage waveform is less than the rise rate of the resonant circuit waveform, the circuit will generate damping.
Due to the fact that the energy of the oscillation circuit has not been completely consumed after each excitation, a new excitation is then applied to superimpose the oscillation voltage time and time again. If the phase of the excitation can be synchronized with the phase of the oscillation waveform, the amplitude of the oscillation voltage will increase until the energy excited is equal to the energy lost in the circuit. Therefore, when the quality factor Q value of the resonant circuit is high, the resonant voltage can also rise very high. Ideally, if the Q value is infinitely high (i.e. the antenna has no loss), the amplitude of the resonant voltage will also rise infinitely high, but this situation does not exist.
The voltage amplitude during resonance in an LC series circuit is closely related to the phase of the excitation waveform, while it is not particularly correlated with the amplitude of the excitation waveform. If the phase or period between voltage square waves is not strictly kept equal, the waveform will experience severe jitter, and the amplitude of the resonant voltage will also decrease significantly. Therefore, measurement methods cannot objectively measure the electromagnetic field strength of interference signals in a certain space.
In addition, it should be pointed out that the receiving antennas used for testing are also divided into electric field induction wires, magnetic field induction antennas, and electromagnetic field induction antennas.
In practical applications, antennas are not specifically distinguished between receiving antennas and transmitting antennas, and both can use the same antenna. Therefore, any charged conductor or conductor through which current flows in a circuit can be regarded as a transmitting antenna.
It can be seen that the magnitude of radiation interference generated by electronic devices is not only related to the amplitude of the interference signal, but also to the size of the induction capacitors C1 and C2, that is, it is related to the area of electric field radiation (the capacitance is proportional to the area size) and the area of magnetic field radiation. Therefore, minimizing the radiation area of the interference signal is a good way to reduce radiation interference.