Extra Class question of the day: spectrum analyzers and oscilloscopes

Two instruments that amateur radio operators frequently use when experimenting or when debugging equipment are the oscilloscope and the spectrum analyzer. How does a spectrum analyzer differ from an oscilloscope? A spectrum analyzer displays signals in the frequency domain; an oscilloscope displays signals in the time domain. (E4A01) What this means is that an oscilloscope will show you have the amplitude of a signal changes with time, while a spectrum analyzer shows you how the amplitude of a signal changes with frequency. The drawing below shows typical displays from an oscilloscope and a spectrum analyzer.

Spectrum analyzers and oscilloscopes

The oscilloscope shows how a signal's amplitude changes with time, while a spectrum analyzer shows how a signal's amplitude changes with frequency.


Because the spectrum analyzer shows how the amplitude of a signal changes with frequency, amplitude is the parameter a spectrum analyzer would display on the vertical axis. (E4A03) Frequency is the parameter a spectrum analyzer would display on the horizontal axis. (E4A02)

Spectrum analyzers are very useful for troubleshooting problems. For example, a spectrum analyzer is used to display spurious signals from a radio transmitter. (E4A04) A spectrum analyzer is also used to display intermodulation distortion products in an SSB transmission. (E4A05) The reason for this is that in both of these cases we are looking for signals that are being erroneously generated.

Whenever frequency is an important part of the measurement, you want to use a spectrum analyzer, if one is available. All of these choices are correct when talking about parameters than can be determined with a spectrum analyzer (E4A06):

  • The degree of isolation between the input and output ports of a 2 meter duplexer
  • Whether a crystal is operating on its fundamental or overtone frequency
  • The spectral output of a transmitter

Because spectrum analyzers are sensitive instruments, you need to be cautious when using them. For example, an important precaution to follow when connecting a spectrum analyzer to a transmitter output is to attenuate the transmitter output going to the spectrum analyzer. (E4A12) Not doing so could damage the spectrum analyzer because its input circuits are not designed to handle high power.

Despite all this talk about spectrum analyzers, the oscilloscope is actually the more versatile instrument,  and will be more useful than the spectrum analyzer for most radio amateurs. For example, the oscilloscope is the instrument used for detailed analysis of digital signals. (E4A11) You can make a number of digital-signal measurements with a scope, including rise time and fall time, as well as analyze how two or more digital signals change in time with regard to one another.

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