Attenuator Calculator
Design Pi and T pad RF attenuators. Enter attenuation and system impedance to get resistor values, or enter resistor values to find attenuation.
About the RF Attenuator Calculator
A resistive RF attenuator is a passive network that reduces signal power by a fixed, precise amount without causing impedance mismatch. Unlike a simple resistor voltage divider, a matched attenuator pad maintains the system impedance (typically 50 Ω) at both ports while achieving the specified attenuation — meaning the return loss at the input remains very high regardless of what is connected to the output.
Pi Pad vs T Pad
Both topologies achieve the same attenuation and impedance match but differ in component arrangement. The Pi pad (π) uses two shunt resistors and one series resistor — better suited for high-frequency applications because the shunt elements naturally absorb stray capacitance. The T pad uses two series resistors and one shunt resistor — better at lower frequencies and preferred in some balanced circuit configurations. For most RF work above 100 MHz, the Pi pad is the standard choice.
Attenuator Design Formulas
For a Pi pad with attenuation A dB in a Z₀ system, define K = 10^(A/20). Then R1 = R3 = Z₀(K+1)/(K−1) and R2 = Z₀(K²−1)/(2K). For a T pad: R1 = R3 = Z₀(K−1)/(K+1) and R2 = 2Z₀K/(K²−1). A matched pad designed for Z₀ has theoretically infinite return loss — the input reflection coefficient is zero when the output is terminated in Z₀.
Practical Applications
Fixed attenuator pads are used to set drive levels between stages, improve isolation between a source and a sensitive load, reduce standing waves on coaxial lines, and calibrate test equipment. A 10 dB pad between a signal generator and a DUT reduces the effect of generator impedance variations by 20 dB — a 10:1 improvement in source flatness. Step attenuators in VNAs and spectrum analysers use cascaded calibrated pads to extend dynamic range.