Originally posted Feb 28, 2014
You may be faster and smarter than you realize
Spectrum and signal analyzers are often used to provide accurate measurements of well-understood signals. Since their earliest days, however, they’ve also been used to answer a more open-ended question: What signals are out there? This question is especially challenging when one is looking for small signals near noise, or transient signals, or signals that are very narrowband and therefore might fall unnoticed between the display bins of a spectrum analyzer.
One logical solution is to use a peak detector as the display detector, as explained in the recent post Detector decisions: see it your way. With the peak detector, no combination of bin spacing, RBW and center frequency will cause a signal to be missed or its amplitude to be incorrectly measured.
However, the peak detector has its disadvantages. In the absence of averaging such as VBW filtering, it can bias the apparent noise level upward and, depending on analyzer settings and signal composition, it can be hard to know how much. The peak detector can also hide small signals that are below the noise peaks.
As a matter of fact—and as you probably know—no single detector is best for all signals and all situations. So, is this one of those times when we must compromise and simply choose a detector with the fewest disadvantages? Instead, perhaps we should follow the old engineering motto we can do better than that and think a bit outside the box.
We can take advantage of the fast information processing and pattern recognition of the human eye-brain system and combine it with our engineering judgment by using a detector choice that is the opposite of data reduction. We can use multiple simultaneous detectors as shown in the figure below.
Using three display detectors simultaneously provides better insight into spectral content. The blue trace is an average detector; the pink trace is a negative peak detector; and the yellow trace is a “normal” detector, which is an intelligent combination of peak and negative-peak functions. Like the peak detector, it prevents signals such as the single yellow spur from being missed.
In this situation, the experienced eye makes use of the extra information to better understand the combination of signals and noise present. It’s easy to detect both the narrow spur and the adjacent channel power of the modulated signal and, at a glance, understand the average power of the modulated signal along with any clues from the negative peaks of the noise.
Agilent X-Series signal analyzers support up to four simultaneous detectors on separate traces, with complementary functions such as max hold and trace averaging. An offset value can be used on individual traces to separate them for easier viewing.
In many situations, a single detector will give you the answers you need, especially when you’re measuring a single type of signal and know what to expect. Just keep in mind that when in uncharted territory it helps to have multiple views to leverage your signal knowledge and intuition.
As mentioned in an earlier post, you can learn more about detector choices and tradeoffs in the new, updated version of application note 150, Spectrum Analysis Basics.