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Fixed Attenuator S-Matrix | Fixed Attenuator scattering matrix

This page describes Fixed Attenuator S-Matrix (scattering matrix). It also covers basics of Fixed Attenuator along with its S-matrix. The applications of attenuator are also mentioned.

What is Fixed Attenuator?

There are two types of attenuators based on the attenuation provided viz. fixed one and variable one. There are two types of attenuators based on their control function viz. analog attenuator and digital attenuator. Analog attenuator needs analog voltage where as Digital attenuator needs digital values as control pins for its operation.

Fixed attenuator provides fixed or constant attenuation in contrast to variable attenuator which provides any attenuation within range as per control lines.

The figure-1 depicts fixed Attenuators. It is used to reduce signal power. It functions opposite to amplifier.

Fixed Attenuator S-Matrix | Fixed Attenuator applications

The Fixed Attenuator S-matrix is as follows. This S-matrix is for ideal attenuator.

Hence attenuator is matched and reciprocal device but it is not lossless. The attenuation of an attenuator is expressed as follows.

Attenuation = -10* Log10|α|2

The typical values of fixed attenuators are 3 dB, 6 dB, 10 dB, 20 dB and 30 dB. They are also known as "pads".

• 6dB attenuator pad attenuates signal by 6dB i.e. Output power is (1/4)th of the input power. Hence one of the applications of attenuator is reduction in power.
• The other application is improvement in the return loss. This is shown in figure-2 below.

As shown attenuator pad of 6 dB is inserted between source and load where matched load is of value equal to 13 dB. Here return loss will increased to value of 25 dB. Here return loss is increased twice by 6 dB which is about 12 dB, once while travelling towards load and again after it is reflected from it back.