Several wireless microphone circuits

Several Wireless Microphone Circuits Editor's Note: This article introduces in detail several representative low-power transmitter circuits within the 88" 108MHz FM broadcast range that are easy to make successfully in amateur situations. Among them are simple single-tube transmitter circuits. There are also stereo transmitter circuits using integrated circuits. They are mainly used for FM wireless headsets, wireless telephone recording and forwarding, remote control, wireless alarms, monitoring, data transmission and campus FM broadcasts. 　　Figure 1 is a more classic one. 5km single-tube FM transmitter circuit. The key component in the circuit is the transmitter transistor, which mostly uses D40, D5O, 2N3866, etc. The operating current is 60-80mA, but the above transistors are difficult to buy, and the price is high, and there are many fakes. . The author chose other transistors for experiments. The readily available transistors C2053 and C1970 are quite good. The actual line-of-sight communication distance is greater than 1.5km. The author has also replaced the D40 transistor with an ordinary transistor 8050, and the operating current is 60-80mA. However, the transmitting distance cannot reach 1.5km. If it is changed to 9018, etc., the operating current will be smaller and the transmitting distance will be shorter. In addition to the transmitting transistor in the circuit, the parameter selection of coil L1 and capacitor C3 is more important. If the selection is improper, it will cause It does not vibrate or the operating frequency exceeds the range of 88--108MHz. Among them, L1 and L2 can use 0.31mm enameled wire to be wound with 5 and 10 turns in a single layer on a round rod of about 3.5mm, and C3 can use 5-20pF porcelain dielectric. Or polyester adjustable capacitor. In actual production, capacitor C5 can be omitted, and L2 can also be replaced with an ordinary inductor coil of 10-100mH. If the launch distance is only a few dozen meters, the battery voltage can be selected as 1.5-3V. If you replace the D40 tube with a cheap one such as 9018, it will consume less power. You can also refer to the article "Simple Long-distance Wireless FM Microphone" in the fifth edition of the 8th issue of "Electronic News" in 2000 and make slight changes as shown in Figure 1. The single-tube transmitter has the characteristics of simple circuit, high output power, and easy production. However, it is inconvenient to connect a high-frequency cable to send the RF signal to the outdoor transmitting antenna. Generally, a 0.7--0.9m rod antenna is directly connected to the For C5 transmitters, due to the Doppler effect, when people move near the antenna, the frequency drift is very serious, causing the normal sound of the receiver to be distorted or silent. If the transmitter is used as a wireless microphone, hold the antenna with your hand. At this time, you can imagine how serious the frequency drift is. 　　Figure 2 shows the 2km FM transmitter circuit. This circuit is divided into three levels: V1, C2--C6, R2, R3 and L1 forms 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 through C7 for amplification. It is amplified by C8 and L2 selects a 44" 54MHz double frequency signal, that is, 88-108MHz. This signal is coupled to V3 by C9 for power amplification. V3 is composed of three 3DGl2 transistors connected in parallel to expand the 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 and L3 are wound with a single layer of 0.31mm enameled wire on a 3.5mm diameter 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 capacitive three-point oscillator similar to the local oscillator circuit of a black and white TV tuner. It has good frequency stability and does not run out of frequency during long-term operation. Practice has proved that in amateur situations, this improved type oscillator can be used The capacitor 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 of the oscillator. It is mainly determined by L1 and C2. By fine-tuning L1, it can cover the 88-108MHZ range. The audio signal is coupled to the base of V1 through R6 and C11. The capacitance between the e and b electrodes of V1 changes with the change of the audio voltage. Change to achieve frequency modulation. In this circuit, L and "L3 are wound with a single layer of 0.31mm enameled wire on a 3.5mm round rod. By adjusting the inter-turn spacing of L1 to fine-tune the oscillation frequency, and then fine-tuning the inter-turn spacing of L2 and L3 to the resonator oscillation frequency, the maximum output power can be obtained. 　　Figure 4 shows the crystal oscillator transmitter circuit. J in the circuit. , VD1, L1, C3, C5, and V1 form a crystal oscillator 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. Vl is a 29" 36MHz crystal The output of the emitter of the oscillating transistor contains rich harmonic components. After amplification by V2, the 3 times frequency signal is selected from the collector network composed of C7 and L2 that resonates at 88-108MHz (that is, the signal at 87" and 108MHz is the strongest). Then amplified by V3; after frequency selection by L3 and C9, a more ideal frequency modulation band signal is obtained. The frequency modulation process is as follows. The change of the audio voltage causes the change of the capacitance between the VD1 poles; because VD1 is connected in series with the crystal J, the oscillation of the crystal is blocked. The frequency also changes slightly. After frequency doubling, the frequency deviation is 3 times the frequency deviation of the 29-36MHz crystal. In practical applications, in order to obtain a suitable modulation degree, a quartz crystal or ceramic oscillator with a larger modulation frequency deviation can be selected. , you can also use a 6-12 frequency multiplier circuit with a slightly more complex circuit. If the input audio signal is weak, you can add a first-level voltage amplifier circuit.