Simple FM wireless microphone transmitter module electronic DIY Kit

The transmission frequency of this circuit is about 90MHz. The receiving equipment uses a common FM radio. Adjust the inductance L to adjust the transmission frequency to avoid the local radio station. The working voltage of the circuit: DC 3V-6V. The transmission distance of this kit is about ten to several tens Meters, the transmission distance is related to the power supply voltage, antenna length matching and impedance matching, regional openness, electromagnetic wave interference, and receiving equipment sensitivity.

1. Performance parameters

Frequency range: 80MHz—108MHz

Working voltage: DC3V—6V

Second, the circuit description

MIC is an electret microphone. Its function is to sense the weak vibration of sound waves in the air and output the same electrical signal as the sound changes. This site uses a microphone with higher sensitivity, which can generally output an audio signal of more than tens of millivolts, which is enough to modulate the frequency of the next-level high-frequency oscillation signal. Note: The microphone has positive and negative poles, and the negative pole is generally connected to the case.
R1 is the bias resistance of the MIC electret microphone. With this resistance, the microphone can output audio signals. This is because the MIC microphone itself has a pole field effect tube amplifier circuit for impedance matching and improving output capabilities.
C2 is an audio signal coupling capacitor, which delivers the sound electrical signal output by the microphone to the next level.
C2 is the base filter capacitor of Q9018. On the one hand, it filters out high-frequency noise. On the other hand, the high-frequency potential of Q9018 is 0. For high-frequency circuits above 50MHz, Q9018 is a common base amplifier circuit. This is Finally, it can form the basis of oscillation. Because the basic condition of the oscillating circuit is that it must have a certain gain, and then it has feedback with a suitable phase (generally positive feedback).
R2 is the base bias resistor of Q9018, providing Q with a smaller base current, and Q will have a larger emitter current through R3. Because the current action in R2 and R3 will produce a voltage drop on their respective resistances and affect each other, the result will automatically stabilize at a certain value state, which is the emitter follower mentioned in the book.
R3 is the emitter resistance of Q9018, which serves to stabilize the DC operating point, and C6 also constitutes the role of high-frequency signal load resistance, which is also part of the entire high-frequency oscillation circuit.
C4 and L form a parallel resonant circuit, which plays the main role of selecting the oscillation frequency. Changing the capacity of C4 or changing the shape of L (including the number of turns) can easily change the transmission frequency.
C7 is a high-frequency signal output coupling capacitor, the purpose is to make the high-frequency signal into radio waves and radiate into the sky. Therefore, the antenna is best to be straight up, and the length is preferably equal to the radio wave frequency wavelength (or an integer multiple), and the surroundings should be wide open, without being blocked by metal objects. Note: The wavelength is equal to the reciprocal of the frequency. When the frequency changes, the wavelength will also change. Furthermore, the specific length of the antenna is also related to the output impedance of the circuit, the thickness of the antenna, etc. In amateur situations, just connect a piece of wire. (If you want to pursue the longest launch distance, you can try more in this area. The component package of this site has been tested by the technical staff of this site, and the effect can easily reach 50 meters away.)
C5 is the feedback capacitor, and the circuit starts to oscillate The key component is it. When analyzing the high-frequency state of this circuit, the collector is the output and the emitter is the input. The output signal is added to the input through C5, which generates strong positive feedback and naturally generates oscillation. This is actually the capacitive three-point oscillation circuit mentioned in the book.
C1 is the power supply filter capacitor, which provides a loop for the AC signal and reduces the AC internal resistance of the power supply.

TX connects to the antenna : a piece of wire can be connected as an antenna, the antenna should not be too long, so as not to affect frequency stability

Component list :

 The following is the actual product

1. PCB empty board, without parts

 

2. Kit parts, assembled by yourself

 

 

Welded finished product

 

 

 

 
debugging:

Connect the power supply and antenna test, the antenna can use 20-40 cm thick wire (the antenna should not be too long, so as not to affect the frequency stability ), not provided in the kit, you need to bring your own. The receiving equipment uses FM radio to adjust the tightness of the inductive coil on the simple wireless microphone, so that the radio can receive the signal of the microphone.