How to Select the Right Microwave Attenuator

Posted by Blaise Yen on May 23, 2017

Selecting the right microwave attenuator for RF testing applications can be a bit daunting. There are a number of device specifications that engineers need to consider, and buying a substandard or inadequate testing device (or even a fleet of devices) can be an expensive mistake.

Before we get into characteristics, it’s worth covering briefly what attenuation is, what it’s used for, and the basic attenuator hardware designs.

Stated simply, attenuation is the opposite of amplification. It creates an intentional and desirable weakening of the power, or amplitude, of an input signal. In RF testing applications, the ability to attenuate a signal—and to do so with reliability and precision—is incredibly valuable for measuring real-world performance under laboratory conditions.

As for design, there are roughly two fundamental types of microwave circuits in RF attenuators. These are the pi and the T, each of which can be balanced and unbalanced. We won’t get into the math or the circuit diagrams in this brief space, but we will note that each of these variants and sub-variants works in different ways toward the same end. This boils down to shared general characteristics that can be compared among any two or more microwave attenuators and will help inform your choice. They include:

  • Attenuation: A microwave attenuator attenuates, naturally. But not all of them do so to the same degree. Depending on your RF testing application, you’ll want to find one that has a reasonable minimum attenuation (down to 0dB) and a practical maximum attenuation (also expressed in dB). The minimum attenuation of the AD-USB2A series of two-channel programmable RF attenuators from Adaura Technologies is 0db; its maximum is 63dB.
  • Accuracy: Perhaps the most important specification after the range of attenuation, accuracy is of course crucial to testing applications. The lower the deviation, the better. For Adaura’s AD-USB4A series of four-channel programmable RF attenuators, the accuracy is rated at +/-(0.60dB +5% of the attenuation setting).
  • Impedance: In most RF testing applications, you’ll want your microwave attenuator to have an impedance of 50Ω. This impedance value is fairly standard on all RF attenuators.
  • Insertion loss: This is the loss in signal power due simply to placing the attenuator in the test setup. In the 0.05 to 3GHz range, the insertion loss for the AD-USB2A is rated between just 2.5 and 6dB. In the 3 to 6GHz range, it’s between 7 and 9dB.
  • Operating frequency: This is the frequency range of the microwave attenuator. When testing RF devices, it’s essential to have a test setup that can accommodate the frequency range of the real-world application of the device under test. For instance, if you’re testing 4G LTE components, you’ll want to have an attenuator that is capable of handling anything within the range of 450 to 5900MHz because it’s such a wide potential spectrum. The AD-USB2A and AD-USB4A both operate between 0.5 and 6GHz.
  • Step size: Steps are the increments by which you can increase or reduce the attenuation across a given range. Generally, the smaller the step size, the more precisely you’ll be able to test a particular device or component, but miniscule step sizes can sometimes be overkill.
  • Switching speed: “Switching” refers to the time it takes for the attenuator to toggle between attenuation states. Here, faster tends to be better.

In modern testing scenarios, the right microwave attenuator is also likely to be a digital programmable RF attenuator because of the flexibility, convenience and precision that these microwave attenuators allow. As mentioned above, the AD-USB2A and AD-USB4A series are two- and four-channel RF attenuators, respectively, which means that they’re capable of testing multiple channels simultaneously. Yet that doesn’t mean they’re more difficult to integrate into a test setup or operate. Both are remarkably compact and connect to a computer via a single supplied USB cable. The included software runs on any Mac, Linux or Windows PC and offers an easy-to-use GUI as well as straightforward command-line functions — including setting one or multiple attenuators to a specified value, as well as a ‘ramp’ command to increase the attenuation with a given dwell time for each step.

Combined with their incredibly competitive price, these features make the AD-USB2A and AD-USB4A series perfect for startups as well as large established manufacturers who are involved in the development and testing of microwave RF components for IoT, cellular and WiFi applications.

Posted in RF Test Engineering.