When operating with a real-time display, it’s important to take care when selecting the RBW filter. Just like in a regular spectrum display, the RBW filter selection will greatly affect the speed of the spectrum measurements. One of the key specifications for an RTSA is the probability of intercept (POI). This specification dictates the minimum signal duration that the instrument is guaranteed to detect. Selecting a narrow RBW will change the POI for the measurement—an important factor to be aware of.
Revealing the whole story
While a much greater amount of information can be obtained from the persistent display compared to a basic spectrum display, it doesn’t reveal the whole story. In modern radios there are many protocols that employ some form of clear channel assessment. Essentially, such radios are capable of determining how busy a channel is and will only transmit when no one else is using the frequency. Even a fast persistent display cannot show the timing relationships between two signals. To determine the timing of signals we need to use the spectrogram function as shown in Figure 4 which allows us to plot the spectrum data over time to determine how often signals are active.