Bridged-T Attenuator Calculator

Overview

The Bridged-T Attenuator is a resistive network topology used to reduce signal amplitude while maintaining a constant characteristic impedance ($Z_0$). As the name suggests, it is a variation of the standard T-pad attenuator, featuring an additional resistive element that "bridges" the two series resistors.

Why use a Bridged-T? Unlike standard Pi or T-pad attenuators that require changing three resistor values to adjust attenuation, the Bridged-T topology often allows for variable attenuation by adjusting just two resistors ($R_1$ and $R_2$), while the two series resistors remain fixed at the system impedance value ($Z_0$). This makes it ideal for building variable attenuators.

Calculator Formulas

Our calculator determines the values for the bridging resistor ($R_1$) and the shunt resistor ($R_2$) based on the required Attenuation ($A_{dB}$) and System Impedance ($Z_0$).

First, we calculate the voltage ratio (K-factor):

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

Then, we calculate the resistor values:

1. Bridge Resistor ($R_1$): $R_1 = Z_0 \times (K - 1)$

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

Where:

• $Z_0$: Characteristic impedance (typically $50\Omega$ or $75\Omega$).
• $A_{dB}$: Desired attenuation in decibels.
• Note: In a standard symmetrical Bridged-T, the two fixed series resistors are equal to $Z_0$.

Applications & Advantages

RF Signal Control Like all attenuators, the Bridged-T network is used to weaken signals from a transmitter to a level suitable for a receiver. This prevents circuit damage and saturation.

Key Advantages over Pi/T Networks:

1. Tunability: The primary advantage of the Bridged-T is its ability to vary attenuation by changing only two resistor values ($R_1$ and $R_2$) while maintaining perfect impedance matching. This is much simpler than Pi or T networks which require adjusting three components simultaneously.
2. Resistor Values: Both $R_1$ and $R_2$ can utilize a wide range of feasible resistor values. Unlike Pi attenuators, where resistance values can become impractical (too low) at high frequencies or specific attenuation levels, Bridged-T networks generally yield more realizable component values.
3. Impedance Matching: It maintains excellent impedance matching ($VSWR \approx 1:1$) even at high attenuation levels, ensuring maximum power transfer between the source and load.