A small FM transmitter circuit

one. Preface: Here I will introduce to you a small FM transmitter that is simple and easy to make. It not only meets the requirements of the transmitting distance, but also can input sound signals using microphones and lines at the same time for background dubbing. The circuit also adds a modulation monitoring meter so that you can better control and use the transmitter. The circuit is as shown below. After reading a brief introduction, do you think it has as many functions as a set of broadcast station equipment? Yes, this little transmitter can help you easily set up an amateur FM radio station! The coverage range is around 500M. If you are excited, take action now! two. 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, which amplifies the weak voltage signal to a sufficient amplitude and enters together with the amplified line input signal of U1A U1B does the mixing. The mixed signal is passed through R17 to modulate the test ratio LC oscillator composed of BG2 field effect transistor and surrounding circuits. The test ratio LC oscillator is characterized by the simplicity of an ordinary capacitor three-point oscillator, and at the same time it has high efficiency and High stability. Good waveform and modulation bandwidth are particularly important for amateurs making FM transmitters. Finally, the post-stage amplifier made of BG3 is amplified and emitted outward through the transmitting antenna, and the broadcast can be listened to with an ordinary FM radio. The other signal is amplified by the amplifier composed of U1C and then rectified by D2 simple wave, and then driven by a DC microammeter to monitor the modulation amplitude of the signal. When using it, it is generally advisable not to exceed 85%. The integrated circuit adopts general-purpose quad operational amplifiers such as LM324 and forms a single power supply reverse input working mode, that is, the positive-phase input voltage is set at half of the power supply voltage. The monitoring meter is 200u type, and BG1.BG3 uses 2SC1815. BG2 uses 3DO2 field effect transistors, and D2 varactor diode uses 1S2267. The antenna is required to use 1/4 wavelength. If a rod antenna is used, it is best not to be less than one meter. Otherwise, the overdue launch effect will not be achieved. three. Postscript: This circuit was not designed by me. It was excerpted and adapted from an unknown magazine I picked up from the junk pile in the early years due to the shortage of electronic information. The original circuit comes with printing plates and assembly drawings. This type of digest magazine is rare nowadays. Thanks again to the original author! Please be careful not to interfere with normal radio broadcasting when using the transmitting circuit. Finally, I wish you a great time.