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?

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. 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.

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.

What Are The Available Types & Technologies Of RF Attenuators

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 also for phased array antennas for signal level control.

RF Coaxial Attenuators

RF Coaxial Attenuators are a subset of RF attenuators where the input and output connector interfaces are coaxial connector types. The most common of these are N-type coaxial attenuators, though SMA, 1.85mm, 2.4mm, 2.92mm, 4.3-10, 7/16 DIN, 7mm, BNC, QMA, QN, and TNC are also common. Coaxial attenuators are available from 3dB to 110 dB attenuation value. These attenuators may be continuously variable, DC bias, fixed, programmable, step, or voltage variable attenuator types. Depending on the coaxial connector type, there are RF coaxial attenuators available that operate from DC to 65 GHz (max frequency for 1.85mm coaxial connectors).

Common attenuation values for 50 Ohm Fixed Coaxial Attenuators range from 3dB to 60 dB. Typically, coaxial attenuators with higher attenuation values and higher power handling are also equipped with built in thermal management technology. This is typically a pre-installed aluminum heat-sink. Some High Power RF Fixed Coaxial Attenuators may have extremely large heatsink that significantly add to the size and weight of the RF attenuator. In extreme cases, forced air and other active thermal management may be used to further  enhance the power handling capability of a high power RF attenuator. RF coaxial attenuators are available with power handling capability ranging from 0.5 Watt to 1000 Watts.

The frequency range of operation for an RF coaxial attenuator is either limited by the design elements within the RF attenuator or the coaxial connector type. The power handling capability may be limited by either the internal design elements of the RF attenuator or also by the coaxial connector type. As there is a relationship between the size of a RF coaxial connector type power handling capability and true transverse electromagnetic mode (TEM) upper frequency limit, higher power handling capability RF coaxial attenuators also tend to use larger size coaxial connector standards, such as N-type, 7/16 DIN and 7mm.

For test and measurement applications, SMA, 1.85mm, 2.4mm, 2.92mm, 3.5mm, and BNC coaxial connector attenuators are the most common 50 Ohm type attenuators. Communications applications most often use 4.3-10, BNC, 7/16-DIN, 7mm, and N-type connectors for 50 Ohm applications. Broadcast and other 75 Ohm applications most often use F, BNC, or N-type coaxial attenuators.

DC bias attenuators are typically coaxial connector type attenuators. These attenuators are also known as DC passing attenuators, as they allow for DC bias energy to be directed to an external port, as opposed to purely just blocking the DC bias energy. There are also DC blocking attenuators that simply prevent a DC connection between the input of the attenuator and the output.

Variable RF Attenuators

It is sometimes the case where an application requires a continuously varying attenuation scale instead of the typical switched attenuator scale that is common for programmable attenuators. In these cases Variable Attenuators are extremely useful as these devices exhibit a continuously variable attenuation level in response to an analog input voltage. Though there are other control types for continuously variable attenuators, the most common is an analog input voltage. This voltage control is enabled by the use of internal circuitry that is able to change the attenuation value based on an input voltage, such as with a PIN diode.

Though continuously variable attenuators are not as accurate as other forms of attenuators, such as switched attenuators, there are some circumstances where only an analog voltage is available for control, which requires additional circuitry to incorporate programmable attenuators. Also, a variable attenuator with continuous voltage control can be readily integrated into a feedback loop for controlling output voltage level, or other applications with similar requirements. Among the many uses for voltage variable attenuators (VVAs) are automatic gain control, calibration correction, and other feedback or process-based functions that require continuous and precise control of the attenuation level.

VVAs are generally designed using diode or transistor elements that are driven in their nonlinear resistance region. Using input control voltage results in the solid-state devices sliding along the nonlinear resistance curve, hence VVAs typically have circuitry that helps to linearize the variable attenuation resulting from this. However, some VVAs do not have linearizing circuitry, and these are known as non-linearized VVAs.

For enhanced control and performance, variable attenuator circuits are often in a pi-type or t-type configuration. In the case of variable attenuators, the resistors in the pi-type and t-type configurations are replaced with the solid-state elements, of which metal semiconductor field effect transistors (MESFETs). In this case, controlling the voltage across the MESFET adjusts the RF resistance, while controlling the current through the PIN diode allows for similar RF resistance control.

It is important in the design of a voltage variable attenuator to ensure that the input and output impedance are consistent while the attenuation changes, otherwise there will be mismatch issues while the attenuation is adjusted. This is the purpose of the pi-type or t-type configurations, and the need to ensure the resistors are synced during variable adjustment. Like with other attenuators, variable attenuators can be configured to be absorptive or reflective.