Your Guide to Digital RF Attenuators

Posted by Blaise Yen on May 23, 2017

In this blog post, we’re going to dive a little bit deeper into the core capabilities of a digital RF attenuator to provide you with some touchstones when assessing their merits and utility in your own testing applications. Attenuators, as you probably already know, control the amplitude, or strength, a signal in a high-frequency RF system.

Some of the other types of RF attenuators include fixed and continuously variable attenuators. A quick recap of their applications will help us highlight how they differ from digital RF attenuators:

  • Engineers often use fixed attenuators to improve the impedance match between two components in a system as long as the attenuation loss can be tolerated. These system components can be a filter or an amplifier.
  • Continuously variable attenuators, by contrast, come in handy when engineers need to emulate the propagation and multipath losses of signals between transmitters and receivers in a wireless link.

Digital RF attenuators have similar functionality but are a great deal more sophisticated. What particularly sets digital RF attenuators apart is that engineers can select a specific amount of attenuation dynamically depending on the state of a logic signal, which is a digital signal with only two types of values. These are the same signals that are used in a computer, and as such, digital RF attenuators can be programmed to produce a given amount of attenuation for a specified period of time using software controls. This has obvious advantages, not least when it comes to automated testing, because the attenuation doesn’t have to be manually selected during each testing phase.

You’ll remember, though, that computers are not all built using the same hardware platform, and so it is with digital RF attenuators. Their logic type can be driven by transistor–transistor logic (TTL) or complementary metal-oxide semiconductor (CMOS).

Regardless of the method they use, digital RF attenuators broadly function on the same principle, which is that the logic signal activates different attenuation states that can be increased or decreased at specific increments, or steps. Internally, these steps are digitally determined according to the least-significant bit (LSB), which selects the smallest amount of attenuation, or the most-significant bit (MSB), which selects the largest amount of attenuation. The interstitial bits therefore select attenuation values between those two extremes. What those values are will depend on how the device is designed. The AD-USB4A series of digital RF attenuators from Adaura, for example, range from 0dB of minimum attenuation to 63dB of maximum attenuation at step sizes of 1dB.

Those minimum/maximum values are important, as is the step size. The wider the attenuation range, the more versatile your digital RF attenuator will be across various applications. And the smaller the step size, the more granular your attenuation testing can be. But another hugely important factor is the frequency range of your digital RF attenuator. If you’re testing a component that is designed for application in a system using IEEE 802.11ac, today’s most common WLAN standard, you’ll want a digital RF attenuator that can accommodate frequencies between 5725 and 5875MHz (plus the 2400 to 2500MHz band for backwards compatibility). For RF components in 4G LTE network devices, testing frequencies anywhere between 450 and 5900MHz are needed.

The last major hardware specification you’ll want to consider is the insertion loss. This is the reduction is signal strength, typically measured in dB, that is a direct result of placing the digital RF attenuator in the path of the testing setup. A loose parallel might be a dam: Dams are designed to let water pass through at controlled rates, but the very existence of the dam itself restricts water flow. With digital RF attenuators, the lower the insertion loss, the better, as this will allow you to get more accurate testing results based on the attenuation you have programmed. Adaura’s AD-USB4A series of digital programmable attenuators has a minimum insertion loss of just 2.5 dB in the 0.05–3GHz frequency range and 7dB in the 3–6GHz range.

One other factor that gets overlooked, however, is ease of use. As the appeal of digital RF attenuators rests in large part on their programmability, the software should strike the right balance between power and simplicity. Adaura Technologies’ two- (AD-USB2A) and four-channel (AD-USB4A) series of RF digital programmable attenuators come with bundled software that runs on any Windows, Mac or Linux computer and offers both command-line and GUI options. At least one reviewer has noted that this is an enormous win for Adaura devices in terms of time to development, as it means that the Adaura digital RF attenuators can be deployed quickly and easily into almost any existing test setup.

Posted in RF Test Engineering.