Three-tube FM wireless microphone production circuit

The microphone uses direct frequency modulation, with a center frequency of 90MHz, a transmitting power of about 0.5W, a maximum frequency deviation of ±50kHz, and a transmitting distance of not less than 50 meters. Circuit block diagram 　　The block diagram and schematic diagram are shown in Figures 1 and 2. The audio signal generated by the electret microphone acts on the emitter junction of the modulator T1 as a modulation voltage. The magnitude of this voltage directly changes the junction capacitance of the transistor's emitter junction. As a part of the loop parameters, the junction capacitance fo is about 45MHz. After frequency multiplication, the output frequency is increased to about 90MHz. After the FM signal is amplified by a high-frequency power amplifier, emitted by the antenna. Circuit Principle: 　　The modulation signal is coupled from the microphone M (CRZ-22) to the base of the modulator via C1, and R1 is the load resistance of the microphone M. The modulator consists of T1, R2, R3, C2, C3, C4, C6 and L1 to form a common base capacitor three-point oscillation circuit. Since the base of this circuit is connected to C2, the base is grounded for high frequency and the collector is grounded for audio (the collector is connected to the power supply via L1). The collector junction capacitor Cc is actually connected in parallel to both ends of the oscillation circuit. Therefore, When the audio signal changes, the oscillation frequency also changes accordingly, thereby obtaining an FM signal. Circuit characteristics 1. The modulator adopts direct frequency modulation method, and its frequency is stable and reliable. 2. Use electret condenser microphone. The microphone contains a field effect tube to form a transmitter follower, which has high sensitivity and wide frequency response. Using this microphone, a modulation voltage with appropriate amplitude can be obtained without adding an audio amplifier. 3. There are adjustment components at all levels for easy debugging. T1 can adjust C3 to change the frequency, T2 can adjust C7 to get the multiplier frequency, and T3 can adjust C10 to get the maximum output. Production and debugging: 　　The circuit printed board diagram is shown in Figure 3. The size of the printed board can be scaled according to needs. Transistors, resistors, capacitors, and microphones can be selected according to the data given in Figure 2; the battery can be a 9V laminated battery; L1, L2, and L3 can be wound with Φ0.33 enameled wire, and the antenna can be made of a soft leather wire about 20cm long; the K button sub-switch. When debugging, proceed as follows: 1. Check the soldered circuit board to ensure that it is installed correctly. First measure the static resistance between the positive and negative power supplies to see if there is a short circuit or open circuit. 2. Measure the total current. The total current measured during power-on is about 50mA. Use tweezers to short-circuit the b and e poles of T1 or short-circuit C3. The current should change significantly. Short-circuiting the b and e poles of T2 and T3 should also change significantly. 3. Measure the carrier frequency. Use an ultra-high-frequency millivolt meter to measure the high-frequency voltage of several volts between the b pole of T2 and the ground. Use a digital frequency meter to observe it. Adjust C3 to a carrier frequency of about 45MHz. Use the same method to test the T3 base and adjust C7 to get a carrier frequency of 90MHz. Then connect the frequency meter and UHF voltmeter between the antenna and ground, adjust C10 so that the frequency is 90MHz and the voltage reaches the maximum value. In this way, the carrier frequency is adjusted. 4. Modulated voltage measurement. Short-circuit C3 first, connect a millivolt meter across both ends of R4, and speak into the microphone. There should be an AC voltage indication, indicating that the microphone and T1 are working properly. If you don’t have an instrument, you can also follow the above steps and test reception while repeatedly adjusting C3, C7, and C10 until the reception distance is far and the sound distortion is minimal. Sun Dimin originally appeared in "Electronic Production"