Attenuator Pad Calculator

Pad topology:

Choose the network shape that fits the matched signal path.

Pi pad T pad

Target attenuation:

{{ formatNumber(Number(attenuation_db), 1) }} dB

Choose 0.1–60 dB; type an exact value or tune the slider.

Exact dB

System impedance:

Select a common matched environment or enter a custom equal port impedance.

Custom impedance:

Equal source and load reference impedance.

ohm

Build values:

The audit recalculates attenuation and input match after preferred-value rounding.

Input power:

Choose the unit that matches the source specification.

Power derating:

50% means select at least twice the modeled dissipation.

Sweep minimum:

Start the sweep from 0.5 to 59 dB.

Sweep maximum:

End the sweep above its minimum and no higher than 60 dB.

Sweep step:

Use 0.5–5 dB increments.

Display precision:

Choose a compact or detailed presentation.

Part	Exact	Build	Difference	Role	Copy
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Build decision

Model boundary

Ideal lumped resistors, equal real port impedances, and a matched load are assumed. At RF, verify resistor tolerance, package parasitics, grounding, layout, and thermal coupling with measurement or simulation.

Formula basis

The voltage ratio is K = 10^A/20. Exact Pi and T values enforce the selected loss while presenting the selected impedance at both terminated ports.

A resistive attenuator reduces signal level while presenting a chosen impedance to both ends of a matched path. That makes it useful between RF instruments, receivers, transmitters, video systems, and other impedance-controlled stages where simply adding one series resistor would disturb the source or load match.

Attenuation in decibels describes a ratio, not a fixed number of volts or watts. A 6 dB pad reduces voltage by about 2:1 and power by about 4:1 in a matched system. The signal power that does not reach the load becomes heat in the pad resistors, so electrical rating is part of the design rather than a separate afterthought.

Fixed attenuator pad between equal source and load impedances, with one series path and one shunt path dissipating signal power.

Pi and T pads are two symmetric three-resistor forms of the same matched-network problem. Their ideal values differ, but both assume the source reference impedance equals the load reference impedance.

Pi and T attenuator topology comparison
Topology	Resistor arrangement	Practical choice
Pi	Input shunt, one series resistor, output shunt.	Convenient when shunt connections fit the physical layout.
T	Input series, center shunt, output series.	Convenient when the signal path favors series parts and one grounded branch.

Exact calculated resistance is rarely the final build value. Standard E-series parts round each resistor to an available nominal value, which changes achieved attenuation and input match. A finer series usually reduces that shift, but component tolerance, board geometry, connectors, and parasitic reactance still matter at high frequency.

Low-loss pads can demand very small series resistance or very large shunt resistance, making lead and trace resistance significant. High attenuation concentrates most input power as heat and may be easier to build as several lower-loss sections. Neither observation creates a universal cutoff; frequency, power, bandwidth, package, and acceptable mismatch decide what is practical.

A calculated pad is a first-pass lumped-resistor design. Confirm the real network with suitable impedance and insertion-loss measurements, and use resistor ratings that remain safe at the operating temperature and duty cycle.

How to Use This Tool:

Begin with the attenuation and equal port impedance required by the real signal path, then audit parts and power separately.

Choose Pi pad or T pad and enter Target attenuation from 0.1 to 60 dB.
Select System impedance for 50, 75, or 600 ohms, or enter a custom equal impedance from 1 to 10,000 ohms.
Choose Build values. Exact values preserve the ideal equations; E12, E24, and E96 round each resistor to the nearest preferred nominal value.
Enter Input power in dBm, dBW, W, or mW and set the intended Power derating. A 50% setting recommends at least twice the modeled hottest-part dissipation.
Compare Component values and Power audit. Check achieved attenuation, input match, each resistor's heat, and the minimum hottest-part rating before using the sweep as a design reference.

Interpreting Results:

Modeled build attenuation is recalculated from the selected build resistors, so it can differ from the target when an E series is used. The input reflection coefficient reports how far the rounded network's input impedance moves from the chosen reference impedance.

Compare exact and build values before ordering. A small percentage change in a sensitive low-loss pad can produce a meaningful attenuation or match error.
Use the hottest resistor, not total pad dissipation, to choose a minimum part rating. Round upward to an available rating and check the manufacturer's temperature derating.
A reflection coefficient above 1% triggers a suggestion to use a finer series or hand-selected values; above 5% also receives a warning badge. These are design-review cues, not universal RF acceptance limits.
The resistor sweep shows ideal values across attenuation settings. It does not re-evaluate preferred-value error or frequency response at every point.

Technical Details:

The equations use a symmetric, purely resistive pad between equal source and load reference impedances. The voltage ratio K comes from the requested attenuation in decibels; the corresponding power ratio is its square.

Formula Core

\begin{aligned} K & = 10^{\frac{A_{dB}}{20}} \\ Power ratio & = 10^{\frac{A_{dB}}{10}} \end{aligned}

For a T pad, the two series resistors are equal and the center resistor is shunt:

\begin{aligned} R_{series} & = Z_{0} \times \frac{K - 1}{K + 1} \\ R_{shunt} & = \frac{2 Z_{0} K}{K^{2} - 1} \end{aligned}

For a Pi pad, the two shunt resistors are equal and one resistor joins the ports:

\begin{aligned} R_{shunt} & = Z_{0} \times \frac{K + 1}{K - 1} \\ R_{series} & = Z_{0} \times \frac{K^{2} - 1}{2 K} \end{aligned}

Z₀ is the equal source and load reference impedance. Exact values are shown without manufacturing rounding; standard values are selected from the nearest E12, E24, or E96 number across adjacent decades.

Transformation Core

After preferred-value selection, the resistor network is solved again. Achieved attenuation comes from the ratio between matched reference output power and modeled output power. Input mismatch is expressed as the magnitude of the reflection coefficient:

Γ = | \frac{Z_{in} - Z_{0}}{Z_{in} + Z_{0}} |

Input power converts to watts as 10^{(dBm − 30)/10} for dBm, 10^dBW/10 for dBW, or mW ÷ 1,000. Each resistor's dissipation is calculated from its modeled voltage drop. Recommended rating equals the hottest resistor's watts divided by the selected derating fraction.

The sweep accepts 0.5 to 59 dB as its minimum, a larger maximum no higher than 60 dB, and steps from 0.5 to 5 dB. The main design accepts 0.1 to 60 dB and positive input power up to 1 GW after unit conversion; the broad upper bound is a validation limit, not an assurance that a passive pad is physically suitable.

Accuracy Notes:

The model omits resistor tolerance around the chosen nominal value, parasitic inductance and capacitance, trace and connector loss, self-heating changes, pulse-energy limits, thermal coupling, source/load mismatch, and frequency-dependent substrate effects. Verify the assembled network across its real frequency and power range. For more than 20 dB, compare one section with cascaded sections; above 30 dB, several lower-loss sections are often more practical.

Worked Examples:

10 dB T pad in a 75 ohm path

The ideal T values are about 38.962 ohms for each series resistor and 52.705 ohms for the center shunt. Nearest E24 values become 39, 51, and 39 ohms. Re-solving that build gives about 10.154 dB attenuation and 74.236 ohms input impedance. At 1 W input with 50% derating, the hottest resistor dissipates about 0.525 W, so the modeled minimum rating is about 1.051 W before rounding up to an available part.

References:

Using the HMC199MS8 as a Low Cost 1-Bit Attenuator, Analog Devices.
RF Demystified—What Is an RF Attenuator?, Analog Devices.
IEC 60063:2015 Preferred number series for resistors and capacitors, International Electrotechnical Commission, March 27, 2015.
NIST Guide to the SI, Chapter 8, National Institute of Standards and Technology.