An attenuator is a two port resistive network designed to weaken or “attenuate” (hence their name) the power being supplied by a source to a level that is suitable for the connected load.
A passive attenuator reduces the amount of power being delivered to the connected load by either a single fixed amount, a variable amount or in a series of known switchable steps. Attenuators are generally used in radio, communication and transmission line applications to weaken a stronger signal.
The Passive Attenuator is a purely passive resistive network (hence no supply) which is used in a wide variety of electronic equipment for extending the dynamic range of measuring equipment by adjusting signal levels, to provide impedance matching of oscillators or amplifiers to reduce the effects of improper input/output terminations, or to simply provide isolation between different circuit stages depending upon their application as shown.
Simple attenuator networks (also known as “pads”) can be designed to produce a fixed degree of “attenuation” or to give a variable amount of attenuation in pre-determined steps. Standard fixed attenuator networks generally known as an “attenuator pad” are available in specific values from 0 dB to more than 100 dB. Variable and switched attenuators are basically adjustable resistor networks that show a calibrated increase in attenuation for each switched step, for example steps of -2dB or -6dB per switch position.
Then an Attenuator is a four terminal (two port) passive resistive network (active types are also available which use transistors and integrated circuits) designed to produce “distortionless” attenuation of the output electrical signal at all frequencies by an equal amount with no phase shift unlike a passive type RC filter network, and therefore to achieve this attenuators should be made up of pure non-inductive and not wirewound resistances, since reactive elements will give frequency discrimination.
Simple Passive Attenuator
Attenuators are the reverse of amplifiers in that they reduce gain with the resistive voltage divider circuit being a typical attenuator. The amount of attenuation in a given network is determined by the ratio of: Output/Input. For example, if the input voltage to a circuit is 1 volt (1V) and the output voltage is 1 milli-volt (1mV) then the amount of attenuation is 1mV/1V which is equal to 0.001 or a reduction of 1,000th.
However, using voltage, current or even power ratios to determine or express the amount of attenuation that a resistive attenuator network may have, called the attenuation factor, can be confusing, so for the passive attenuator its degree of attenuation is normally expressed using a logarithmic scale which is given in decibels (dB) making it easier to deal with such small numbers.
Degrees of Attenuation
An attenuators performance is expressed by the number of decibels the input signal has decreased per frequency decade (or octave). The decibel, abbreviated to “dB”, is generally defined as the logarithm or “log” measure of the voltage, current or power ratio and represents one tenth 1/10th of a Bel (B). In other words it takes 10 decibels to make one Bel. Then by definition, the ratio between an input signal (Vin) and an output signal (Vout) is given in decibels as:
We can produce a set of constant values called “K” values for different amounts of attenuation as given in the following table.
Attenuator Loss Table
and so on, producing a table with as many “K” values as we require.
Fixed value attenuators, called “attenuator pads” are used mainly in radio frequency (Rf) transmission lines to lower voltage, dissipate power, or to improve the impedance matching between various mismatched circuits.
Line-level attenuators in pre-amplifier or audio power amplifiers can be as simple as a 0.5 watt potentiometer, or voltage divider L-pad designed to reduce the amplitude of an audio signal before it reaches the speaker, reducing the volume of the output.
In measuring signals, high power attenuator pads are used to lower the amplitude of the signal a known amount to enable measurements, or to protect the measuring device from high signal levels that might otherwise damage it.
In the next tutorial about Attenuators, we will look at the most basic type of resistive attenuator network commonly called a “L-type” or “L-pad” attenuator which can be made using just two resistive components. The “L-pad” attenuator circuit can also be used as a voltage or potential divider circuit.