According to the different working frequencies, they can be divided into high-frequency connectors and low-frequency connectors. Low-frequency connectors usually refer to connectors with frequencies below 100MHz; high-frequency connectors refer to connectors with frequencies above 100MHz. The structure of such connectors must consider the leakage and reflection of high-frequency anti-static shoes, PU nylon coated gloves, and anti-static clothing electric fields.
According to the different appearance structures, it can be divided into round connectors, rectangular connectors, printed circuit board connectors, ribbon flat cable connectors, etc.
Several commonly used connectors
Circular connector: also known as aviation plug and socket, it has a standard spiral locking mechanism, and the number of contacts ranges from two to hundreds.
Rectangular connector: The rectangular arrangement of rectangular connectors can make full use of space, and the current capacity is also large, so it is widely used for the interconnection of ampere-level current signals in the machine.
Printed board connector: In order to facilitate the replacement and maintenance of printed board circuits, printed circuit boards or printed circuit boards and other components are often interconnected using printed board connectors, which are divided into reed type and pinhole type according to their structural form.
Strip flat cable connector: The strip flat cable connector is made of dozens of wires with polyvinyl chloride as the insulation layer bonded together side by side. It takes up little space, is light and flexible, is easy to wire, and is not easy to confuse.
Detection of switch components and connectors
The key points of detection of switch components and connectors are reliable contact and accurate conversion. Generally, visual inspection and multimeter measurement can meet the requirements. First of all, the appearance inspection is required. For non-sealed switch parts and connectors, the appearance inspection can be carried out first. The main thing is to check whether the whole is complete and whether there is any damage. Whether the contact part is damaged, deformed, loose, oxidized or loses elasticity. The band switch should also be checked for accurate positioning, dislocation, short circuit, etc. Secondly, the contact resistance and insulation resistance are measured. Set the multimeter to the 100×100 position and measure the DC resistance between the two contacts. This resistance should be close to zero, otherwise it means that the contact is poor. Set the multimeter to the R×1k or R×10k position and measure the resistance between the contacts and the "ground" after the contacts are disconnected.
This value should tend to infinity, otherwise the insulation performance of the switch and connector is not good. The third test is load capacity, that is, the rated current value that the connector can pass under a given voltage under specified conditions. The fourth test is mechanical performance. After the crimped terminal is installed in the shell, and the external lock is removed, the insertion force and extraction force required to plug and unplug it with the socket along the axial direction are called the plug-in and pull-out force. Fix the correctly crimped connector in the shell, stretch the wire body at a certain speed along the direction of the connector, and the minimum pulling force required to make the connector come out of the shell is the terminal retention force. Fix the pin seat, apply a thrust to the pin from the top, and the thrust required to cause the pin to displace with the shell is the pin retention force.
Electroacoustic devices refer to devices that can convert between electrical signals and sound signals. Commonly used electroacoustic devices include microphones, speakers and headphones.
1. Microphone
1) Types of microphones and circuit symbols
Microphones are also called microphones, etc. In circuits, they are often represented by symbols B or BM. There are many types of microphones in circuits. According to the driving method, they can be divided into electric microphones, capacitor microphones, piezoelectric microphones, electromagnetic microphones, carbon particle microphones and semiconductor microphones. According to the signal transmission method, it can be divided into wired microphones and wireless microphones. According to the directivity, it can be divided into cardioid, acute cardioid, supercardioid, bidirectional (8-shaped) and non-directional (omnidirectional), etc. According to the purpose, it can be divided into measurement microphones, vocal microphones, instrument microphones and recording microphones, etc.
2. Main parameters indicating microphone performance
Sensitivity. This refers to the magnitude 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 (0dB=1000mV/Pa). The sensitivity of general electret microphones is mostly in the range of 0.5~10mV/Pa or -66~-40dB. The higher the sensitivity of the microphone, the greater the amplitude of the audio signal output under the same sound.
Frequency response. This refers to the characteristic that the sensitivity of the microphone changes with the sound frequency, and is usually represented by a curve. Generally speaking, when the sound frequency exceeds the upper and lower limit frequencies given by the manufacturer, the sensitivity of the microphone will drop significantly. The frequency response of electret microphones is generally flat. The frequency response of ordinary products is good (i.e. the sensitivity is relatively balanced) in the range of 1001Hz to 10kHz, the frequency response of good quality microphones is 40Hz to 15kHz, and the frequency response of high-quality microphones can reach 20Hz to 20kHz. Output impedance. This refers to the AC impedance of the output end of the microphone at a certain frequency (1kHz). After the impedance transformation of the internal field effect tube, the output impedance of the electret microphone is generally less than 3k.Q.
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. Generally, the inherent noise of the electret microphone is very small, which is ∥ level voltage.
Directivity. It is also called directivity, which refers to the characteristic that the sensitivity of the microphone changes with the direction of the incident sound wave. The directivity of the microphone is divided into three types: unidirectional, bidirectional and omnidirectional. The sensitivity of the front of the unidirectional microphone to the sound wave is significantly higher than that in other directions, and according to the shape of the directional characteristic curve, it can be divided into three types: cardioid, supercardioid and super-directional. The sensitivity of the bidirectional microphone in the front and rear directions is higher than that in other directions. Omnidirectional microphones have basically the same sensitivity to sound waves from all directions. Most of the machine-mounted electret microphones are omnidirectional microphones.
