What is an Attenuator – Design, Types and Applications

Wireless communication technology has paved the way for very interesting inventions. It is also known as ” over the air” communication. This technology made mobile and interplanetary communication a reality. The first mobile communication invented in 1880 was ” Photophone”. It used sunlight to transfer the audio from one point to another. In telecommunication systems, some form of energy like radio waves or acoustic energy is used to transfer the information one place to another. Here no wires are used and medium of propagation is usually air. There are some challenges faced by this technology that degrades its efficiency and reliability. One such challenge is Attenuation. The device used for attenuation is Attenuator.

What is Attenuator?

Signals are sent from one place to another through a medium. These signals can be data signals, voltage signals, current signals, etc. When the distance traveled by the signal increases the strength of the signal gradually decreases. This gradual loss of intensity of signals through the medium is called Attenuation.

Though viewed as a challenge for long-distance transfer of signals, this phenomenon is found to be useful in many other tasks. The device which is designed to reduce the power of signals without disturbing its waveform is called as “Attenuator”.

The attenuator is highly used after signal generator circuits. It helps in attenuating or reducing the strength of high-level signals before applying them to the Antenna circuits. An attenuator is a two-port electronic device It is designed using resistors to weaken or attenuate a signal. Attenuators are passive circuits, they work without any power supply. These are available both as fixed attenuator with a fixed attenuation level and as a continuously changing attenuator. Contrary to amplifiers gain percentage, attenuator gives loss percentage. The amount of attenuation is measured in decibels.

Design of Attenuator

Attenuators are passive two-port electronic circuits. These are purely designed using resistors. Here, resistors are arranged as a voltage divider network. Attenuator design depends on the line geometry of the connecting wires between devices. Depending on whether a line is balanced or unbalanced, attenuators used with the line are required to be balanced or unbalanced. Attenuators used with coaxial lines are unbalanced form. Attenuators used with twisted-pair are of the balanced form.

Attenuator circuit is both linear and reciprocal based on the application, the attenuator can be unidirectional or bidirectional. When the attenuator circuit is made symmetrical, there will be no difference between the input port and the output port. In that case as a general rule, the left port is considered as input and the right port is considered as an output.

Attenuators are also found as built-in circuits in signal generators as well as stand-alone circuits. Stand-alone attenuators are placed in series between a signal source and load circuit on the signal path. In such a case in addition to providing attenuation, it must match the source impedance and load impedance. Attenuators are found in Radio communication and transmission lines to reduce the power of the signal.

Types of Attenuator

Attenuators are available as both fixed attenuators and adjustable attenuators. Fixed attenuator networks are known as ‘attenuator Pads”. These are available for specific values from 0dB to 100dB. Attenuators are commonly found in Radio Frequency and Optical applications. Radio Frequency attenuators are used in the electronic circuits whereas optical attenuators find applications in fiber optics.

Few common layouts of attenuator are- T configuration, pI configuration, and L configuration.  These configurations are of an unbalanced type. The balanced type of T configuration and pI configurations are denoted as ‘H’ configuration, O configuration respectively. The balanced type is a symmetrical circuit whereas unbalanced types are asymmetrical circuits.

The RF-based design of the attenuator is of six types. They are Fixed type, Step type, Continuously Variable type, Programmable Type, DC Bias type and DC Blocking type.

Fixed Type

In Fixed type attenuators the resistor network is locked at a predetermined attenuation value. These are laid down in the signal path to attenuate the power of the transmitted signal. These can be unidirectional or bidirectional based on their application requirement. These can be available as either surface mount, waveguide or coaxial types. In a chip-based design, the different types of materials deposited on the thermally conductive substrate develop the resistance. This resistance value depends on the dimensions of the chip and the materials used for chip production.

Step Type

These attenuators are similar to fixed attenuators. But in this type, a push-button is provided to adjust the attenuation values. These provide attenuation values only from the pre-calibrated steps. Depending upon the application, the attenuator can be used in either chip, waveguide or Coaxial formats.

Continuously Variable Type

In Continuously variable type, attenuation value can be changed manually to any attenuation value from the given specified range. In this type, the resistors present in the attenuator network are restored with the solid-state elements, for example, MOSFET or PIN diode. Compared to the passive resistor network, by changing the voltage in the FET devices attenuation can be varied with greater resolution. Here it is possible to vary the attenuation using either manually or with the help of electronic signals.

Programmable Type

This type is also popularly called as the ‘Digital step attenuator’. This component is controlled by a computer-driven external control signal. These are controlled by the TTL logic circuits with a step size range such as 2,4,6,……,32. If the applied voltage across this attenuator is found to be lesser than 1V, logic level 0 is attained. For voltages of 3V and higher, logic level 1 is given. The above logic levels are used to control the single-pole and double-throw switches that connect a number of attenuators on the signal path. This type is also available in the USB designs with the installed software.

DC Bias Type

This type of attenuator has capacitance both at the input port and output port of the device which blocks the DC voltages. Thus, apart from attenuating the RF signals, this type passes the DC signals.

DC Blocking Type

This type is similar to the DC Bias type. The only difference between these two is the way the DC signal is totally blocked without any alternative path running towards the output port.

Optical Attenuators

These are similar to RF attenuator but instead of electrical signals, these attenuate the light waves. This attenuator absorbs or dissipates light according to the attenuation values without changing the waveform. Similar to the RF attenuators, optical attenuators are also designed as fixed, variable, programmable, etc….These are designed based on the application requirement. Fixed optical attenuators use doped fibers to disperse the light given as input. Variable and programmable optical attenuators are closely related to the RF variable and RF programmable attenuators.

Attenuation in Networking

Attenuation is the reduction of signal strength. This can be found with both analog and digital signals. Attenuation is measured in decibels. In fiber optic cables the attenuation is measured as a number of decibels per foot. The cable with less attenuation per unit distance is considered to be more efficient.

Attenuation is seen in communication systems when signals are transmitted over long distances. In the computer networking context, attenuation is the loss in communication or data signals strength when transmitted over long distances. As the rate of attenuation decreases the transmitted data becomes more distorted. The main causes of attenuation in the computer network are-

• Range – Both in wired and wireless communication when a signal is transmitted over long distances the strength of the signal gradually decreases.
• Interference- Interference of any form such as physical obstructions decreases the strength of transmitted signals.

Typical values for line attenuation on the DSL  network ranges from 5dB to 50dB. Here attenuation is measured as the signal loss between the provider’s access point and home. Lower the value of attenuation better the signal quality. For Wi-Fi networks, dynamic rate scaling is observed. This automatically adjusts the connection’s maximum datarate up or down depending upon the transmission quality of the line.

Applications of Attenuators

Some of the notable applications of Attenuators are as follows-

• Attenuators are used as volume control equipment in broadcasting stations.
• For testing purposes in laboratories, to obtain smaller voltage signals, attenuators are used.
• Fixed attenuators are used to improve the impedance matching in circuits.
• These are used to protect the circuits from damages caused by high voltage values.
• RF attenuators are used for the protective dissipation of power in measuring RF signals.
• Optical attenuators are applied in fiber optic communication to properly match transmitter and receiver levels.

FAQs

1). What does an RF attenuator do?

To protest systems from the damage caused by the high power signals that are too high to process by the circuit, RF attenuators helps to reduce the amplitude level of the input signal.

2). What is Passive attenuator?

A Passive attenuator is an attenuator circuit purely made up of resistors. This circuit does not require any power supply to function.

3). How is attenuation measured?

Attenuation is measured as the units of decibels per unit length of the medium.

4). What is the cause of attenuation in optical fibers?

In optical fibers, two main causes of attenuation are absorption and scattering.

5). What is the use of Attenuator for TV signals?

Attenuator used for TV signals to adjust the signal power and reduce interference.

Attenuator helps in the reduction of signal levels. Here the power dissipation of the device depends on the surface area and mass of the resistor material used in its network. Some of the important characteristics of RF attenuator are its accuracy, low SWR, flat-frequency response and repeatability.