FM transmitter circuit diagram (detailed explanation of FM radio/FM IF/MW radio transmitter circuit diagram)

FM transmitter circuit diagram (1)
The working principle and circuit diagram of the FM transmitter meet the requirements of the transmission distance, and it can also use the microphone and the line to input the sound signal at the same time for background dubbing. The circuit has also added a modulation monitoring header to better grasp and use the transmitter. The circuit is as shown in the figure below. After reading the brief introduction, do you think it has as many functions as a set of broadcasting station equipment. Yes, this small transmitter can help you easily set up an amateur FM radio station! The coverage is about 500M.

Circuit principle and component selection:
After the microphone picks up the signal, it enters the voltage negative feedback single-tube amplifier composed of BG1 and peripheral circuits through C1 coupling, and amplifies the weak voltage signal to a sufficient amplitude to enter U1B together with the amplified line input signal of U1A for mixing. The mixed signal is modulated by R17 all the way through the test frequency LC oscillator, which is composed of BG2 FET and surrounding circuits. The test frequency LC oscillator is characterized by its simple three-point oscillator with ordinary capacitors and high efficiency and High stability. Good waveform and modulation bandwidth, which is especially important for amateur production of FM transmitters. Finally, the post-amplifier made of BG3 is amplified and transmitted out through the transmitting antenna, and you can listen to the broadcast with ordinary FM radio. The other signal is amplified by the amplifier composed of U1C and then driven by the D2 simple wave rectifier to drive the microammeter with a direct current to monitor the modulation amplitude of the signal. Generally, the modulation degree should not exceed 85% during use. The integrated circuit uses general four operational amplifiers such as LM324 to form a single power supply reverse input operation mode, that is, the voltage of the positive phase input terminal is set at half of the power supply voltage. The monitor head is 200u type, BG1.BG3 selects 2SC1815. BG2 uses 3DO2 field effect tube, D2 varactor diode uses 1S2267. The antenna is required to use 1/4 wavelength. If a whip antenna is used, it is better not to be less than one meter. Otherwise, the overdue launch effect will not be achieved.

FM transmitter circuit diagram (2)
Circuit diagram of classic 15W FM transmitter

FM transmitter circuit diagram (3)
This wireless microphone has reasonable circuit design, beautiful appearance, long sound transmission distance, long service life, economical and low power consumption, and is very suitable for ordinary FM radio reception.

Production of oscillating coil L: Use Ф0.5mm enameled wire on a Ф5mm straight shank drill to wind 4T and take it off.

Adjustment of the oscillation coil L: Turn on the radio (placed in the FM section) and the microphone switch, then hold the microphone and adjust the knob while speaking into the microphone until your own voice is heard from the radio. If you still cannot receive your own voice in the entire frequency band (ie 88~108MHz), carefully move the oscillating coil L. When you move, you only need to open or reduce the distance between each turn of the coil. Be careful when adjusting. If adjusting the tightness of the coil still has no effect, weld down L to add one turn or decrease one turn (due to the influence of electronic component parameters), and continue the above adjustment after re-welding.

Before preparing for installation and production, please use a multimeter to screen the quality of each component. If possible, measure the capacitance of each ceramic capacitor, so that it is foolproof, and the installation is successful. The quality should be guaranteed during welding, and there should be no false welding, false welding, and wrong welding.

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