Spectrum Analyzer Basics Tutorial

- RF spectrum analysers are an essential tool for RF and other engineers, providing a view of the spectrum of signals with their amplitudes and frequencies.

Spectrum analyzers are widely used within the electronics industry for analysing the frequency spectrum of radio frequency, RF and audio signals. Looking at the spectrum of a signal they are able to reveal elements of the signal, and the performance of the circuit producing them that would not be possible using other means.

Spectrum analysers are able to make a large variety of measurements and this means that they are an invaluable tool for the RF design development and test laboratories, as well as having many applications for specialist field service.

An image of a typical bench top, rack mountable spectrum analyzer offering top performance
A typical bench spectrum analyser
An image of a handheld version of a spectrum analyser offering portability and performance for applications where space and portability are needed
A handheld spectrum analyzer

Why spectrum analysis?

The most natural way to look at waveforms is in the time domain - looking at how a signal varies in amplitude as time progresses, i.e. in the time domain. This is what an oscilloscope is used for, and it is quite natural to look at waveforms on an oscilloscope display. However this is not the only way in which signals can be displayed.

A French mathematician and physicist, named Jean Baptiste Joseph Fourier, who lived from 1768 to 1830 also started to look at how signals are seen in another format, in the frequency domain where signals are viewed as a function of their frequency rather than time. He discovered that any waveform seen in the time domain, there is an equivalent representation in the frequency domain. Expressed differently, any signal is made up from a variety of components of different frequencies. One common example is a square waveform. This is made up from signal comprising the fundamental as well as third, fifth, seventh, ... harmonics in the correct proportions.

In exact terms it is necessary that the signal must be evaluated over an infinite time for the transformation to hold exactly. However in reality it is sufficient to know that the waveform is continuous over a period of at least a few seconds, or understand the effects of changing the signal.

It is also worth noting that the mathematical Fourier transformation also accommodates the phase of the signal. However for many testing applications the phase information is not needed and considerably complicates the measurements and test equipment. Also the information is normally not needed, and only the amplitude is important.

By being able to look at signals in the time domain provides many advantages and in particular for RF applications, although audio spectrum analyzers are also widely used. Looking at signals in the frequency domain with a spectrum analyzer enables aspects such as the harmonic and spurious content of a signal to analyzed. Also the width of signals when modulation has been applied is important. These aspects are of particular importance for developing RF signal sources, and especially any form of transmitter including those in cellular, Wi-Fi, and other radio or wireless applications. The radiation of unwanted signals will cause interference to other users of the radio spectrum, and it is therefore very important to ensure any unwanted signals are kept below an acceptable level, and this can be monitored with a spectrum analyzer.

Spectrum analyzer basics

There are many different types of RF test equipment that can be used for measuring a variety of different aspects of an RF signal. It is therefore essential to choose the right type of RF test equipment to meet the measurement requirements for the particular job in hand.

Test Instrument Type Frequency measurement Intensity / amplitude measurement Application
Power meter N Y Use for accurate total power measurements
Frequency counter Y N Used to provide very accurate measurements of the dominant frequency within a signal
Spectrum analyser Y Y Used primarily to display the spectrum of a radio frequency signal. Can also be used to make power and frequency measurements, although not as accurately as dedicated instruments
RF network analyser Y Y Used to measure the properties of RF devices

Properties of RF measuring instruments in common use

The spectrum analyzer is able to offer a different measurement capability to other instruments. Its key factor is that it is able to look at signals in the frequency domain, i.e. showing the spectrum, it is possible to see many new aspects of the signal.

An analyser display, like that of an oscilloscope has two axes. For the spectrum analyser the vertical axis displays level or amplitude, whereas the horizontal axis displays frequency. Therefore as the scan moves along the horizontal axis, the display shows the level of any signals at that particular frequency.

This means that the spectrum analyser, as the name indicates analyses the spectrum of a signal. It shows the relative levels of signals on different frequencies within the range of the particular sweep or scan.

The display format for a spectrum analyser
General format of the display on a spectrum analyzer

In view of the very large variations in signal level that are experienced, the vertical or amplitude axis is normally on a logarithmic scale and is calibrated in dB in line with many other measurements that are made for signal amplitudes. The horizontal scale conversely is normally linear. This can be adjusted to cover the required range. The term span is used to give the complete calibrated range across the screen. Terms like scan width per division may also be used and refer to the coverage between the two major divisions on the screen.

By Ian Poole

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