RF Attenuators

An attenuator is a passive network that reduces signal power by a precise, frequency-independent amount. Unlike a simple resistive divider, a well-designed attenuator maintains its characteristic impedance at all ports across a wide frequency range. Attenuators are used to set signal levels, improve source or load impedance, isolate stages from reflections, and calibrate measurement systems.

Key Parameters

Parameter	Symbol	Typical values
Attenuation	A (dB)	1–40 dB in 1 dB steps for switched pads; fixed values for inline pads
Characteristic impedance	Z₀	50 Ω (RF/microwave), 75 Ω (cable TV/video)
Power rating	P_max (W)	0.1–50 W for most SMD/connectorised types
Frequency range	DC–f_max	DC to 18 GHz for SMA types; 40+ GHz for 2.92 mm types
VSWR		Typically < 1.2:1 across rated bandwidth

Pi (Π) Attenuator

The Pi pad uses two equal shunt resistors flanking a series resistor. It is the most common topology for RF PCBs. For attenuation A dB with system impedance Z₀:

Pi attenuator (shunt-series-shunt) and T attenuator (series-shunt-series). K = 10^(A/20).

\(K = 10^{A/20}, \quad R_{shunt} = Z_0\dfrac{K+1}{K-1}, \quad R_{series} = Z_0\dfrac{K^2-1}{2K}\)

T Attenuator

The T pad uses two equal series resistors with a shunt resistor to ground at the midpoint. Better suited for cable assemblies where both conductors need to be attenuated symmetrically:

\(R_{series} = Z_0\dfrac{K-1}{K+1}, \quad R_{shunt} = \dfrac{2Z_0 K}{K^2-1}\)

L-pad (Impedance-Matching Attenuator)

An L-pad matches two different source and load impedances while also providing attenuation. Unlike the Pi and T, the L-pad is inherently asymmetric. The attenuation is fixed once the impedances are set; it cannot be chosen independently. One series resistor and one shunt resistor are needed:

\(R_{series} = \sqrt{Z_S Z_L}\,\dfrac{\sqrt{Z_S/Z_L}-1}{1}, \quad R_{shunt} = \dfrac{Z_S Z_L}{R_{series}+Z_S}\)

(exact formula depends on which port is the high-Z side).

Bridged-T Attenuator

The Bridged-T uses the system impedance Z₀ as one of its elements, making it efficient for large attenuation values with fewer components. With a bridging resistor \(R_1 = Z_0(K-1)\) and a shunt resistor \(R_2 = Z_0/(K-1)\) plus two Z₀ arms.

Temperature and Power Dissipation

Each resistor in the attenuator dissipates a fraction of the incident power. For high-power attenuators, the power in each element must be calculated from the impedance equations. The series element in a Pi pad dissipates \((1 - 1/K^2)\) of the input power.

Standard Pi attenuator values. R_shunt = Z₀(K+1)/(K−1), R_series = Z₀(K²−1)/2K.

Practical Notes

Standard RF attenuator values: 1, 2, 3, 6, 10, 20, 30 dB.
Switched attenuators in LNAs and signal generators use PIN diode or MEMS switching to change attenuation state.
Surface-mount 0402/0603 resistors are used up to ~6 GHz; above that, thin-film chip attenuators are preferred.
Place bypass capacitors on the shunt resistors to maintain flat response into high frequencies.

🔧 Related Calculators

Attenuator Designer → dBm ↔ Watts Converter →