Pi Attenuator Calculator

Enter the Impedance and required attenuation level to get values for R1 and R2

  • dB
  • Ω

Result

  • R1
    Ω
  • R2
    Ω
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What is Pi attenuator?

Pi type attenuator is a type of passive attenuator that uses a resistive network circuit that replicates the Greek letter "Π" (Pi) i.e., it has one series resistor and two parallel shunt resistors, which are connected to the ground.

An attenuator is a device used to reduce the power of the source without distorting its signal waveform. It is connected in series with the signal source and load. In order to work efficiently the attenuator must be matched with the source’s and signal’s impedances. For example, the attenuator is used to lower the amplitude of the signal to the desired value to enable measurements or to protect the measuring device from signal levels that might damage it. The attenuator is also called a pad. The pi type attenuators are ideal for use in radio, electronic circuits, communication, and microwave transmission line applications to weaken a stronger signal and to provide impedance matching.

The pi type attenuator circuit is an unbalanced circuit. In an unbalanced circuit/configuration, the impedance of two transmission lines (up & bottom) does not have equal impedance with respect to the ground, also the bottom line is grounded/earthed. The unbalanced attenuator is suitable to connect with an unbalanced transmission line, such as a coaxial cable that feeds the load.

Pi type unbalanced symmetrical configuration (both shunt resistors are equal value)
Pi type unbalanced symmetrical configuration (both shunt resistors are equal value)
Pi type unbalanced symmetrical configuration (both shunt resistors are equal value)
Pi type unbalanced symmetrical configuration (both shunt resistors are equal value)

However, the pi attenuator can be converted to a balanced circuit by placing half the series resistance in both top line & bottom line (return path). Such a balanced circuit has two transmission lines with equal impedances along their lengths with respect to ground. The balanced attenuator is suitable to connect with a balanced transmission line, such as a twisted-pair cable that feeds the load.

Symmetrical Pi Attenuator:

As we know, the pi type attenuator is an unbalanced circuit, and it acts as a symmetrical attenuator when both the shunt resistor's values are equal to each other (i.e., R2 =R2). The symmetrical attenuator has the same value of the characteristic impedance (Zo = V/I) at the input, and output sides, allowing for its input and output terminals to be transposed with each other.

Unbalanced Symmetrical pi attenuator connected between source and load (Zo (i.e., ZIN = Zout) = ZS = ZL)
Unbalanced Symmetrical pi attenuator connected between source and load (Zo (i.e., ZIN = Zout) = ZS = ZL)

The symmetrical pi type attenuator is ideal to connect between a signal source and load to reduce the signal's power when the source impedance (ZS) and load impedance (ZS) is equal (i.e., ZS = ZL = Zo).

Unsymmetrical pi attenuator:

The unbalanced pi type attenuator act as an unsymmetrical attenuator when both the shunt resistors’ values are unequal to each other (i.e., R2≠ R3). The unsymmetrical pi type attenuator is ideal for insertion between the source and load impedance (ZS ≠ ZL) to reduce the signal power level as well as for providing impedance matching between the source and load.

Unsymmetrical pi attenuator
Unsymmetrical pi attenuator

Note: Balanced pi attenuator (also called “O” pad) is a symmetrical pi attenuator. Thus, balanced pi type attenuator is also suitable to connect between a signal source and load to reduce the signal power when the source impedance (ZS) and load impedance (ZL) are equal (i.e., ZS = ZL = Zo). Here, Zo represents the characteristic impedance of the balanced pi attenuator.

What is Pi attenuator calculator?

How to calculate the resistance value of Pi type unbalanced symmetrical attenuator?

This calculator is used to calculate the value of resistor R1 & R2 of pi type symmetrical attenuator (shown below) for the required value of attenuation (in dB) and for matching impedance (Zo = Zs =ZL Ω). Here, Zo is the characteristic impedance of pi type symmetrical attenuator, which is equal to the value of source (Zs) and load impedance (ZL).

Unbalanced symmetrical pi attenuator (shunt resistors equal value R1=R1)
Unbalanced symmetrical pi attenuator (shunt resistors equal value R1=R1)

The following formula is used to calculate the value of resistor R1 & R2 of pi type symmetrical attenuator (shown above) for the desired value of attenuation (dB) and matching impedance/characteristic impedance (Zo Ω).

  • Where:
  • R1 (Ω) = Value of shunt resistor in the Pi attenuator
  • R2 (Ω) = Value of series resistor in the Pi attenuator
  • Zo = Characteristic impedance of the pi type attenuator (shown above) in Ω
  • dB = Required attenuation level in dB (i.e., decibels)
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