What is a Spectrum Analyzer?
To many electrical engineers, a spectrum analyzer looks just like an oscilloscope except with more features and graphics. However, while both an oscilloscope and a spectrum analyzer display a signal’s amplitude on the vertical axis, the difference between them is what’s shown on the horizontal axis; an oscilloscope displays time, whereas the spectrum analyzer shows frequency. The figure below shows multiple frequency measurements being displayed on Rigol’s DSA815-TG Spectrum Analyzer.
Figure 1: Spectrum analyzers display frequency measurements on the horizontal axis. (Image source: Rigol Technologies[1])
According to Keysight Technologies, a spectrum analyzer “measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. Its primary use is to measure the power of the spectrum of known and unknown signals.”[2] In other words, a spectrum analyzer allows users to “analyze a spectrum,” where a spectrum is defined as a collection of sine waves combined to produce a time-domain signal.
As an example, let's observe a signal on an oscilloscope (Figure 2).
Figure 2 : A signal displayed on an oscilloscope (Image source: Agilent Technologies[3])
While this signal is obviously not a pure sinusoidal waveform, a spectrum analyzer determines each of the individual sinusoidal waveforms that make up this signal. And after the spectrum analyzer has identified these waveforms, it plots the amplitude versus frequency of each individual waveform. As you can see in Figure 3, the signal from Figure 2 is made up of only two sinusoidal waveforms.
Figure 3: Relationship between a time domain signal (of which is displayed on an oscilloscope) and frequency domain signals (of which are displayed on spectrum analyzers) (Image source: Agilent Technologies[3])
Types of Spectrum Analyzers: Technology Types and Form Factors
There are two main categories of spectrum analyzers: swept-tuned analyzers and real-time analyzers, also referred to as real-time spectrum analyzers, or RTSA. Both types, which have been used for many years, display amplitude on the vertical axis and frequency on the horizontal axis, but how they go about “analyzing a spectrum” is what distinguishes them.
Given that a swept-tuned spectrum analyzer is “nothing more than a frequency-selective voltmeter with a frequency range that’s tuned (swept) automatically,”[4] it’s not at all surprising to realize that these traditional types of analyzers “descended from radio receivers.”[4] And because swept-tuned spectrum analyzers “cannot evaluate all frequencies in a given span simultaneously,”[4] they are primarily used for measuring steady-state or repetitive signals. These analyzers have successfully served the compliance engineering community (think pre-compliance testing and EMC/EMI testing) for several decades.
Unlike swept-tuned spectrum analyzers, real-time spectrum analyzers can evaluate all frequencies simultaneously. A real-time spectrum analyzer works by first acquiring data in the time domain and then converting that data into the frequency domain by use of the fast Fourier transform (FFT).
Spectrum analyzers come in a variety of form factors, including benchtop (Figure 4), handheld (Figure 5), and portable.
Figure 4: Teledyne LeCroy’s T3SA3200 Benchtop Spectrum Analyzer offers a frequency range from 9 kHz to 3.2 GHz. (Image source: Teledyne LeCroy[5])
Figure 5: Seeed Technology’s RF Explorer Model 2.4G is a 2.35 GHz to 2.55 GHz Handheld Spectrum Analyzer. (Image source: Seeed Technology)
Benchtop models typically outperform their handheld counterparts but can carry a higher price tag. Handheld spectrum analyzers are both less expensive and smaller, but they offer only a subset capability relative to benchtop analyzers. Portable analyzers are simply those (including some benchtop versions) that can be taken into the field thanks to their battery packs.
Conclusion
While all (we hope!) electrical engineers know what an oscilloscope is and how to use one, it can be surmised that only some electrical engineers have ever used a spectrum analyzer. Although oscilloscopes and some spectrum analyzers (the benchtop versions) may look similar in both form factor and display, they are quite different; a spectrum analyzer presents its acquired data in an amplitude-versus-frequency fashion, whereas an oscilloscope displays its information in an amplitude-versus-time method. However, just like oscilloscopes, various spectrum analyzer types are available depending on one’s needs and budget.
