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Spotting Difficult Noise Figure Measurements

Blog Post created by benz on Jun 10, 2017

  A quick, intuitive look at what will make them challenging

Noise is fundamental in much of what RF engineers do, and it drives cost/performance tradeoffs in major ways. If you’ve read this blog much, you’ve probably noticed that noise is a frequent focus, and I’m almost always working to find ways to reduce it. You’ve also noticed that I lean toward an intuitive explanation of RF principles and phenomena whenever possible.

In the most recent post here, Nick Ben discussed four fundamentals of noise figure. It’s a useful complement to my previous look at the measurement and the two main ways to make it.

As engineers, we work to develop a keen sense of when we might be venturing into difficult terrain. This helps us anticipate challenging tasks in measurement and design, and it helps us choose the best equipment for the job. In this post I’ll summarize factors that might make noise figure measurements especially troublesome.

First, the most common challenge in noise figure measurements: ensuring that the noise floor of the measurement setup is low enough to separate it from the noise contributed by the DUT. These days, the most frequently used tool for noise figure measurements is a spectrum or signal analyzer, and many offer performance and features that provide an impressively low noise floor for noise figure measurements.

Measurement examples of noise floor for a broad frequency span on Keysight PXA signal analyzer. Lower traces include effect of internal and external preamplifiers.

Internal (middle trace) and external (bottom trace) preamplifiers can dramatically reduce the noise floor of signal analyzers (scale is 4 dB/div). The measurements are from a Keysight PXA X-Series signal analyzer, which also includes a noise subtraction feature as another tool to reduce effective analyzer noise floor.

My instinct is to separate noise figure measurements into four general cases, resulting from two characteristics of the DUT: high or low noise figure versus high or low gain.

I should note that this is something of an oversimplification, and not useful for devices such as attenuators and mixers. For the sake of brevity in this post I’ll limit my discussion to RF amplifiers, and in a future post deal with other devices and the limits of this approach.

Because analyzer noise floor is a critical factor in the measurements, it’s probably no surprise that you’ll have an easier time measuring devices with a relatively high level of output noise. This includes devices that have a poor noise figure, no matter their gain. Less obviously, it also includes devices with a very good noise figure, as long as their gain is high enough.

The intuitive thing to keep in mind is that large amounts of gain will amplify DUT input noise by the same amount, resulting in output noise power large enough to be well above the analyzer’s noise floor.

Thus, the most difficult measurements involve devices with modest gain, especially when their noise figure is very good (low). The resulting noise power at the DUT output is also low, making it difficult to distinguish the noise power at the DUT output from that of the signal analyzer.

In his recent post, Nick also brought up the problems that interference and other (non-noise) signals can cause with noise figure measurements. Shielding your setups and ensuring connection integrity can help, and signal analyzers can identify discrete signals and avoid including them in the noise figure results.

One more complicating factor in noise figure measurements is the impedance mismatches that occur in two places: between the noise source and the DUT, and between the DUT and the analyzer. This problem is generally worse at higher frequencies, making it increasingly relevant in 5G and other millimeter-wave applications. The most thorough way to handle the resulting errors in noise power and gain measurements is to use the “cold source” noise figure method implemented in vector network analyzers.

Noise figure measurements will challenge you in many other ways, but those mentioned above should give noise figure novices a better sense of when it’s most important to be careful with the measurements and cautious in interpreting the results.

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