High Fortune Model (AM) Wireless Microphone Circuit, LM389 AM Transmitter
Keywords: lm389, adjustment microphone
This article describes the wireless microphone, using a modulation (amplitude modulation) mode, operating frequency range 525 ~ 1605 kHz, and receiving the radio AM. Compared to traditional FM (frequency modulation) wireless microphones (operating at 80-108 MHz FM band). The requirements for the modulated frequency parameters of components and components are greatly reduced. The circuit components are less, easy to install and debug, and increase the power supply voltage by carefully adjusting and appropriately increasing. The transfer distance is reached several tens of meters. In addition to being used as a stage wireless microphone, it can also be used as a telephone wireless ring, and the baby is crying monitoring, television sound forwarding and wireless call doorbell.
I. Working principle
The circuit consists of the integrated circuit LM389 as the core element. The principle is shown in the drawings. The speech signal is acquired by the microphone MIC and converted into an electrical signal, and the base of the VT3 is coupled to the base current source circuit after amplifying the volume potentiometer RP2 and C5. VT1, VT2 together constitute a radio frequency oscillator, L, C6 is a parallel high-frequency oscillation tuning loop, and C7 is an oscillation feedback capacitor. This type of circuit oscillation frequency is stable, no spurious modulation and modulation harmonic and its intermodulation distortion, the line modulation characteristics is significantly better than other radio frequency oscillation circuits, thus making signal wireless transmission has a high fidelity. Degree. In the drawings, RP1 is a high-frequency transistor of the bias voltage adjustment potentiometer, VT1, VT2, and VT3 of the microphone MIC.
Second, component selection
MIC uses a domestic CRZ2 type electret microphone. Its operating voltage is 1.5 to 12V, and the frequency range is 50-10000Hz. Sensitivity. 1882.20mv / pa. C6 uses CBM-223 or CBM-270 small tuning capacitance, if the two capacitance R uses one of them. In the actual production, since the C6, L circuit selects only one in-band in the medium band in the medium band in 525 ~ 1605kHz, the C6 can also use one 0 ~ 250PF high-frequency fixed capacitance and a fine-tunable capacitor to replace it. L Wrap the magnetic rod in a MX-400 manganese zinc ferrite with a φ0.01 mm × 7 strand line. Spend 60, magnetic rod model Optional Y10 × 50 (circular) or B5 × 10 × 50 (flat), and the like. If the coil is rotated around a 20 ~ 30 on the magnetic rod, a 1.2 m long cord is connected, and the other end is grounded (the battery negative), the emission distance can be increased. Other components parameters are as follows, no special requirements.
Third, debugging method
It is installed in all components. After the check is correct. It is possible to debug. Methods as below:
1 Microphone MIC bias. RP1 is adjusted so that the voltage between the two ends of the MIC is 3V, namely 1 / 2VCC.
2. RF oscillation level. The wire is first short of C6, adjusting R4 to make VT1, and the VT2 collector current is 1 to 1.2 mA. If it is not equal, adjustable R7 to make them equal. If the two tube collector current cannot be equal. Note Differences between two tube performance parameters. The test should be disposed. The VT3 base bias voltage is about 1 V, and is determined by R2 and R3, which generally does not need to be adjusted.
3. Monitor. The audio signal size determines the amplitude modulation depth and is determined by the RP2. If the modulation is weak. The radio will not receive the signal; if the adjustment is too strong, the received signal will result in distortion. Generally when there is no test instrument. RP2 makes the sound received by the radio, and does not block distortion.
4 Send a distance. Adjusts the position of C6 and L on the magnetic rod, and gradually pulls the distance. Make the reception distance farthest. The radio sounds flour, it is not disturbed, and repeatedly commissioned several times. The best working state is best. Author: Shen Zhi
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