Making Frequency Modulation Transmitting Circuit with 3DA87C
Making a FM transmitter circuit with 3DA87C Introduce a long-range FM transmitter circuit made with a common transistor 3DA87C. This circuit is also an ordinary three-point oscillation circuit. The article shows that the long-distance transmitter circuit uses high-current transmission and can reach 1KM in open areas. The author purchased the components and assembled the test according to the schematic diagram. The components were in accordance with the requirements in the article. The transistor should be selected with a blue dot. The magnification is greater than 80 times, but it is found in the experiment that its frequency is not normal at 88-108MHZ. Within the FM band, no matter how the capacitance and inductance are adjusted, the frequency point is lower than 88MHZ, which is about 70MHZ. The author is a TESUN radio with a TV sound reception function to receive it normally. It is because the fT cutoff frequency parameter value of the transistor is not enough, so that its oscillation frequency cannot be raised.
In order to increase the transmission distance and make its frequency fall within the range of the normal FM radio, it is necessary to look for other high-frequency transistors. Many electronic newspapers often introduce D40, C1971, C1972 as high-frequency oscillation or power amplifier circuits. The high-power output is used to increase the transmission distance, but such high-frequency transistors are difficult to buy on the market, and even if they are bought, they are mostly fakes and cannot be used. Later, it was found to use C3355. The cut-off frequency of this transistor is several thousand MHZ, and its power is 600MW. It is enough for the FM band. Then the circuit is improved to make a long-distance FM transmission circuit.
Component selection: Capacitors C2, C3, C4 are high-frequency ceramic dielectric capacitors, Ct is 5 / 25P high-frequency semi-adjustable capacitors, and can also be replaced with high-frequency ceramic dielectric of the same value after testing with a digital multimeter after debugging Capacitance, L is enameled wire with a diameter of 0.9mm 8mm
The round tube is reborn from 6 turns around, and then pulled apart about 2CM, the middle tap, the transmitting antenna adopts a TV antenna or is replaced by a wire of the same length.
The longest distance from the actual debugging to the best transmission power is not less than 500 meters.
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