A collection of amateur FM transmitter circuits

The poster said [Repost] A collection of amateur FM transmitter circuits A collection of amateur FM transmitter circuits (top) Figure 1 is a more classic 1.5km single-tube FM transmitter circuit. The key component in the circuit is the emitter transistor, which is mostly D40, D50, 2N3866, etc., and the operating current is 60~80mA. However, the above transistors are difficult to purchase, the price is high, and there are many fakes. The author chose other triodes for experiments. The relatively easily available triodes C2053 and C1970 are quite good, and the actual line-of-sight communication distance is greater than 1.5km. The author has also replaced the D40 tube with an ordinary triode 8050. The operating current is 60-80mA, but the emission distance cannot reach 1.5km. If it is replaced with 9018, etc., the operating current will be smaller and the emission distance will be shorter. In addition to the transmitter transistor in the circuit, the parameter selection of the coil L1 and capacitor C3 is more important. If the selection is improper, it will not vibrate or the operating frequency will exceed the range of 88~108MHz. Among them, L1 and L2 can use ∮0.31mm enameled wire, single-layer flat-wound 5 and 10 turns on a round rod of about ∮3.5mm, and C3 can use 5-20pF porcelain or polyester adjustable capacitors. In actual production, capacitor C5 can be omitted, and L2 can also be replaced by an ordinary inductor coil of 10 to 100mH. If the launch distance is only a few tens of meters, you can choose the battery voltage to be 1.5~3V, and replace the D40 tube with a cheap 9018, etc., which will consume less power. You can also refer to "Electronic Newspaper" Issue 8, 2000 The article "Simple Long-distance Wireless FM Microphone" has been slightly modified since the fifth edition. The single-tube transmitter introduced in Figure 1 has the characteristics of simple circuit, high output power, and easy production. However, it is inconvenient to connect high-frequency cables to send the radio frequency signal to the outdoor transmitting antenna. Generally, it is a 0.7-0.9m rod antenna. If it is directly connected to C5 for transmitting, due to the Doppler effect, when people move near the antenna, the frequency drift phenomenon is very serious, causing the sound of the receiver that is originally receiving normal sound to be distorted or silent. If this transmitter is used as a wireless microphone, you can imagine how serious the frequency drift will be when holding the antenna. Figure 2 shows the circuit of a 2km FM transmitter. This circuit is divided into three levels: oscillation, frequency multiplication and power amplification. In the circuit, V1, C2~C6, R2, R3 and L1 form a capacitor three-point oscillator. Its oscillation frequency is mainly determined by the parameters of C3, C4 and L1. Its oscillation frequency is 44~54MHz. The signal is output from the center tap of L1. , and then coupled to V2 for amplification through C7, C8 and L2 select the double frequency signal of 44~54MHz, that is, 88~108MHz. This signal is coupled to V3 for power amplification by C9, and V3 is powered by three 3DG1 2 transistors connected in parallel can Expand output power. When the circuit is working normally, the current is about 80~100mA. The three 3DG12s that make up V3 can be equipped with appropriate heat sinks to prevent overheating. During production, L1~L3 are wound with a single layer of ∮0.31mm enameled wire on a ∮3.5mm round rod. Figure 3 shows a practical 50m FM wireless headset transmitter circuit. The circuit is divided into oscillation and signal amplification parts. L1, C2~C5, V1, etc. form an improved capacitor three-point oscillator similar to the local oscillator circuit of the black and white TV tuner. It has good frequency stability and does not run out of frequency for a long time. Practice has proved that this improvement can be used in amateur situations. A capacitive three-point oscillator is fully capable. After the author directly soldered the collector of V1 with an electric soldering iron for a few seconds, when the temperature of the transistor was very high, the reception with an ordinary radio was still normal and there was no frequency deviation. 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 the e and b electrodes of V1 causes changes in the oscillation frequency as the audio voltage changes, achieving frequency modulation. In this circuit, L1~L3 are single-layer flat-wound with ∮0.31mm enameled wire on ∮3.5mm round rod. The oscillation frequency is fine-tuned by adjusting the inter-turn spacing of L1, and then the inter-turn spacing of L2 and L3 is fine-tuned to resonate at the oscillation frequency to obtain the maximum output power. Figure 4 shows the crystal oscillator transmitter circuit. In the circuit, J, VD1, L1, C3~C5, and V1 form a crystal oscillation circuit. Because quartz crystal J has good frequency stability and is less affected by temperature, it is widely used in cordless phones and AV modulators. V1 is a 29~36MHz crystal oscillator transistor. The emitter output contains rich harmonic components. After amplification by V2, the collector consists of C7 and L2 and selects a 3 times frequency signal (i.e. 87~108MHz) from a network that resonates at 88~108MHz. The signal is the strongest), and then amplified by V3, and a more ideal FM band signal is obtained after frequency selection by L3 and C9. The process of frequency modulation is as follows. Changes in the audio voltage cause changes in the inter-electrode capacitance of VD1. Since VD1 is connected in series with crystal J, the oscillation frequency of the crystal also changes slightly. After tripling the frequency, the frequency deviation is 29~36MHz crystal 3 times the frequency deviation. In practical applications, in order to obtain a suitable modulation degree, you can choose a quartz crystal or ceramic oscillator with a large modulation frequency deviation, or you can use a 6 to 12 frequency multiplier circuit with a slightly more complex circuit. If the input audio signal is weak, a first-level voltage amplifier circuit can be added.