Many RF circuits for audio and video systems are designed for **75 ohm **impedance. This tool calculates resistor values to build an attenuator that matches this value of impedance.

To use this tool, enter the desired value of dB attenuation. Both input and output impedances are set to 75 ohm. It can however be changed to any arbitrary value.

The tool computes resistor values in two different configurations:

- Pi Attenuator
- T Attenuator

**Pi Attenuator Calculator**

**Formula**

**R _{1} = Z_{o} [10^{dB/20} + 1] / [10^{dB/20} – 1] **

**R _{2} = Z_{o}/2 [10^{dB/10} – 1] / [10^{dB/20}]**

where **R1** is the shunt resistor (to ground) and **R2** is the series resistor.

**Z _{o}**

**= 75 Ω**

To build this attenuator you will need

**two shunt resistors**– one at the input and the other at the output**one series resistor**

**T Attenuator Calculator**

**Formula**

**R _{1} = Z_{o} [10^{dB/20} – 1] / [10^{dB/20} + 1] **

**R _{2} = (2*Z_{o})*[10^{dB/20}] / [10^{dB/10} – 1]**

where **R1** is the series resistor and **R2** is the shunt resistor (to ground).

**Z _{o}**

**= 75 Ω**

To build this attenuator you will need

**two series resistors**– one at the input and the other at the output**one shunt resistor**

If the characteristic impedance of the attenuator in a 75 ohm system is not the same, then there will be mismatch loss.