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The Principle and Design of the Attenuator

Attenuators are widely used in various electronic devices. The official definition of the attenuator is "a circuit used to introduce a predetermined attenuation within a specified frequency range", which is generally indicated by the number of decibels of the introduced attenuation and the number of ohms of its characteristic impedance. 


Attenuators can be divided mainly into passive attenuators and active attenuators. Among them, passive attenuators can be divided into fixed attenuators and adjustable attenuators. Active attenuators are generally used in conjunction with thermal elements to form a variable attenuator.


1. The main purpose of the attenuator


Attenuators are widely used in electronic equipment. Its main functions are: to adjust the size of the signal in the circuit; in the comparison method measurement circuit, it can be used to directly read the attenuation value of the network under test; to improve impedance matching, if some circuits require when there is a relatively stable load impedance, an attenuator can be inserted between this circuit and the actual load impedance to buffer impedance changes. Usually, the attenuator is between the signal source and the load. The attenuator is a four-terminal network composed of resistive elements. Its characteristic impedance and attenuation are constants independent of frequency, and the phase shift is equal to zero.


2. The principle of the attenuator


The attenuator consists of a potentiometer and is used in debugging and level adjustment. It is required that the input and output impedance of the attenuator should match the interface end, and should be 75 ohms in the cable TV system. The frequency characteristics of the attenuator should meet the requirements of the frequency range of the system. Within the frequency range, the attenuation of the attenuator should be independent of the frequency. Therefore, in the composition of common resistive elements, the frequency range is different, and the form of the attenuator is also different.


3. Designs of the fixed attenuator


Commonly used fixed attenuators have several structures such as symmetrical T-type, bridge T-type, and inverted L-type (asymmetric type). Among the above-fixed attenuators, T-type, T-type, and bridge T-type belong to symmetrical attenuators, which are mainly used for attenuation. The inverted L type is an asymmetric attenuator and is mainly used for impedance matching.


The inverted L-shaped asymmetric attenuator constitutes an impedance matcher. The difference from the symmetrical attenuator is that the inverted L type cannot specify the attenuation amount. After the input and output impedances are determined, the attenuation amount is also determined. In the bridge T-type fixed attenuator, the value of two resistors is the characteristic impedance (input and output resistance), and the calculation formula is simple, which is very convenient to form an adjustable attenuator.

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