# 50 Ohm Attenuator Calculator

Most RF circuits are designed for 50 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 impedance is set to a default value of 50 Ω. It can however be changed to any arbitrary value.

The tool computes resistor values in two different configurations:

• Pi Attenuator
• T Attenuator

It also includes tables of values for both types of attenuators.

## Pi Attenuator Formula

R1 = Zo [10dB/20 + 1] / [10dB/20 – 1]

R2 = Zo/2 [10dB/10 – 1] / [10dB/20]

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

Zo = 50 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 Formula

R1 = Zo [10dB/20 – 1] / [10dB/20 + 1]

R2 = (2*Zo)*[10dB/20] / [10dB/10 – 1]

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

Zo = 50 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 50 ohm system is not the same, then there will be mismatch loss.

## Background

Resistive attenuator pad types are commonly used in RF circuit design to reduce the signal strength between the input and output of a circuit. An attenuator is a device that reduces the amplitude of a signal, and in the case of resistive attenuator pads, this is achieved by using resistors.

There are several types of resistive attenuator pads, including Pi and Tee. These attenuator circuits are designed to match the input and output impedance of the system, ensuring efficient signal transfer.

The characteristic impedance of the resistive attenuator pad is determined by the resistance value used in the circuit. By selecting the appropriate resistance, the desired attenuation level can be achieved.

Tee attenuators shaped like a T consist of two resistors connected in series between the input and output, with a third resistor connected between the junction of the first two resistors and ground.

Pi attenuators shaped like Π, consist of two resistors connected in parallel between the input and output, with a third resistor connected between the junction of the first two resistors.

Purely resistive attenuators, as the name suggests, use only resistors in the circuit to achieve the desired level of attenuation.

## Pi Attenuator Table

Below is a table of resistor values for attenuation values from 1dB to 50 dB. The impedance value is 50 ohm.