The Passive Attenuator is a purely resistive network that is used to weaken or “attenuate” the signal level of a transmission line while improving the impedance match, making passive attenuators the opposite of amplifiers.
Passive Attenuators are electrically connected between the source supply and the load with the amount of attenuation induced being of a fixed amount. The connected attenuator section can provide fixed attenuation, impedance matching or isolation between the source and the load. As a passive attenuator only has resistive elements within its design, the attenuated signal does not suffer from distortion or phase shift.
Passive attenuator designs can be either fixed, stepped or variable, with fixed attenuators being known as “pad attenuators” with commonly used attenuation networks ranging from 1dB to 20dB. The amount of attenuation presented by the attenuator pad is determined by the voltage ratio between the input source signal and the output load signal with this ratio being expressed in terms of decibels. The ratio between an input signal (Vin) and an output signal (Vout) is given in decibels as:
This voltage ratio can also be derived from the attenuation in decibels. A factor known as the “K-factor” can be used in the calculation of an attenuators resistive elements. As the “K”-factor corresponds to a given amount of attenuation in decibels, tables can be produced which gives the value of “K” as shown.
“K”-factor Tables for Attenuator Loss
A voltage attenuation of 6dB will be 10 (6/20) = 1.9953
Passive Attenuators can be symmetrical or asymmetrical in form, and can be either a balanced or unbalanced type. Common passive attenuator circuits include “T-type”, “Pi-type” and “Bridged-T” type as shown below.
L-pad Attenuator Design
The L-pad Attenuator is the simplest attenuator design consisting of only two resistive elements and is more commonly known as a voltage divider circuit. The L-pad attenuator is an unbalanced asymmetrical attenuator circuit that can only impedance match in one direction. The balanced equivalent of the “L-pad Attenuator” is called a U-pad attenuator.
T-pad Attenuator Design
The T-pad Attenuator is so called because its configuration resembles the letter “T”. The “T-pad Attenuator” is a symmetrical attenuator that can be used either for attenuation between equal impedances or impedance matching between unequal impedances. When the attenuation is high, the parallel shunt arm impedance becomes small. The balanced equivalent of the T-pad attenuator is called an H-pad attenuator.
Bridged-T Attenuator Design
The Bridged-T Attenuator is variation on the standard T-pad design which has an additional resistive element forming a bridged network across the two series resistors and gains its name from the fact the input sources signal appears to “bridge” itself across the T-pad network without affecting the characteristic impedance of the circuit.
The “Bridged-T Attenuator” is a symmetrical purely resistive attenuator that can conveniently be used as a variable attenuator or a switchable attenuator. It is also possible to construct a balanced version of the Bridged-T attenuator circuit.
Pi-pad Attenuator Design
The Pi-pad Attenuator is so called because its resistive configuration resembles that of the Greek letter “π” (pi). The “Pi-pad Attenuator” is the most common symmetrical passive attenuator that can be used between equal impedances or impedance matching between unequal impedances.
A single Pi-pad attenuator can achieve much higher levels of attenuation compared to the equivalent T-pad attenuator and when the attenuation is high, the series arm impedance in large. The balanced equivalent of the Pi-pad attenuator is called an O-pad attenuator.