What is an RF Attenuator?

An RF attenuator is a component used to reduce signal power in a controlled manner, measured in decibels (dB). This reduction is typically achieved by converting part of the RF signal into heat through resistive materials. The typical application of attenuators is to protect sensitive components in a system, improve the signal-to-noise ratio, and adjust signal levels for optimal performance.

Types of Attenuators

RF Attenuators in large are either fixed or variable. Each type of attenuator then has different subcategories defined by their design and use. They can be connectorized, surface mounted or used in waveguide systems.
Main considerations for choosing your attenuator:

• Frequency Range
• VSWR
• Insertion Loss
• Power Capability
• Connectivity
• Attenuation Range and Flatness

Let’s look at the common types of attenuators

Fixed Attenuators

Fixed attenuators, as their name suggests, are fixed or unchanging. These are used in applications that don’t require changing levels of attenuation or where an occasional replacement of one attenuator with another is acceptable. Fixed attenuators are typically designed using either resistive networks or resistive/absorptive materials.

Chip Attenuators

Chip attenuators are designed using resistor networks. They can be used at very high frequencies depending on their design and the materials used. Resistive network designs are either balanced and symmetrical or unbalanced and asymmetrical. Balanced resistor networks provide equal resistance on input and output, reducing RF noise and ensuring better signal quality and stability. Unbalanced resistor networks are simpler and easier to design. They are more cost-effective and versatile due to their variety of configurations (see L, T and Pi configurations).

Coaxial Fixed Attenuators

Connectorized coaxial fixed attenuators are typically designed using resistive discs or rods that, depending on the power rating, have a heatsink attached to remove the absorbed heat as the signal travels through the attenuator and is being attenuated. Most fixed attenuators are available in standard increments, e.g. 3dB, 6dB, 10dB, 20dB, etc. and can cover a wide frequency range, e.g. DC-18GHz, DC-40GHz, etc. These fixed attenuators can be either directional or bidirectional. Bidirectional attenuators can be utilized with the input being the output and vice versa. They are essentially nondirectional.

Waveguide Fixed Attenuators

Waveguide Fixed Attenuators lower the amplitude of the signal most commonly by either inserting an absorptive material into the path of the signal inside the waveguide or by coupling a portion of the signal off and dissipating it into absorptive material or a termination. The latter design is generally able to handle more powerful signals by adapting the power rating of the termination for specific needs.

The attenuation of this type of design is directly related to the coupling value. Both types of designs can be bidirectional, but the design that couples and dissipates the signal into a termination is often directional, especially in high-power designs. The frequency range of waveguide fixed attenuators is limited by the constraints of the waveguide.

Variable Attenuators

Variable attenuators have the benefit of being adjustable to the required attenuation. There are different types of variable attenuators that vary in how the attenuation is changed. Each type has advantages and disadvantages when it comes to the resolution, speed of adjustment, insertion loss and VSWR, as well as power handling.

Let’s compare coaxial variable attenuators.

Voltage Variable Attenuators (VVA)

VVAs use an analog control voltage to adjust the attenuation continuously. They support remote control and fast switching speeds but tend to have higher insertion loss than mechanical types. Below is an example of a schematic of a VVA.

Mechanical Step Attenuators

Mechanical Step Attenuators provide attenuation using fixed size steps to increase or decrease the attenuation. Below schematic shows the individual attenuation sections that get either switched in or out of the RF line. Step Attenuators consist of a series of fixed attenuators that add up to a total amount of attenuation for this passive component. The size of each step is defined by the attenuation section with the smallest amount of attenuation. Adjustment of this attenuator is done manually.

Digital Step Attenuators

Digital Step Attenuators are computer controlled programmable attenuators that change attenuation in defined step sizes with a wide attenuation range and good flatness across the frequency range. Just like the VVAs they have a high switching speed and higher insertion loss than other types.

Above schematic shows a digital step attenuator with its individual sections that get either added into or taken out of the RF signal line. Each attenuator section is a fixed attenuator that is controlled using a computer program.

Manual Continuously Variable Attenuators

Unlike step attenuators, these devices allow for smooth, continuous adjustment of attenuation levels, providing precise control over signal strength. They operate over a broad frequency range which makes them suitable for various applications. These attenuators offer very low insertion loss as well as low VSWR. They provide attenuation that is flat over the frequency range. Users can adjust attenuation manually, with options like locking knobs or dial mechanisms. They are ideally suited for applications that require precise adjustment.

This type of attenuator is designed to increase attenuation by sliding an absorptive material around an RF conductor. As the absorptive material surrounds the conductor, the attenuation increases. Higher frequencies will attenuate more than the low frequencies unless it is tuned for flat attenuation over specified frequency ranges. Due to the nature of the design, maximum attenuation below 0.5GHz is generally under 10dB. At higher frequencies, attenuation of up to 60dB is easily achieved. VSWR is typically better than 1.5:1 with an Insertion Loss better than 0.5dB.

Impulse Technologies offers a wide range of models covering frequency ranges from 0.8GHz all the way to 26.5GHz with attenuation up to 60dB. The Continuously Variable Attenuators are available as level adjust models that are best suited for a specific attenuation range over a broad frequency range but where flatness of the attenuation over the frequency range is not important. Below is an example of how Level Adjust models compare to Frequency Flat models.

In this example, you will see the Level Adjust model having almost a 9dB difference when looking at the start and the end of the frequency range. The Frequency Flat model on the other hand has only about 2dB difference between the low end and the high end of the frequency range. So, if attenuation over a frequency range needs to be the same, the frequency flat model will be your best choice. There are limits to how wide a frequency range can be to achieve a flat attenuation response.

Every model is available with 3 different adjustment options, standard screwdriver adjustment, knob control and turns counting dial. The turns counting dial is a great option when returning to certain attenuation levels is desired.

Conclusion

Choosing the right attenuator depends on the specific needs of your system. Key factors include frequency range, power handling, and how precisely the attenuation needs to be controlled. Fixed attenuators are ideal for stable conditions where a consistent signal reduction is required. Variable attenuators are better suited for setups that demand flexibility or frequent adjustments. Each type offers its own benefits, whether based on design, configuration, or ease of use. A clear understanding of these differences helps ensure you select the most effective option for your application.