Collection of amateur FM transmitting circuits
Source:hidaicy
2024-03-27 04:00:00
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Poster said [Poster] Amateur FM transmission circuit collectionA Collection of Amateur FM Transmitting Circuits (I) Figure 1 is a classic 1.5km single tube FM transmitter circuit. The key element in the circuit is the transmitting triode, which mostly uses D40, D50, 2N3866, etc., and the working current is 60~80mA. However, the above triodes are difficult to purchase, and the price is high, with many fakes. The author selects other triodes for experiment, which is quite good compared with the easily available C2053 and C1970 triodes. The actual line of sight communication distance is more than 1.5 km. The author has also replaced D40 tube with common triode 8050, and the working current is 60~80mA, but the emission distance is less than 1.5km. If it is replaced with 9018, the working current is smaller and the emission distance is shorter. In addition to the transmitting triode in the circuit, the parameter selection of coil L1 and capacitor C3 is more important. If not properly selected, the vibration will not start or the operating frequency will exceed the range of 88~108MHz. L1 and L2 can be wound horizontally for 5 turns and 10 turns in a single layer on a round bar of about Φ 3.5mm with Φ 0.31mm enamelled wire, and C3 can use 5~20pF porcelain medium or polyester adjustable capacitor. In actual production, the capacitance C5 can be omitted, and L2 can also be replaced with 10~100mH ordinary inductance coil. If the transmission distance is only a few tens of meters, the battery voltage can be selected as 1.5~3V, and the D40 tube can be replaced with a cheap 9018, etc., which will consume less power. You can also refer to the article "Simple Remote Wireless FM Microphone", the fifth edition of the 8th issue of Electronic News in 2000, and make a slight change. The single tube transmitter described in Figure 1 has the characteristics of simple circuit, large output power and easy production, but it is not convenient to connect high-frequency cables to send RF signals to the outdoor transmission antenna. Generally, 0.7~0.9m rod antenna is directly connected to C5 for transmission. Due to Doppler effect, when people move near the antenna, frequency drift is very serious, which makes the sound of the receiver with normal reception distorted or silent. If you use this transmitter as a wireless microphone, you can imagine how serious the frequency drift is when you hold the antenna. Figure 2 shows the circuit of 2km FM transmitter. This circuit is divided into three stages: oscillation, frequency multiplication and power amplification. In the circuit, V1, C2 ～ C6, R2, R3 and L1 form a capacitive three-point oscillator. Its oscillation frequency is mainly determined by the parameters of C3, C4 and L1. Its oscillation frequency is 44 ～ 54MHz. This signal is output from the center tap of L1, and then coupled to V2 through C7 for amplification. C8 and L2 select a 44 ～ 54MHz frequency doubling signal, that is, 88 ～ 108MHz, This signal is coupled by C9 to V3 for power amplification. V3 is composed of three 3DG12 triodes in parallel, which can expand the output power. When the circuit works normally, the current is about 80~100mA. The three 3DG12 that make up V3 can be added with proper heat sink to prevent overheating. L1 ～ L3 shall be horizontally wound in a single layer on a Φ 3.5mm round bar with Φ 0.31mm enamelled wire during fabrication. Figure 3 shows the transmission circuit of a practical 50m FM wireless headset. The circuit is divided into oscillation and signal amplification. L1, C2 ～ C5, V1, etc. constitute an improved capacitive three-point oscillator similar to the local oscillator circuit of the black and white TV tuner. The frequency stability is good, and the frequency does not run for a long time. Practice has proved that the improved capacitive three-point oscillator is successful in amateur situations. A few seconds after the collector of V1 is directly soldered with an electric soldering iron, under the condition that the temperature of the triode is very high, the reception with an ordinary radio is still normal, and there is no frequency run. The frequency of the oscillator is mainly determined by L1 and C2. By fine tuning L1, it can cover the range of 88~108MHz. The audio signal is coupled to the base of V1 through R6 and C11. The capacitance between e and b poles of V1 changes with the change of audio voltage to cause the change of oscillation frequency and realize frequency modulation. In this circuit, L1 ～ L3 is wound horizontally in a single layer on a Φ 3.5mm round bar with Φ 0.31mm enamelled wire. Adjust the L1 inter turn spacing to fine tune the oscillation frequency, and then fine tune the L2, L3 inter turn spacing to resonate with the oscillation frequency to obtain output power. Figure 4 shows the circuit of crystal oscillator transmitter. In the circuit, J, VD1, L1, C3~C5, V1 form the crystal oscillation circuit. Quartz crystal J is widely used in cordless telephones and AV modulators due to its good frequency stability and less temperature influence. V1 is a 29~36MHz crystal oscillating triode, and the emitter output contains rich harmonic components. After V2 amplification, select the 3-fold frequency signal (i.e. the strongest signal in 87~108MHz) in the network where the collector is composed of C7 and L2 and resonates in 88~108MHz. After V3 amplification, select the frequency of L3 and C9 to get the ideal FM frequency band signal. The process of frequency modulation is like this. The change of audio voltage causes the change of the capacitance between the poles of VD1. Because VD1 is connected in series with crystal J, the oscillation frequency of the crystal also changes slightly. After frequency tripling, the frequency deviation is three times of the frequency deviation of 29 ～ 36MHz crystal. In practical application, quartz crystal or ceramic oscillator with large modulation frequency deviation can be selected to obtain proper modulation system, or 6-12 frequency multiplication circuit with slightly complex circuit can be used. If the input audio signal is weak, a voltage amplifier circuit can be added. |
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