Originally posted May 3, 2016
Understanding how it limits dynamic range and what to do about it
A talented RF engineer and friend of mine is known for saying “Life is a microcosm of phase noise.” He’s an expert in designing low-noise oscillators and measuring phase noise, so I suppose this conceptual inversion is a natural way for him to look at life. He taught me a lot about phase noise and, although I never matched his near-mystical perspective on it, I have been led to wonder if noise has mass.
A distinctly non-mystical aspect of phase noise is its effect on optimizing distortion measurements, and I recently ran across an explanation worth sharing.
A critical element of engineering is developing an understanding of how phenomena interact in the real world, and making the best of them. For example, to analyze signal distortion with the best dynamic range you need to understand the relationships between second- and third-order dynamic range and noise in a spectrum analyzer. These curves illustrate how the phenomena relate:
The interaction of mixer level with noise and second- and third-order distortion determine the best dynamic range in a signal analyzer. The mixer level is set by changing the analyzer’s input attenuator.
From classics such as Application Note 150, you probably already know the drill here: In relative terms, analyzer noise floor and second-order distortion change 1 dB—albeit in opposite directions—for every 1 dB change in attenuation or mixer level, and third-order distortion increases 2 dB for each 1 dB increase in mixer level. Therefore, the best attenuation setting for distortion depends on how these phenomena interact, especially where the curves intersect.
The optimum attenuator setting does not precisely match the intersections, though it is very close. The actual dynamic range at that setting is also very close to optimum, though it is about 3 dB worse than the intersection minimum suggests, due to the addition of the noise and distortion.
That’s where your own knowledge and insight come in. The attenuation for the best second-order dynamic range is different from that for the best third-order dynamic range, and the choice depends on your signals and the frequency range you want to measure. Will analyzer-generated second-order or third-order distortion be the limiting factor?
Of course, you can shift the intersections to better locations if you reduce RBW to lower the analyzer noise floor, but that can make sweeps painfully slow.
Fortunately, because you’re the kind of clever engineer who reads this blog, you know about technologies such as noise power subtraction and fast sweep that reduce noise or increase sweep speed without the need to make other tradeoffs.
Another factor may need to be considered if measuring third-order products, one that is often overlooked: analyzer phase noise.
In this two-tone intermod example with a 10 kHz tone spacing, the analyzer’s phase noise at that same 10 kHz offset limits distortion measurement performance to -80 dBc. Without this phase noise the dynamic range would be about 88 dB.
I suppose it’s easiest to think of the analyzer’s phase noise as contributing to its noise floor in an amount corresponding to its phase noise at the tone offset you’re using. Narrower offsets will be more challenging and, as usual, better phase noise will yield better measurements.
That’s where clever engineering comes in again. Analyzer designers are always working to improve phase noise, and the latest approach is a major change to the architecture of the analyzer’s local oscillator (LO): the direct digital synthesizer LO. This technology is now available in two of Keysight’s high-performance signal analyzers and will improve a variety of measurements.
The focus of this post has been on two-tone measurements but, of course, many digitally modulated signals can be modeled as large numbers of closely spaced tones. Phase noise continues to matter, even if the equivalent distortion measurements are ACP/ACPR instead of IMD.
Once again, noise is intruding on our measurement plans—or maybe it’s just lurking nearby.
Perhaps this post only proves that my perceptions of phase noise still don’t reach into the mystical realm. Here’s hoping your adventures in phase noise will help you achieve second- and third-order insights.