Spectrum Analyzer Types

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

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.
    Read more about the Swept spectrum analyser
  • 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. Read more about the FFT spectrum analyser
  • Real-time analysers:   These test instruments are a form of FFT analyser. One of the big issues with the initial FFT analyser types was that they took successive samples, but with time gaps between the samples. This gave rise to some issues with modulated signals or transients as not all the information would be captured. Requiring much larger buffers and more powerful processing, realtime spectrum analyser types are able to offer the top performance in signal analysis. Read more about the Real-time spectrum analyser
  • 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 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 test instruments and this technology is able to operate at frequencies up to many GHz. However it 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 / swept instrument 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 be 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.

By Ian Poole

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