The spectrum density measurement feature is used to monitor time-varying and superimposed signals by capturing and analyzing continuous spectrums over time. Rare or superimposed events, which cannot be discovered using the ordinary Spectrum Measurement Mode, can be visualized in a spectrum density chart. Therein, the horizontal axis shows the frequency (FFT points) and the vertical axis represents the power level, which resembles the spectrum chart. In addition, the probability of an occurring power level at a certain frequency is displayed using a pre-defined color range. That means each color defines a certain probability of occurrence. The spectrum density is also called a spectral histogram.

Starting with an empty spectrum density, each event accounts for the histogram proportionally to the number of currently captured spectrums. When the spectrum counter reaches the user defined memory size, new events account proportionally to the memory size. Hence, elder events are lost over time by reducing their contribution to the histogram. In case a probability value reaches a certain threshold, the value in the spectrum will disappear from the screen. The disappearance is visualized as a “persistence” or “after glowing” effect.

The accuracy of a spectral density chart increases with the number of captured spectrums. Consequently, the progress of accuracy increase depends on the sweep time of the remote spectrum monitor, the network bandwidth, and network delay.