R&S FSH
How a Spectrum Analyzer Works
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Operating Manual 1309.6275.12 - 14
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9 How a Spectrum Analyzer Works
Basically, it is possible to measure and analyze RF signals either in the time domain or
the frequency domain.
Measurements in the time domain show signal variations over time. You can perform
these with an oscilloscope, for example. Measurements in the frequency domain show
the frequency components of a signal. To perform measurements in the frequency
domain, you can use a spectrum analyzer.
Both modes are essentially equivalent because applying the Fourier transform to any
signal converts it into its spectral components. Depending on the signal characteristic
to be measured, one method is usually more appropriate than the other. With an
oscilloscope, it is possible to tell whether a signal is a sine wave, a square wave with a
certain on/off ratio or a sawtooth wave. However, detecting superimposed low-level
signals or monitoring the harmonic content of the signal is easier with a spectrum or
signal analyzer.
Figure 9-1 shows the theoretical basis of the two measurement methods. In the time
domain, an oscilloscope would, for example, show a section of the signal that is a
square wave. The same signal, when viewed with a spectrum analyzer, would show a
line spectrum (the fundamental and its harmonics).
Figure 9-1: Visualization of time domain and frequency domain
Applying the Fourier transform to the periodic square wave transforms it into the
frequency domain. The spectrum analyzer would show the fundamental (or frequency
of the square wave) and its harmonics.
The spectrum analyzer uses a narrow bandpass filter for measurements in the
frequency domain. Only at frequencies containing a signal there is a reading that gives
the amplitude of the frequency component.
Figure 9-2 shows the basic principle of how a spectrum analyzer works.
