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Frequency vs. Recency in Advanced Signal Analyzer Displays

Blog Post created by benz on Sep 14, 2016

Originally posted Aug 31, 2014

Is “recency” really a word?

My spell checker nags me with a jagged red underline, but yes, “recency” is a legitimate word. And it isn’t one of those newly invented for a marketing campaign words: Merriam-Webster traces it back to 1612, and others go back even further.

It’s a good word that means exactly what it sounds like: the quality of being recent. In our world of highly dynamic signals and spectral bands, this quantity is becoming ever more useful.

Of course, recency-coded displays have been around for a long time, though more commonly in oscilloscopes than spectrum analyzers. Traditional analog variable-persistence displays naturally highlighted recent phenomena, as the glow from excited phosphors decayed over time. Extending this decay time by an adjustable amount made the displays even more useful.

A current term for this sort of display is “digital persistence” and in the 89600 VSA software it produces this display of oscillator frequency and amplitude settling:

Digital-persistence spectrum of an oscillator as it settles to new frequency and amplitude values. The brighter traces are more recent, though recency could also be indicated by color mapping.

Digital-persistence spectrum of an oscillator as it settles to new frequency and amplitude values. The brighter traces are more recent, though recency could also be indicated by color mapping.

A good complement to recency is “frequency,” which—in this context—is defined as how often specific frequency and amplitude values occur in a spectrum measurement.

Common terms for this sort of display are frequency of occurrence or density or DPX or cumulative history. It’s a kind of historical measure of probability, and for the balance of this post I’ll just use the term density.

Thus, recency is a measure of when something happened, while density is a measure of how often.

In real-time analyzers—and analog persistence displays—the two phenomena are generally combined in some way. However, although related, they indicate different things about the signals we measure.

Because the 89600 VSA provides them independently, as separate traces with separate controls, I’ll use it for another example and discuss combined real-time analyzer displays in a future post. Here’s a frequency or density display of the infamous 2.4 GHz ISM band:

Off-air spectrum density measurement of 2.4 GHz ISM band, including brief Bluetooth hops and longer WLAN transmissions. Signal values of amplitude and frequency that occur more often are represented by red and yellow, while less-frequent values such as those from the Bluetooth hops are shown in blue.

Off-air spectrum density measurement of 2.4 GHz ISM band, including brief Bluetooth hops and longer WLAN transmissions. Signal values of amplitude and frequency that occur more often are represented by red and yellow, while less-frequent values such as those from the Bluetooth hops are shown in blue.

This pure density display represents a great deal of information about the time occupancy of the ISM band, showing the relatively long duration of the WLAN frames and the brevity of the Bluetooth hops. However, it offers nothing about signal timing: how many bursts, whether they overlap or not, or even whether the Bluetooth hops are sequential.

That leads, perhaps, to a suggestion. While both display types present a lot of information at once,—and can show very infrequent signals or behavior—they are optimal for different measurement purposes: if you want to know when something happened, with an emphasis on the recent, use persistence; if you want to distinguish signals by how often they appeared, use density.

It’s an over-simplification to say that persistence is best for viewing signals and density is best for viewing spectral bands, but that’s not a bad place to start.

If you’ve used a real-time analyzer you probably noticed that the density displays are usually a kind of hybrid, with an added element of persistence. And you’ve probably heard at least a little about spectrogram displays, which add the time element in a different and very useful way. They’re all excellent tools, and will be good subjects for future posts.

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