What Is a Resonant Circuit?
For a passive one-port network that includes capacitors, inductors, and resistive elements, the ports may appear capacitive, inductive, and resistive. When the voltage U and current I of the circuit ports appear in phase and the circuit is resistive. Called the resonance phenomenon, such a circuit is called a resonance circuit. [1]
- In a resistor R, inductor L, and capacitor C
- Its dynamic equation is F = -kx. The phenomenon of resonance is that the current increases and the voltage decreases. The closer to the resonance center, the ammeter voltmeter power meter rotates faster, but the difference from short circuit is that no zero sequence quantity will appear.
- When the series loop reactance is equal to zero, the circuit is said to have series resonance. The RLC series resonance circuit is shown in Figure 9-9.
- The circuit shown in Figure 3-13 is a GLC parallel circuit, which is another typical resonant circuit. in parallel
- The application of resonance circuits in electronic technology is very extensive. Because of its frequency selectivity, it is often used as a load for high-frequency and intermediate-frequency amplifiers in transmitting and receiving equipment; the resonant circuit is an important part of the oscillator; the resonant circuit is an absorption circuit in electronic circuits to filter out interference signals Here are some examples. [3]
Selection of Resonant Circuit Signal
- The input circuit of an AM radio is shown in Figure 6-16. In the circuit, L 1 is the receiving antenna of the radio input circuit, L 2 and C are the resonant circuit to form the radio frequency selection circuit, and L 3 sends the selected radio signal to the radio receiving circuit.
- Radio input circuit
- The radio antenna receives electromagnetic waves from different stations in the air, and adjusts C to make L 2 and C resonate on the carrier frequency of a desired station. At this time, the maximum current flows on L 2 and the station signal is selected. By adjusting C so that L 2 and C resonate on the carrier frequencies of different stations, programs of different stations can be received. [3]
Filtering of resonance circuit signals
- The TV sets the 38MHz IF signal after being mixed by the high-frequency tuner. If the 38MHz signal of the external signal enters the TV, it will cause serious interference to the TV's IF signal, so the external 38MHz signal must be filtered. In addition, the method is shown in Figure 6-17. The LC series resonant circuit is connected in parallel with the input of the TV, and the LC loop is resonant at 38MHz. According to the characteristics of the series resonance, it presents a very small resistance to the 38MHz signal, which is equivalent to short-circuiting the 38MHz signal to the ground, preventing foreign IF signals such as other TVs from entering the TV to interfere with the operation of the local IF amplifier. Prevent the IF signal of this unit from radiating out through the antenna to interfere with the operation of other machines. Because the LC circuit has a very high impedance to the TV signal, it will not affect the normal operation of the TV. [3]
- TV intermediate frequency suppression circuit
Measurement of resonant circuit components
- Q meter is a meter used to measure parameters such as figure of merit, inductance, capacitance, etc. It uses the characteristics of the resonant circuit to work. Its principle can be illustrated in Figure 6-18.
- Q table schematic
- The signal source uses a high-frequency signal generator with variable frequency and output voltage. When measuring, change the frequency while keeping the power supply output voltage constant.
- Measure the Q value of the inductive coil L x , connect L x with a standard capacitor C b in series and connect it to the output of the high-frequency signal generator, adjust the capacitance of C b or the frequency of the high-frequency signal generator to make the circuit reach resonance At this time, the voltage across C b reaches a maximum value and is equal to Q times the power supply voltage. The standard capacitor C b can be connected in parallel with a voltmeter or a Q meter. It should be connected to read the Q value. Generally, the output frequency of the Q meter and the capacitance value of the standard capacitor C b are marked.
- When using series resonance, the known resonant frequency f 0 and the standard capacitance C b can be used to obtain the inductance of the coil L x under test. [3]