In an AC circuit with resistor R, inductor L, and capacitor C components, the voltage at both ends of the circuit is generally different in phase from the current. If the parameters or power frequency of the circuit components (L or C) are adjusted, they can be made to be in phase, and the entire circuit exhibits pure resistance. This state of the circuit is called resonance. In the resonant state, the total impedance of the circuit reaches or approximates an extreme value. The purpose of studying resonance is to understand this objective phenomenon and fully utilize the characteristics of resonance in science and application technology; At the same time, it is necessary to prevent the harm it may cause. According to the different circuit connections, there are two types: series resonance and parallel resonance.
Series resonance (also known as series variable frequency resonance)
The equivalent impedance is minimized during series resonance, and the impedance is pure resistance. Although the size of the series resistance does not affect the natural frequency of the series resonance circuit. But it has the function of controlling and adjusting the amplitude of current and voltage during resonance.
Parallel resonance
It is another typical resonant circuit. The definition of parallel resonance is the same as that of series resonance. The working condition when the voltage U on the port is in phase with the input current I is called resonance. Due to its occurrence in parallel circuits, it is called parallel resonance.
What value can LLC have in those areas?
The application of resonant circuits in electronic technology is very extensive. Due to its frequency selectivity, it is often used as a load for high-frequency and intermediate frequency amplifiers in transmitting and receiving devices. The resonant circuit is an important component of an oscillator. Resonant circuits are used as absorption circuits in electronic circuits to filter out interference signals, etc.
Signal selection
The input circuit of a certain AM radio, in which L1 is the receiving antenna of the radio input circuit, L2 and C are resonant circuits to form the radio frequency selection circuit, and L3 sends the selected radio signal to the radio receiving circuit.
The radio antenna receives electromagnetic waves emitted from different radio stations in the air, adjusts C to make L2 and C resonate on the carrier frequency of a desired radio station, and at this time, the maximum current flows through L2 to select the signal of this radio station. Adjust C to make L2 and C resonate on the carrier frequency of different radio stations to receive programs from different radio stations.
Signal filtering
After being mixed by a high-frequency tuner, the TV outputs a 38MHz intermediate frequency signal. If there is a 38MHz signal from the external signal entering the TV. It will cause serious interference to the intermediate frequency signal of the television, so it is necessary to filter out the external 38MHz signal. Connect the LC series resonant circuit in parallel with the input terminal of the television, and the LC circuit resonates at 38MHz. According to the characteristics of series resonance, it presents a very small resistance to the 38MHz signal, which is equivalent to short circuiting the 38MHz signal to ground, preventing external intermediate frequency signals such as other televisions from entering the television and interfering with the operation of the local intermediate frequency amplifier, and also preventing the local intermediate frequency signal from radiating outwards through the antenna and interfering with the operation of other machines. Due to the high impedance of the LC circuit to the television signal, it will not affect the normal operation of the television.
Component measurement
Q meter is an instrument used to measure parameters such as quality factor, inductance, capacitance, etc. It works by utilizing the characteristics of resonant circuits.
The signal source uses a high-frequency signal generator with variable frequency and output voltage. During measurement, the frequency is changed while maintaining a constant power output voltage.
Measure the Q value of the inductor coil Lx, connect Lx in series with a standard capacitor Cb, and then connect it to the output terminal of the high-frequency signal generator. Adjust the capacitance of Cb or the frequency of the high-frequency signal generator to achieve resonance in the circuit. At this point, the voltage across Cb reaches its maximum value and is equal to Q times the power supply voltage. The two ends of the standard capacitor Cb can be connected in parallel with a voltmeter or a Q meter, and it is recommended to connect it to read the Q value. The output frequency of a general Q-meter and the capacitance value of the standard capacitor Cb are both marked with scales.
When using series resonance, the inductance of the measured coil Lx can be obtained with the known resonance frequency f0 and standard capacitance Cb.