RF and Microwave Attenuator Fundamentals

RF attenuators are fundamental components of RF and Microwave circuits and systems. Often found in virtually every RF application, attenuators play a vital role in receivers, transmitters, test and measurement systems etc. The main purpose of attenuators is to reduce the signal strength, such as prior to the signal reaching a sensitive circuit element. Attenuators can be fabricated using various technologies and knowledge of the available options can help an engineer choose the best attenuator for their application. One of the most common types of RF attenuators, are RF coaxial attenuators, which are used inline to RF coaxial systems to reduce the signal level at the output of the attenuator.

What Are Attenuators?

RF attenuators simply decrease the wanted or unwanted signal strength along a signal path. They can be used to decrease the output signal of a device-under-test before a sensitive test and measurement receiver, to ensure a more conformal impedance match, or to ensure precise control of the signal amplitude at the output of a transmitter. The attenuation level of a device—the amount of signal power/voltage lost through the device—is commonly measured in either decibels (dB) or as a voltage ratio.

RF attenuators are often placed at the output and input of reflective components/devices, such as some filters, mixers, etc. In this way the reflected voltage doesn’t build to a standing wave and instead decays rapidly. Other use cases for RF attenuators include using attenuator values in calibration steps with signal sources to ensure that a precise signal level is achieved at the output of a test system.

As RF attenuators help to dampen the significance of reflections between poorly matched ports, RF attenuators are often used to provide some level of impedance matching between disparate port impedance. This isn’t a perfect match scenario, but RF attenuators do alleviate some of the issues associated with connecting unmatched ports, such as causing more rapid decay of a standing wave and preventing a standing wave developing.

What Types and Technologies of Attenuators are Available?

The most common attenuators are broadband attenuators. But some attenuator types and technologies may have frequency dependent performance and limitations. Though terminations also reduce the signal strength at the load of a system, attenuators differ from terminations as they are in-line to the signal path.

Attenuators are based on passive resistors, absorptive material/techniques, PIN diodes, or field-effect transistor (FET) technologies. Additionally, attenuators can be developed from coaxial transmission line, stripline, surface mount, or even waveguide interconnect technologies. The performance and physical properties of these different technologies vary widely. The quality of construction and costs also contribute to the range in performance, thermal, and physical properties.

Attenuators also come as fixed attenuators or adjustable types. The adjustable types of attenuators can be switched attenuators with discrete levels of attenuation, or as continuously variable attenuators with analog adjustment. Both types can be designed with electrical or mechanical control. Some attenuators are programmatically controlled through digital signals and even software.

Common RF Attenuator Types

DC Bias Attenuators
Fixed Attenuators
Programmable Attenuators
Step Attenuators
Variable Attenuators
Voltage Variable Attenuators
Waveguide Direct Read Attenuators
Waveguide Variable Attenuator

Attenuators input and output impedance can vary depending on which application they are designed for. This could be a common 50Ω, 75Ω, or a custom impedance value for coaxial connector type attenuators or surface mount technology (SMT) attenuators. Also, some attenuator designs enable DC bias passing, and are known as DC bias passing attenuators.

Additionally, depending upon the attenuator technology, an attenuator may be reflective or non-reflective. A reflective attenuator reflects the attenuated signal energy, instead of absorbing it. The amount of signal energy reflected is a function of the attenuation level.

Key RF Attenuator Performance Specifications

• Frequency range [Hz]
• VSWR [ratio]
• Insertion loss [dB] *lowest attenuation value
• Attenuation value(s) [dB]
• Impedance [Ohms]
• Velocity of propagation [%c]
• Power handling (CW, peak) [Watts]
• DC bias voltage [V]
• DC bias current [A]
• DC bias power [Watts]
• DC Bias resistance [Ohms]
• Attenuation accuracy [+/-dB]
• Attenuation step size [dB]
• Switching speed [ms, us, ns etc.]
• Connector type 1 and 2
• Temperature range of operation [deg C]

RF attenuators are sometimes arranged into a larger matrix. These are called matrix attenuators. Often these matrix attenuators have methods for controlling the input paths to the attenuators and output paths from the attenuators. This is effectively a combination of a switch matrix with an attenuator matrix. These types of attenuator arrangements are commonly used for handover test systems for wireless system test, and for phased array antennas for signal level control.