Working principle of audio transmitter (88-18MHZ)

As shown in the figure, the whole transmitter consists of low-frequency amplification, varactor bias and oscillation circuit. After the audio signal is amplified by BG1, it is sent to the varactor, so that its capacity changes with the signal, and the oscillation circuit is directly frequency-modulated, eliminating the parasitic amplitude modulation phenomenon of using transistor junction capacitor frequency modulation, with few harmonic components. The frequency modulated RF signal radiates directly from L1 to space.

Selection of components

The inductor L1 is wound with ∮1mm enameled wire on ∮8mm round tube for 5 turns, which is hollow. L2 is made of ∮ .1 ~ .3 mm enameled wire wound 1 turns on a carbon film resistor with a 1/4w resistance greater than 1kΩ, and the joints are welded at both ends of the resistor. C2, C11, C4, C3 and C5 should adopt high-quality ceramic capacitors. The resistors are all 1/8W, and the decoupling capacitors of .1μF are ordinary ceramic capacitors.

installation and debugging

after the welding is checked, turn on the+6V power supply, and measure the voltage between the emitter of 914 and the ground. If it is not correct, adjust R1. Re-measure the voltage between the emitter of 918 and the ground. If it is not right, adjust R6. Re-measure that there is no voltage at the two poles of varactor diode, or the voltage difference between the two poles is less than 1～2V V.. If the voltage difference between the two poles is too large, adjust R3 or R4. After adjusting the above items, turn on the FM section of the radio to receive the signal transmitted by the transmitter, and pull it to a distance of more than 5 meters. If the signal transmitted by the transmitter cannot be found, adjust L1, stretch L1, increase the fundamental frequency, compress L1, and decrease the fundamental frequency, so that the transmitter can avoid the local radio station and receive it reliably. Input the audio signal, and carefully adjust W1 so that the music will not be distorted at high volume. After debugging, put it into a strong plastic box. The transmitter used for listening to music or TV sound is the same as FM radio signal, clear and stable, and it does not need an antenna, so it is small in size. Because it is used at home, there is no need to worry about the transmission distance.

experimental comments

this transmitter is simple to manufacture, reliable in performance and low in cost. In the concrete production, epoxy board with small high-frequency loss is selected. After debugging, all coils are sealed with high-quality high-frequency wax, and the effect is very good. The capacitor of 5pF can be replaced by double-sided copper clad plate (calculated by 1pf = 1mm2).