Attenuator Pad
Understanding Attenuator Pads
Fixed attenuator pads are among the simplest and most commonly used RF components. They provide precise, repeatable attenuation independent of frequency (up to their rated bandwidth). Every RF lab has a collection of fixed pads for signal level adjustment and impedance padding.
Pad Topologies
- T-pad: Series-shunt-series resistor network. Equal input and output impedance.
- Pi-pad: Shunt-series-shunt resistor network. Equal input and output impedance.
- L-pad: Asymmetric network for matching between different impedances.
Applications
- Level setting: Reduce transmitter output to match receiver dynamic range.
- VSWR improvement: A 3-6 dB pad between a mismatched source and load improves the effective VSWR by twice the pad value.
- Isolation: Reduce interaction between cascaded components.
R1 = R3 = Z0 x (K-1)/(K+1)
R2 = Z0 x 2K/(K^2-1)
where K = 10^(dB/20)
3 dB pad: R1=R3 = 8.55, R2 = 141.9 ohms
6 dB pad: R1=R3 = 16.6, R2 = 66.9 ohms
10 dB pad: R1=R3 = 25.97, R2 = 35.14 ohms
20 dB pad: R1=R3 = 40.9, R2 = 10.1 ohms
Frequently Asked Questions
What is an attenuator pad?
An attenuator pad is a fixed resistive network that reduces signal power by a precise amount while maintaining impedance match. Common values are 3, 6, 10, and 20 dB. They use T-pad or Pi-pad resistor configurations.
Why use a pad instead of just a resistor?
A single resistor creates an impedance mismatch. A pad uses a specific resistor network (T or Pi) that maintains the correct impedance (50 or 75 ohms) at both ports while providing the desired attenuation. This ensures system VSWR is not degraded.
Does an attenuator pad reduce noise figure?
Adding an attenuator before a receiver degrades the system noise figure by the attenuation value. A 3 dB pad adds 3 dB to the system NF. However, in some cases the VSWR improvement from the pad reduces noise figure uncertainty, providing a net measurement benefit.