T-Pad Attenuator Calculator (Tee Attenuator)

Overview

The T-pad Attenuator (or Tee Attenuator) is a passive resistive network used to reduce signal power while maintaining impedance matching. As its name implies, the circuit topology resembles the letter "T".

Circuit Structure: A standard symmetrical T-pad consists of three resistors:

• Two Series Resistors ($R_1$): One connected to the input and one connected to the output.
• One Shunt Resistor ($R_2$): Connected between the two series resistors and ground.

Calculator Formulas

This calculator computes the required resistor values ($R_1$ and $R_2$) based on the desired attenuation ($A_{dB}$) and the system characteristic impedance ($Z_0$).

1. Calculate the K-factor: First, determine the voltage ratio ($K$) from the decibel value:

$K = 10^{\frac{A_{dB}}{20}}$

2. Calculate Resistor Values: Once $K$ is found, substitute it into the following equations:

• Series Resistors ($R_1$): $R_1 = Z_0 \times \frac{K - 1}{K + 1}$

• Shunt Resistor ($R_2$): $R_2 = Z_0 \times \frac{2K}{K^2 - 1}$

Where:

• $R_1$: The value for the two series resistors (Input and Output arms).
• $R_2$: The value for the central shunt resistor (Grounded arm).
• $Z_0$: System characteristic impedance (typically $50\Omega$ or $75\Omega$).
• $A_{dB}$: Desired attenuation in decibels (dB).

Applications

Why use a T-Pad? Attenuators are essential in RF (Radio Frequency) design for two main reasons:

1. Signal Level Control: They reduce the power delivered from a source to a load, preventing saturation or damage to sensitive receiver circuits.
2. Impedance Matching: They help isolate two circuit stages. By providing a fixed impedance ($Z_0$) looking into the attenuator, they minimize signal reflections (VSWR) between the source and the load.

Topology Advantage: The T-pad is favored for its simplicity. Compared to complex "Bridged-T" or "Balanced" networks, the T-pad is easier to manufacture, especially when etching thin-film circuits or laying out PCBs for high-frequency applications.