The Benefits of Variable RF Attenuators

Posted by Blaise Yen on May 23, 2017

In testing applications where the power of a signal needs to be continuously or regularly varied, variable RF attenuators come into play. In these devices, the resistor networks commonly found in fixed or step attenuators tend to be swapped out for solid-state metal-oxide semiconductor field-effect transistors (MOSFETs) or PIN (so named because a p-type and n-type semiconductor region sandwich an intrinsic semiconductor region) diodes. The attenuation can be controlled by a variety of methods—manual or electronic—that regulate the voltage across the transistor or the current across the PIN diode. Digital programmable attenuators make use of a combination of fixed resistive pads with PIN diode switches.

Variable RF attenuators come in particularly handy in RF testing applications where the required or optimal amplitude of the signal is still being determined or if it should fall within a particular range. Broadly speaking, this makes variable RF attenuators useful in the development and manufacturing of mobile phones and other cellular devices, IoT devices that make use of WiFi, Bluetooth or proprietary RF communication, as well as cutting-edge MU-MIMO and WiFi MIMO devices, all of which have to satisfy rigorous industry standards while meeting real-world demands in both enterprise- and consumer-level environments. From an industry standpoint, these technologies are either well established or becoming increasingly ubiquitous in consumer electronics, automobiles, aerospace, logistics, education, home appliances, manufacturing and many other fields.

Some specific usage scenarios for variable RF attenuators might include:

  • Reducing the signal amplitude: This is the most fundamental example of what all varieties of attenuators are designed to do. Using variable RF attenuators in this way can limit signal power within a circuit simply to keep from exceeding a certain upper threshold.
  • Controlling a signal amplitude: In this scenario, a variable RF attenuator can be used to dial in a signal to the desired target amplitude. This makes sense when the signal generator creates an accurate signal but at a higher amplitude than is necessary or desired.
  • Improving the impedance match: A properly matched RF attenuator (almost always 50Ω impedance, although 75Ω is sometimes seen) will ensure that input and output impedances are kept close to the circuit level. On occasion the introduction of other components into a testing system will cause a degradation in performance should the impedance fluctuate as a result, and such degradation can jeopardize the integrity of the signal and the test process.

The particular benefits of variable RF attenuators are mostly apparent by what they are not — that is, fixed and passive. Unlike fixed RF attenuators, which are only capable of creating a set amount of attenuation, variable RF attenuators offer more flexibility and allow the engineer to control the amount of attenuation dynamically depending on the environment, the application or the test goal. To the layperson, it might seem like attenuation of any kind would be unwanted. But the most powerful signal is not always the optimal signal. In many RF testing applications, deliberate attenuation is essential when precisely managing signal amplitudes in order to prevent the overload and saturation that might cloud or skew test results.

Used by industry leaders such as Itron, Google, NASA, Verizon, Intel, Mimosa and many others, Adaura Technologies’ digital programmable attenuators combine the benefits of variable RF attenuators with an unparalleled ease of programmability. Their rich feature set and affordability make them ideal for both of the major use cases in the RF industry:

  • In small startups. Bootstrapped IoT and wireless startups don’t always have the wherewithal to afford cutting-edge testing equipment, so they make do with very limited high-end equipment, they rely on legacy equipment or they simply do without it altogether. This either creates a serious production bottleneck, or it leaves their products partially (or even completely) untested for real-world scenarios.
  • In large companies. The sheer magnitude of production in large companies naturally drives up the cost of testing equipment. This can lead to massive outlays for premium high-performance equipment for massively parallel testing or a switch to less expensive—and potentially less capable—alternatives.

Adaura Technologies’ AD-USB2A two-channel series of RF digital programmable attenuators, or the AD-USB4A four-channel variant, can be implemented quickly and inexpensively in testing scenarios for RF devices without compromising quality and thoroughness of testing.

Designed around Hittite’s HMC642 broadband GaAs digital IC for high input and accurate step size, both the AD-USB2A and AD-USB4A series of RF digital programmable attenuators allow their multiple channels to be controlled and powered over a single USB port via a supplied 3′ USB cable, which reduces the complexity of the test setup. And their intuitive GUI enables test engineers to program fixed attenuations or flexible ramp functions through simple serial-port commands on any Windows, Mac or Linux PC. Those features give you plenty of variability in your testing options while eliminating detrimental variability in build quality and performance.

Posted in RF Test Engineering.