High-fidelity AM wireless microphone circuit

The wireless microphone introduced in this article adopts the AM (amplitude modulation) method, with an operating frequency range of 525 to 1605kHz, and is received by the AM radio band. Compared with the traditional FM (frequency modulation) wireless microphone (operating in the FM band of 80-108MHz), the requirements for the FM high-frequency parameters of the components and the distributed capacitance of the component board are greatly reduced. There are few circuit components, and it is easy to install and debug. Through careful adjustment and appropriate increase of the power supply voltage, the transmission distance can reach tens of meters. In addition to being used as a stage wireless microphone, it can also be used for telephone wireless ringing, baby crying monitoring, TV audio forwarding and wireless call doorbell, etc.

1. Working Principle

The circuit is composed of integrated circuit LM389 as the core component. The working principle is shown in the attached figure. The voice signal is obtained by the microphone MIC and converted into an electrical signal. After amplification, it is coupled to the base of VT3 in the differential current source circuit through the volume potentiometer RP2 and C5. VT1 and VT2 together form a radio frequency oscillator, L and C6 are parallel high-frequency oscillation tuning circuits, and C7 is an oscillation feedback capacitor. This circuit has a stable oscillation frequency, no stray amplitude modulation and frequency modulation harmonics and their intermodulation distortion, and the line modulation characteristics are significantly better than other radio frequency oscillation circuits, so that the wireless transmission of the signal has a higher fidelity. In the attached figure, RP1 is the bias voltage adjustment potentiometer of the microphone MIC, and VT1, VT2, and VT3 are high-frequency transistors inside LM389.

2. Component Selection

MIC uses a domestic CRZ2 electret microphone. Its operating voltage is 1.5~12V, and the frequency response range is 50-10000Hz. The sensitivity is >20mv/pa. C6 uses CBM-223 or CBM-270 small tuning capacitors. If it is a double capacitor, R uses one of the two groups. In actual production, since the C6 and L circuits only select a frequency point without a station in the 525~1605kHz medium wave band, C6 can also be replaced by a 0~250pF high-frequency fixed capacitor and a fine-tuning capacitor in parallel. L is wound with Φ0.01mm×7 strands of yarn wrapped wire on a 50mm long MX-400 manganese-zinc ferrite medium wave magnetic rod. It is wound flat with 60 turns. The magnetic rod model can be selected from Y10×50 (round) or B5×10×50 (flat). If a 20-30-turn coil is wound flat on the magnetic bar, one end is connected to a 1.2m long soft wire, and the other end is grounded (battery negative pole), the transmission distance can be increased. Other component parameters are as marked in the attached figure, and there are no special requirements.

3. Debugging Methods

After all components are installed and checked, you can power on and debug. The method is as follows:

1 Microphone bias voltage. Adjust RP1 so that the voltage across MIC is 3V, i.e. 1/2Vcc.

2. RF oscillation stage. First, short-circuit C6 with a wire, and adjust R4 to make the collector currents of VT1 and VT2 equal to 1-1.2 mA. If they are not equal, adjust R7 to make them equal. If the collector currents of the two tubes cannot be equal, it means that the performance parameters of the two tubes are quite different. They should be exchanged for testing. The base bias voltage of VT3 is about 1V, which is determined by the voltage divider of R2 and R3, and generally does not need to be adjusted.

3. Modulation. The amplitude modulation depth is determined by the size of the audio signal, which is determined by adjusting RP2. If the modulation is too weak, the radio will not receive the signal; if the modulation is too strong, the received signal will be distorted. Generally, when there is no test instrument, adjust RP2 so that the sound received by the radio is loud and clear, and there is no blockage or distortion.