Why Use a Waveguide Variable Attenuator?

Waveguide-based high-frequency systems often require some control of amplitude or signal level. Waveguide attenuators can deliver that means of control, with fixed attenuators providing a single value attenuation setting and variable attenuators providing a range of attenuation control across a desired frequency range.

AttenuatorWaveguide variable attenuators can be especially useful in test-and-measurement systems and in applications where the final desired level may not be precisely known or may fall within a range of levels. But selecting a waveguide variable attenuator for an application is not a trivial task, and it can be helpful to know a bit about some of the main attributes of these components and some key performance parameters. Since there are several kinds of waveguide variable attenuators, it can also be helpful to know about the different ways in which the attenuation can be controlled. Typical options to control attenuation include; either a manual controlled version or a motor controlled version. The motor controlled version uses voltage to adjust the attenuator and attenuation value. Knowing what options are available allows the customer to pick the best waveguide variable attenuator for their particular application. Waveguide variable attenuators are available with a wide range of attenuation values, some capable of adjusting attenuation settings from 0 to 30 dB or more with impressive stability and reliability across waveguide frequency bands.

As with other waveguide components, the size of the waveguide in a variable attenuator will determine the usable frequency range. Within a given Waveguide sizes’ frequency range, two different constructions of variable attenuators will offer various attenuation ranges. Sidewall Variable Attenuators have an attenuation range of up to 40dB, whereas Topwall Variable Attenuators typically have an attenuation range from 0 to 15-20dB. As mentioned before, there are typically two different ways to control the attenuation range. Often, some form of movable structural element within the waveguide, such as a rotary vane or moving resistive card will cause the attenuation variation. The attenuation element may be moved manually or under motorized power. Manual variable attenuators typically employ an adjustable screw control to adjust the level of attenuation, while motor-driven waveguide variable attenuators may provide continuously variable control of the attenuator using voltage-tuned motors or they may tune in steps using, for example, a stepper motor for control of the attenuation element and attenuation. Many suppliers of mechanically tuned waveguide variable attenuators will offer a choice of the number of turns required by the tuning element to achieve the full attenuation setting.

Additionally, waveguide variable attenuators are available with fully electronic control of attenuation, for example, using PIN diodes to tune the attenuation level of the component. Manually tuned waveguide variable attenuators offer the benefits of simplicity and straightforward control. Motor- or diode-controlled electronically adjusted waveguide variable attenuators offer the option of remote control and rapid resetting of attenuation level, as needed.

Various methods can be employed to display the attenuation level set on one of these components, from simple mechanical dials to more sophisticated electronic readouts. The choice of attenuation display will typically be determined by the needs of a particular application. Depending upon the need for accuracy, waveguide variable attenuators are available as calibrated or uncalibrated models. Calibrated attenuators are typically verified by a supplier at different spot frequencies and attenuation settings; although these frequencies and attenuation settings can be chosen by a customer to best match the needs of an application.

Once such basic requirements as frequency range/waveguide size, attenuation range, and manual or electronic control of attenuation have been decided, specifying a waveguide variable attenuator is a matter of comparing key electrical and mechanical parameters for different available models. The electrical performance of different attenuators can be compared in terms of insertion loss, voltage standing-wave ratio (VSWR), and power-handling capability.

For many applications, stability is an important characteristic: How much will the attenuation and/or the phase response of the waveguide variable attenuator vary with frequency, or with attenuation setting, or even with temperature. In other applications, the attenuation flatness versus frequency might be of interest. Variations of ±1dB or ±2dB in attenuation can be typical across the full waveguide frequency range. (I.e. -10 to 15 GHz for a WR75) These variations, in attenuation, can also be impacted by the amount of total attenuation offered by a component. A waveguide variable attenuator offering an attenuation adjustment range of 0 to 15dB can be expected to provide less variation with frequency than a unit with an attenuation adjustment range of 0 to 40dB.

Most waveguide variable attenuators are somewhat limited in RF/microwave power-handling capability compared to fixed waveguide attenuators operating near or within the same frequency range. A wide range of waveguide variable attenuators are designed for input power levels in the 1-W or less continuous-wave (CW) power range, with higher-power units designed for input power levels of typically 4 to 5 W. In contrast, high-power waveguide fixed attenuators covering similar frequencies may be rated for 40 W or more of CW power.

The power-handling capability can be influenced by a number of factors, including the type of attenuation element used in the attenuator, the types of materials (I.e. -aluminum or brass) used in the construction of the attenuator or even the type of finish applied (such as a corrosion-resistant finish or silver plating).

The mechanical construction of a waveguide variable attenuator may be critical, for some applications. Waveguide size is a function of the wavelength of a frequency of interest, the physical size of most waveguide variable attenuators (such as the length) will be mostly determined by the operating frequency, with lower-frequency attenuators being considerably larger than higher-frequency attenuators, even though they may typically be rated for the same power-handling capabilities. The type of flanges used on the waveguide input and output ports may be of significance or the drive mechanisms’ construction.

Contact MDL for your waveguide variable-attenuator questions or requirements at 1-781-292-6684 or visit our website at www.mdllab.com.

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