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Fixed Pi and Tee Attenuators - Equations

 

Fixed attenuators can be designed to have either equal or unequal impedances and to provide any amount of attenuation (theoretically) equal to or greater than the configuration's minimum attenuation - depending on the ratio of Z1/Z2. Attenuators with equal terminations have a minimum attenuation of 0 dB. Unequal terminations place a lower limit on the attenuation.

K = power in / power out, or in decibels = 10 * log (K) dB

 

Attenuator equation - RF Cafe, or in decibels

KMIN  = 10 * log (Kmin) dB

 

An online attenuator calculator is provided below.

Unbalanced Tee (T) Attenuator

These equations apply to the two forms of Tee attenuators at the left.

 

Attenuator equation - RF Cafe
If Z1 = Z2, then:

Attenuator equation - RF Cafe

Unbalanced T attenuator equation
Balanced Tee (T) Attenuator
 Balanced T attenuator equation - RF Cafe
Unbalanced Pi (p) Attenuator

These equations apply to the two forms of Pi attenuators at the left.

 

Attenuator equation - RF Cafe
If Z1 = Z2, then:

Attenuator equation - RF Cafe

 Unbalanced Pi attenuator equation - RF Cafe
Balanced Pi (p) Attenuator
 Balanced Pi attenuator equation - RF Cafe

Note: Only enter values in the yellow cells or risk overwriting formulas!

An RF Cafe visitor wrote to say that he thought the above equations might be in error when unequal source and load termination resistances are used. The image below shows the mathematical steps that prove the equations are correct. It uses a source resistance of 50 ohms and a load resistance of 100 ohms, with an attenuation of 10 dB. Resistor values for both the "T" and ""Pi" attenuators were determined using the attenuator calculator on RF Cafe (which uses these equations). 

Attenuators with unequal terminations - RF Cafe

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