Pi Attenuator Calculator

What Is a Pi Attenuator?

A pi attenuator (pi pad) is a three-resistor network shaped like the Greek letter Pi. It reduces signal power between a source and load while maintaining impedance matching. RF engineers use pi pads to control amplitude, isolate circuit stages, and match unequal impedances.

Pi Attenuator Formulas

For equal source and load impedance $Z_0$:

$$K = 10^{\frac{\text{atten}}{20}}$$ $$R_1 = Z_0\left(\frac{K+1}{K-1}\right)$$ $$R_2 = Z_0\left(\frac{K^2-1}{2K}\right)$$

Example: a 40 dB pad on 50 Ω gives $K = 100$, so $R_1 \approx 51\,\Omega$ and $R_2 = 2500\,\Omega$.

For unequal impedances, separate formulas apply for input shunt $R_1$, series $R_2$, and output shunt $R_3$ using source impedance $Z_S$ and load impedance $Z_L$.

Frequently Asked Questions

When should I use equal vs unequal impedance?

Use equal impedance when source and load are the same (e.g. 50 Ω to 50 Ω). Use unequal when you need impedance matching between different values, such as 75 Ω to 50 Ω.

What does the K factor represent?

K is the voltage ratio corresponding to the attenuation in decibels. For 40 dB, K equals 100 because $10^{40/20} = 100$.

Are R1 and R3 the same in equal-impedance pads?

Yes. In the equal-impedance topology, both shunt resistors share the same value and connect to ground at the input and output.

Can a pi pad provide gain?

No. Attenuators only reduce signal level. They cannot amplify. Negative dB values are not meaningful for passive pi pads.

What applications use pi attenuators?

Signal generators, RF test benches, impedance matching between stages, and isolation between sensitive circuit blocks.

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What Is a Pi Attenuator?

Pi Attenuator Formulas

Frequently Asked Questions

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