Attenuator Calculator

Tee, Pi, bridged-T, and reflection pads

Operating Mode

Required Parameters

System impedance

Ohm

Attenuation

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Quick Answer

An RF attenuator reduces signal power by a precise dB amount while maintaining impedance matching. For a T-pad in 50 ohm: R_series = Z0 x (K-1)/(K+1), R_shunt = 2Z0K/(K^2-1), where K = 10^(dB/20).

Documentation

RF Attenuator Calculator

Design T-pad, Pi-pad, and Bridged-T resistive attenuators for RF and audio applications. Calculate resistor values for a given attenuation and characteristic impedance.

T-Pad Attenuator

For impedance Z0 and attenuation A (dB):

R1 = Z0 x (K - 1) / (K + 1) where K = 10^(A/20)
R2 = 2 x Z0 x K / (K^2 - 1)

Pi-Pad Attenuator

The dual of the T-pad, preferred when the shunt elements need grounding.

Design Tips

Use 1% tolerance resistors for accurate attenuation
Consider power handling for each resistor element
Cascading attenuators adds their dB values

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Design Notes

Resistive attenuators are frequency-independent in theory, but parasitic inductance and capacitance of physical resistors limit the usable frequency range. Through-hole pads work to about 100 MHz; SMD 0402 pads extend to 6 GHz. For broadband precision, use thin-film attenuator chips rated for your frequency range. Always verify return loss (>20 dB) and ensure each resistor can handle its share of the dissipated power.

Common Mistakes

1
Using a simple voltage divider instead of a matched-impedance pad — this causes reflections and VSWR problems.
2
Ignoring resistor power dissipation — at high attenuation, nearly all input power is dissipated as heat in the resistors.
3
Using wirewound resistors at RF frequencies — parasitic inductance destroys the flat frequency response above a few MHz.

Engineering Handbox

1. Calculate K = 10^(6/20) = 10^0.3 = 1.995\n2. R_series = 50 x (1.995-1)/(1.995+1) = 50 x 0.332 = 16.6 ohm\n3. R_shunt = 2 x 50 x 1.995 / (1.995^2 - 1) = 199.5 / 2.98 = 66.9 ohm

Verification6 dB T-pad: two 16.6 ohm series resistors and one 66.9 ohm shunt resistor. Use 16.5 and 68 ohm standard E96 values.

Knowledge Base

What is a Pi attenuator used for?

Pi attenuators reduce signal power while maintaining the characteristic impedance (50 or 75 ohms) of the transmission line. They prevent reflections and VSWR problems by presenting a matched impedance to both the source and load. Common uses: protecting sensitive mixer/detector inputs, setting test equipment levels, and improving amplifier stability.

How do I choose between T and Pi pads?

T-pads have two series resistors and one shunt — better when you need lower shunt capacitance for high-frequency work. Pi-pads have two shunt resistors and one series — easier to implement on PCBs and offer better ground-plane connection. At frequencies below 1 GHz, the difference is minimal. Choose based on PCB layout convenience.

What is the formula for a T-pad attenuator?

For a symmetrical T-pad: R_series = Z0 × (K-1)/(K+1), R_shunt = 2 × Z0 × K / (K²-1), where K = 10^(dB/20) and Z0 is the system impedance. Example: 6 dB in 50Ω gives K = 2.0, R_series = 16.6Ω, R_shunt = 66.9Ω.

What is a Bridged-T attenuator?

A Bridged-T attenuator adds a bridge resistor across the series path, allowing continuous attenuation adjustment with a potentiometer. It maintains impedance matching across the adjustment range. Useful for variable attenuator designs in test equipment.

What resistor tolerance should I use for attenuators?

For lab/test use: 1% (E96) metal film resistors minimum. For production RF: 0.1% precision resistors for attenuators above 20 dB. At high attenuation, small resistor errors cause large dB errors.

What is the maximum frequency for resistive attenuators?

Standard through-hole resistive pads work to about 100 MHz. SMD 0402/0201 pads extend to 6 GHz+. Above 6 GHz, use specialized thin-film attenuator chips (e.g., Mini-Circuits) that control parasitic capacitance and inductance.

What is a reflection attenuator?

Unlike absorptive T/Pi pads that dissipate power as heat, reflection attenuators use mismatched impedances to reflect power back to the source. Simpler design but cause VSWR issues on the reflected port. Used mainly in specific RF test setups.

How do I cascade multiple attenuators?

Simply add the dB values. A 3 dB pad followed by a 6 dB pad gives 9 dB total attenuation. However, ensure each individual pad matches the system impedance — cascading mismatched pads causes cumulative reflection errors.

What power rating do attenuator resistors need?

Total power dissipated equals the input power minus the output power. For a 10 dB pad with 1W input: output = 0.1W, so resistors dissipate 0.9W total. Distribute this across the T or Pi resistors proportionally and choose wattage ratings with at least 2× safety margin.

What is return loss in an attenuator?

Return loss measures how well the attenuator matches the system impedance. A perfect 50Ω match has infinite return loss. Practical attenuators should achieve >20 dB return loss (VSWR < 1.22). Higher attenuation pads inherently have better return loss because the attenuated reflections from the load are further reduced.

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