Attenuator Calculator (Pi & T Pad)

How it works

K = 10^(dB ÷ 20) · Pi pad: R_shunt = Z₀ (K + 1)/(K − 1), R_series = Z₀ (K² − 1)/(2K) · T pad: R_series = Z₀ (K − 1)/(K + 1), R_shunt = Z₀ × 2K/(K² − 1)

A matched resistive attenuator reduces signal level by a fixed number of decibels while presenting the system impedance Z₀ at both ports, so it also improves the match between mismatched stages. Pi and T pads with the values above are electrically equivalent; the Pi pad is easier to build with standard resistors at high attenuation, the T pad at low attenuation. Make sure the first resistor can dissipate most of the input power, and use non-inductive (thin-film or SMD) resistors at RF.

Worked example

A 10 dB Pi pad for a 50 Ω system: K = 10^(10/20) = 3.162, so the two shunt resistors are 50 × 4.162 ÷ 2.162 = 96.25 Ω and the series resistor is 50 × 9 ÷ 6.325 = 71.15 Ω (E24: 100 Ω and 68 Ω). 1 W in comes out as 100 mW. The T version would use 25.97 Ω series and 35.14 Ω shunt resistors.

Sources: Wikipedia — Attenuator (electronics): Pi and T pad formulas · Wikipedia — Pi pad

Frequently asked questions