The 2020-2024 Extra License question pool asks you to identify characteristics of a spectrum analyzer, a common RF engineering signal measurement device:
E4A02: Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?
A. RF amplitude and time
B. RF amplitude and frequency
C. SWR and frequency
D. SWR and time
Let’s first frame this question about a measurement device with a general description of the types of signals that hams commonly wish to measure, and the correct response will precipitate right out of the signal description. In the General License Course we introduce a 3D model of waveform signals from which either a time domain view or a frequency domain view may be considered. This 3D model is reprised in the Extra License Course book, and we'll use it again here in examining Extra question E4A02. (For an examination of time and domain views of signals, see this Ham Radio School article: Time & Frequency Domain Views.)
The 3D model depicts a complex signal comprised of a few thousand hertz of bandwidth. In this particular depiction the model is depicting an audio band from approximately 200 Hz to 3000 Hz, typical of an audio band that might modulate a SSB RF signal. For viewing clarity, only a sampling of a few specific frequency waveforms are shown across this bandwidth, else the image would be too cluttered with 3000+ waveform images. Further, you can imagine a similar complex RF signal by bumping up the frequency values by several megahertz.
In the model, waveforms are imagined to be moving across the axis that indicates their frequencies, and each waveform of different frequency varies in amplitude as it flows along, commensurate with the variations of power in each frequency. For this audio signal, perhaps encoding an operator’s voice, the lower frequencies that are typical of vowel sounds are usually of greater amplitude than the higher frequencies more commonly produced by consonant sounds. If our model was dynamic instead of statically welded in an image on this page, we would observe the waveforms flowing happily along, each one growing and shrinking in height as the characteristics of the voice’s sound ebbed and flowed and enunciated a variety of words.
An oscilloscope is a measurement device that displays the time domain view of a signal. When an AC signal is fed into the input of the oscilloscope, the scope measures the voltage of the signal over a brief time period that is usually selected by the scope operator. The oscilloscope plots the measured voltage of the signal on the vertical axis of the display, thereby depicting the amplitude of the input signal over time that is plotted on the horizontal axis. The image is from the perspective of our model viewer on the left, a sine wave varying over time -- the time domain view.
The model viewer on the right, however, is getting a much different picture. He has a view across the range of frequencies contained in the signal, seeing each frequency as a vertically plotted bar. He observes the changing amplitude of each different frequency as the changing height of each frequency’s bar over time. However, he has no “time axis” to view. While the amplitudes of the frequencies are depicted on the vertical axis similar to the oscilloscope plot, the horizontal axis is now depicting frequency, or the signal spectrum. This view is very similar to that observed on many audio frequency equalizer displays, on which the amplitudes of frequencies dance up and down with the voice or music signal routed through the equalizer. This is the frequency domain view.
The view of our friend on the right is that depicted by a measurement device called a spectrum analyzer, and these devices display a range of frequencies across the horizontal axis and amplitude on the vertical axis.
The answer to Extra Class question E4A02, "Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?" is “B. RF amplitude and frequency.”
-- Stu WØSTU