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Home > Circuit Diagram > Communication Circuit > Low power FM radio transmitter circuit (1000M)

Low power FM radio transmitter circuit (1000M)

Source:lyhfeitian
2024-03-28 14:00:00 7

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Li Shifeng


The author uses the existing components on hand and comprehensively refers to<& Lt; Beijing Electronic News>& Gt; We have built a long-distance FM radio transmitter which works in the 88 － 108MHZ frequency band and plays music in our spare time.

The circuit principle is shown in the attached figure. Figure (1) shows the power supply part. After the mains voltage is reduced and rectified, the voltage is stabilized to obtain stable 12V DC power for RF circuit. RF circuit consists of high-frequency oscillator, buffer amplifier, final power amplifier and antenna. High frequency oscillator is used to generate carrier frequency signal. The frequency point falls within 88-108MHZ, and the transmission frequency can be changed by completing frequency sensitivity. The RF signal is output by the emitter of VT1 and sent to the buffer amplifier composed of VT2, L2, C22, R4, etc. for power boost, which can reduce the impact of the final amplifier circuit on the oscillator. The final stage is the high frequency C narrowband amplification, which further amplifies the RF power, and is coupled to the transmitting antenna through C25 to radiate to the surrounding space. If the output signal amplitude of the connected sound source is too large, the attenuation resistor shall be connected in series to avoid sound distortion.

The circuit board can be made of copper clad plate. Pay attention to the influence of distributed capacitance when wiring. The figures without unit indication of capacitance in the figure are all in "pF", which should be consistent with high-frequency porcelain chip capacitance. Ultra high frequency NPN type silicon tube for VT1-- VT3, such as 9018, B> 60. Icm=50mA. fr>= 600MHZ VT3 can also use medium power transmitting tubes C2053, BF96S, etc., and the transmitting distance may be longer. L1-L3 is made by wrapping 00.8mm enameled wire tightly on a 04mm screwdriver for 4 turns and then pulling out. The antenna is a rod antenna with a length of 1/4 (or 1/2) of the frequency wavelength. If the transmission frequency is 100MHz, the antenna length is 0.7m (or 1.5m)

It shall be installed step by step during fabrication. The RF part is equipped with an oscillator, a buffer amplifier, and the turn to turn distance of L1 is adjusted to make the frequency point fall at the place where there is no station. The black probe of the pointer multimeter is used to contact the collector of VT2, and L2 is adjusted to deflect the pointer (i.e. power). If the meter is printed, the pens can be wound together until the meter is not printed. Use the same method to adjust L3 to make the final stage output power. Use the FM radio to search for the transmission signal at a distance of more than 10 meters from the transmitter, estimate the transmission frequency, connect the antenna, and adjust the length properly to put it into use.

When the measured power supply voltage of the machine is 12V (in fact, it can work normally within 6-15V, the higher the voltage is, the farther the distance is), the working current is only about 45mA, and the transmission frequency is about 104MHZ. Place it on the balcony on the third floor, and receive it with the universal machine (internal chip CXA1019M) without the obstruction of high buildings. The distance is 1000 meters.



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