Attenuator Calculator & RF Guide
Calculate resistor values for Pi, T, and Bridged-T attenuator circuits.
Select Attenuator Type
Pi (Π) Attenuator
An attenuator is a passive electronic device that reduces the power of a signal without appreciably distorting its waveform. It is essentially a network of resistors designed to lower the signal level by a specific amount, typically measured in decibels (dB). They are crucial in radio frequency (RF) and audio applications for matching impedances, preventing signal overload, and extending the dynamic range of measuring equipment.
The calculations are based on standard formulas. Let A be attenuation in dB and Z₀ be system impedance.
Pi (Π) Formula:
R1 = Z₀ * (10^(A/20) + 1) / (10^(A/20) - 1)
R2 = (Z₀/2) * (10^(A/10) - 1) / 10^(A/20)
T Formula:
R1 = Z₀ * (10^(A/20) - 1) / (10^(A/20) + 1)
R2 = 2 * Z₀ * 10^(A/20) / (10^(A/10) - 1)
Bridged-T Formula:
R1 = Z₀
R2 = Z₀ * (10^(A/20) - 1)
R3 = Z₀ / (10^(A/20) - 1)
Common Attenuator Topologies
Pi (Π) Attenuator
Named for its resemblance to the Greek letter Π, this configuration has one series resistor and two parallel shunt resistors. It's an unbalanced circuit commonly used for impedance matching between unequal source and load impedances.
T Attenuator
The T-pad consists of two series resistors and one parallel shunt resistor to ground. Like the Pi attenuator, it is unbalanced. It is frequently used in RF applications and provides a good general-purpose attenuation solution.
Bridged-T Attenuator
This is a modified T-pad that maintains a constant impedance across a wide range of frequencies, making it ideal for variable attenuators. It uses four resistors and offers excellent performance in high-frequency applications.
Frequently Asked Questions
Decibels (dB) are a logarithmic unit used to express the ratio of two values of a physical quantity, often power or intensity. A 3dB attenuation means the signal power is halved, while a 10dB attenuation reduces the power to one-tenth of its original value. It's a convenient way to represent very large or very small ratios in a simple number.
System impedance is the characteristic impedance of a transmission line or system. For an attenuator to work correctly without causing signal reflections, its impedance must match that of the source and the load. Common values are 50 Ohms for most RF systems and 75 Ohms for video and cable TV systems.
The choice depends on the application. Pi attenuators are often preferred for impedance matching between different impedances. T attenuators are great general-purpose circuits. Bridged-T attenuators are best for applications requiring a constant impedance over a variable attenuation range, such as in test equipment.
