First turn the multimeter to the diode position, measure the continuity between the two pins and the shell, and measure the connection between the right pin and the shell, so the right pin is the negative pole, and the left pin is the positive pole, as seen from the welding plate of the microphone The copper skin of the right foot is connected to the shell, so it can also be judged that the right foot is negative. Then measure the quality of this microphone. When the multimeter is set to 2K in ohms, the red test lead is connected to the positive electrode of the microphone, and the black test lead is connected to the negative electrode. The multimeter displays 1K, and then blows to the mouth. The resistance value of the multimeter changes, indicating that the microphone is good. The greater the change in blowing, the better the sensitivity. This measurement can only test the sensitivity of the microphone and roughly whether it is good or bad, other parameters such as distortion and noise cannot be judged.
Principle of electret microphoneWhen the acoustic wave causes the electret film to vibrate and produce displacement; the distance between the two plates of the capacitor is changed, which causes the capacitance of the capacitor to change. Since the number of charges on the electret always remains constant, according to the formula: Q = CU When C changes, it will inevitably cause a change in the voltage U at both ends of the capacitor, thereby outputting an electrical signal and realizing an acoustic-electric conversion.
The total charge of the electret is constant. When the plate retreats under sound wave pressure, the capacitance decreases, and the voltage between the two poles of the capacitor will increase inversely. On the contrary, when the capacitance increases, the voltage between the two poles of the capacitor will increase. It will decrease inversely.
Stationary microphone structureThe internal structure of the electret microphone is shown in figure (a), which is mainly composed of two parts: "acoustic-electric" conversion and impedance conversion.
The key element of the "acoustic-electric" conversion is the electret vibrating diaphragm, which uses a very thin plastic diaphragm as a substrate, evaporates a pure metal film on one side, and then passes through a high-voltage electric field to "elect" After treatment, an opposite charge that can be maintained for a long time is formed on both sides-this is the origin of the term "electret" (also known as "permanent charge body").
Main parameters of electret microphone(1) Working voltage (UDS). This refers to the minimum DC working voltage that must be applied to both ends of the microphone when the electret microphone is working normally. This parameter differs depending on the model, even the same model has a greater dispersion, usually the typical values ​​given by the manufacturer are 1.5V, 3V and 4.5V.
(2) Working current (IDS). This refers to the DC current that the electret microphone passes through when it is static. It is actually the static current of the internal field effect tube. Similar to the operating voltage, the discreteness of the operating current is also greater, usually 0.1 to 1 mA.
(3) Maximum operating voltage (UMDS). This refers to the maximum DC voltage that the drain and source of the FET inside the electret microphone can withstand. When the limit voltage is exceeded, the FET will be damaged by breakdown.
(4) Sensitivity. This refers to the size of the audio signal voltage that the microphone can generate under a certain external sound pressure, and its unit is usually mV/Pa (millivolt/Pa) or dB (1dB=1000mV/Pa). Generally, the sensitivity of electret microphones is mostly in the range of 0.5～10mV/Pa or -66～-40dB. The higher the microphone sensitivity, the greater the amplitude of the audio signal output under the same sound level.
(5) Frequency response. Also called frequency characteristic, it refers to the characteristic that the sensitivity of the microphone changes with the change of the sound frequency, which is usually expressed by a curve. Generally speaking, when the sound frequency exceeds the upper and lower limit frequency given by the manufacturer, the sensitivity of the microphone will decrease significantly. The frequency response of electret microphones is generally flat. The frequency response of ordinary products is better (ie, the sensitivity is more balanced) in the range of 100Hz～10kHz, better quality microphones are 40Hz～15kHz, and high quality microphones can reach 20Hz～20kHz.
(6) Output impedance. This refers to the AC impedance at the output of the microphone at a certain frequency (1kHz). The output impedance of the electret microphone is generally less than 3kΩ after the impedance transformation of the internal field effect tube.
(7) Inherent noise. This refers to the noise signal voltage output by the microphone when there is no external sound. The greater the inherent noise of the microphone, the greater the noise mixed in the output signal during operation. In general, the inherent noise of electret microphones is very small, with a voltage of microvolts.
(8) Directivity. Also called directivity, it refers to the characteristic that the sensitivity of the microphone changes with the incident direction of sound waves. The directivity of the microphone is divided into three types: unidirectional, bidirectional and omnidirectional. The frontal sensitivity of unidirectional microphones to sound waves is significantly higher than other directions, and according to the shape of the directional characteristic curve, it can be subdivided into three types: cardioid, super cardioid and super directional; the sensitivity of bidirectional microphones in the front and back directions All are higher than other directions; omnidirectional microphones have basically the same sensitivity to sound waves from all directions. Most commonly used machine-mounted electret microphones are omnidirectional microphones.
Four Connection Methods for Resident Group
The corresponding microphone lead-out ends are divided into two-terminal and three-terminal types. In the figure, R is the load resistance of the field effect tube. Its value is directly related to the microphone's DC bias, and has a greater impact on the working parameters such as microphone sensitivity. Big impact.
The two-terminal output mode is to connect the field effect tube into a drain output circuit, which is similar to a common emitter amplifier circuit of a transistor. Only two lead wires are needed, and a drain resistance R is connected between the drain D and the positive electrode of the power supply. The signal output from the drain has a certain voltage gain, so the sensitivity of the microphone is relatively high, but the dynamic range is relatively small. Most electret microphones currently on the market are connected in this way. (SONY uses this type of microphone on MD).
The three-terminal output mode is to connect the field effect tube into a source output mode, which is similar to the emitter output circuit of a transistor, with three leads. The drain D is connected to the positive electrode of the power supply, and a resistor R is connected between the source S and the ground to provide the source voltage, and the signal is output from the source through the capacitor C. The output impedance of the source output is less than 2K, the circuit is relatively stable, and the dynamic range is large, but the output signal is smaller than the drain output.
Pin identification of electret microphoneThe pin identification method of electret microphone is very simple, whether it is in-line, lead or solder pin type, its bottom surface is generally a printed circuit board, as shown in the figure.
For an electret microphone with 2 parts of copper coated on the printed circuit board, the copper coating connected to the metal shell should be the "ground terminal", and the other copper coating should be the "power/signal output terminal" (with "drain D The difference between "output" and "source S output"). For electret microphones with three copper-clad parts on the printed circuit board, except for the copper-clad that communicates with the metal shell, which is still the "ground terminal", the remaining two parts of the copper-clad are "S end" and "D end" respectively. Sometimes the printed circuit board of the leaded microphone is encapsulated inside the shell and cannot be seen. At this time, it can be identified by the lead: the shielded wire is the "ground terminal", and the two core wires in the middle of the shielded wire are the "D end" ( Red wire) and "S terminal" (blue wire). If there is only one core wire (such as the domestic CRZ2-9 type), the lead must be "power/signal output".
Notes on electret microphone selectionThe price of electret microphones is very low. Replace them when they are damaged. Pay attention to the following points regarding the selection of electret microphones:
(1) The two-pin and three-pin electret microphones cannot be directly replaced, and generally, the circuit is not replaced.
(2) This type of microphone is not divided into models. Microphones with the same number of pins can be replaced, but there are differences in performance.
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