Spectrum Analyzer Types

- overview and notes about the different types of spectrum analyser with their advantages and disadvantages for different applications.

Spectrum analyzer tutorial includes:

Just as in the case of other instruments, there are a number of types of spectrum analyzer that can be seen in the manufacturers catalogues.

The different types of spectrum analyzer are used in different applications - some are more appropriate for some applications, while others are more appropriate for others.

Additionally there are some significant differences in cost between the different types of analyser, making

Summary of spectrum analyser types

There are a number of different categories for spectrum analyzers. Some of the main spectrum analyser types are noted below:

• Swept or superheterodyne spectrum analysers:   The operation of the swept frequency spectrum analyzer is based on the use of the superheterodyne principle, sweeping the frequency that is analysed across the required band to produce a view of the signals with their relative strengths. This may be considered as the more traditional form of spectrum analyser, and it is the type that is most widely used.
• Fast Fourier Transform, FFT analysers:   These spectrum analyzers use a form of Fourier transform known as a Fast Fourier Transform, FFT, converting the signals into a digital format for analysis digitally. These analysers are obviously more expensive and often more specialised.
• Audio spectrum analyzer:   Although not using any different basic technology, audio spectrum analyzers are often grouped differently to RF spectrum analyzers. Audio spectrum analyzers are focussed, as the name indicates, on audio frequencies, and this means that low frequency techniques can be adopted. This makes them much cheaper. It is even possible to run them on PCs with a relatively small amount of hardware - sometimes even a sound card may suffice for some less exacting applications.

Spectrum analyser advantages and disadvantages

Both swept / superheterodyne and FFT spectrum analyzer technologies have their own advantages. The more commonly used technology is the swept spectrum analyser as it the type used in a general-purpose analysers enabling these analyzers to operate up to frequencies of many GHz. However a swept frequency analyser is only capable of detecting continuous signals, i.e. CW as time is required to capture a given sweep, and they are not able to capture any phase information.

FFT analyzer analyser technology is able to capture a sample very quickly and then analyse it. As a result an FFT analyzer is able to capture short lived, or one-shot phenomena. They are also able to capture phase information. However the disadvantage of the FFT analyzer is that its frequency range is limited by the sampling rate of the analogue to digital converter, ADC. While ADC technology has improved considerably, this places a major limitation on the bandwidths available using these analyzers.

In view of the fact that both FFT and superheterodyne analyzer technologies have their own advantages, many modern analyzers utilise both technologies, the internal software within the unit determining the best combinations for making particular measurements. The superheterodyne circuitry enabling basic measurements and allowing the high frequency capabilities, whereas the FFT capabilities are introduced for narrower band measurements, and those where fast capture is needed. An analyzer will often determine the best method dependent upon factors including the filter settling time and sweep speed. If the spectrum analyser determines it can show the spectrum faster by sampling the required bandwidth, processing the FFT and then displaying the result, it will opt for an FFT approach, otherwise it will use the more traditional fully superheterodyne / sweep approach. The difference between the two measurement techniques as seen by the user is that using a traditional sweep approach, the result will seen as sweep progresses, when an FFT measurement is made, the result cannot be displayed until the FFT processing is complete.

These different types of spectrum analyzer technology are described in more detail in further pages of this tutorial.

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