1. Principle of wireless microphone
The electronic microphone first converts various sound signals into audio electrical signals, and then this electrical signal modulates the high-frequency signals generated by the electronic oscillator. Finally, the high-frequency signal is transmitted into the air through the antenna. If the transmitting frequency is designed in the FM radio band, and then cooperate with any FM radio to receive the high-frequency signal, and restore the sound signal from the high-frequency signal, thereby completing various purposes.
2. Circuit diagram
The attached figure is the circuit diagram of the FM wireless microphone. The high-frequency transistor V1 and the capacitors C3, C5, and C6 form a capacitor three-point high-frequency oscillator. The load C4 and L of the collector of the triode form a resonator, and the resonance frequency is the emission frequency of the FM microphone. According to the parameters of the components in the figure, the transmitting frequency can be between 88~108MHz, which just covers the receiving frequency of the FM radio. By adjusting the value of L (coil L), the transmitting frequency can be easily changed to avoid the FM radio. The transmitted signal is connected to the antenna through C4 and then transmitted.
R4 is the base bias resistance of V1, which provides a certain base current to the triode, so that Vl works in the amplifying area. R5 is a DC feedback resistance that stabilizes the operating point of the triode. The principle of frequency modulation of this FM microphone is to achieve the purpose of frequency modulation by changing the capacitance between the base and emitter of the triode. When the sound voltage signal is applied to the substrate of the triode, the capacitance between the base electrode and the emitter of the triode will change synchronously with the size of the sound voltage signal, and at the same time, the emission frequency of the triode will be changed to realize frequency modulation.
The microphone MIC can collect external sound signals. This is an electret microphone, which has very high sensitivity and can collect faint sounds. At the same time, this microphone must have a DC bias to work, and the resistor R3 can provide a certain DC bias , The greater the resistance of R3, the weaker the microphone's sensitivity to collect sound. The smaller the resistance, the higher the sensitivity of the microphone. The AC sound signal collected by the microphone is sent to the base of the transistor through C2 coupling and R2 matching. In the circuit, the two diodes Dl and D2 are in anti-parallel connection, which mainly serves as a two-way limiting function. , The diode’s turn-on voltage is only 0.7V. If the signal voltage exceeds 0.7V, the diode will be turned on and shunted. This ensures that the amplitude of the sound signal can be limited between plus and minus 0.7V. Excessive sound signals will cause the transistor to pass Modulation, resulting in sound distortion or even not working properly.
CK is an external signal input socket. External sound signal sources such as TV earphone socket or Walkman earphone socket can be introduced to the FM transmitter through a dedicated connection line. The external sound signal is sent to the FM transmitter through R1 attenuation and Dl, D2 limiting. The base of the triode is frequency modulated. So this kit can be used not only as a wireless microphone, but also as a TV wireless headset.
The light-emitting diode D3 in the circuit is used to indicate the working status, and it will light up when the FM microphone is energized, and R6 is the current-limiting resistance of the light-emitting diode. C8 and C9 are power supply filter capacitors. Because large capacitors are generally made by winding technology, the equivalent inductance is relatively large. A small capacitor C8 in parallel can reduce the high-frequency internal resistance of the power supply. This circuit is very common.
Kl and K2 in the circuit are a kind of switch, it has three different positions (not shown here, just add explanation), turn off the power when dialed to the far left, the far right is Kl, K2 is turned on for FM microphone use, in the middle The position is that K1 is connected, K2 is disconnected, and it is used as a wireless repeater.
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