Spectrum analyser basics

- an overview or tutorial about the basics of a spectrum analyser: what one is; how to use a spectrum analyser; and how one works.

A spectrum analyzer is a very important item of test equipment for someone designing or repairing electronic equipment that uses radio frequency signals. The spectrum analyzer has some similarities to an oscilloscope because it provides a graphical display. However rather than giving a display of amplitude and time like an oscilloscope, the spectrum analyzer gives a display of frequency against the amplitude (typically power).

Essentially a spectrum analyser scans a band of frequencies set by the user, and notes the signal levels, displaying them on a screen. In this way it is possible to obtain a plot of the signals appearing in any band of frequencies.

Uses for Spectrum analyzers

Spectrum analyzers are normally complicated pieces of equipment which take a little while to get used to using. However after a little familiarisation, they can become very powerful tools for testing and RF equipment. The spectrum analyzer can be used for a number of tasks:

• Looking at the frequency spectrum of a signal to see items like the following:



• The overall spectrum of a modulated signal to see whether it is wide enough or too narrow, etc



• To investigate whether any spurious or unwanted signals are present



• To find out whether a signal is on the right frequency, and not in another band for example.



• To investigate general problems with a signal. Often it can just help looking at a signal to see what a problem is. With RF signals a spectrum analyzer can prove to be the eyes for the person investigating the problem.



• Sometimes spectrum analyzers can be used to measure power, although they are not as accurate as proper power meters.



• Sometimes spectrum analyzers can be used to measure frequency, although they are not always as accurate as frequency counters

Spectrum analyzer controls

There are a number of different controls and interfaces on a spectrum analyser. Although these instruments may appear to be complicated, it is possible to make good use of them after a little practice as it is necessary to use the controls correctly.

• The display   The purpose of a spectrum analyzer is to provide a plot or trace of signal amplitude against frequency. The display has a graticule which typically has ten major horizontal and ten major vertical divisions.
  
  The horizontal axis of the analyzer is linearly calibrated in frequency with the higher frequency being at the right hand side of the display. The vertical axis is calibrated in amplitude. This scale normally uses a logarithmic scale, although it is often possible to have other scales including linear ones. This is because it enables signals over a much wider range to be seen on the spectrum analyser. Typically a value of 10 dB per division is used. This scale is normally calibrated in dBm (i.e. decibels relative 1 milliwatt) and therefore it is possible to see absolute power levels as well as comparing the difference in level between two signals.




• Setting the frequency   To set the frequency of a spectrum analyser, there are two selections that can be made:
  
  
  • Centre frequency
  
  
  • Span
  
  
  
  The two selections are independent of each other and made on two separate controls. The centre frequency selection sets the frequency of the centre of the scale to the chosen value. It is normally where the signal to be monitored would be located. In this way the main signal is in the centre of the display and the frequencies either side can be monitored. The span selection on the spectrum analyzer is the extent of the frequency coverage that is to be viewed or monitored. The span may be given as a bandwidth per division on the graticule, or the total span that is seen on the calibrated part of the screen, i.e. within the maximum extents of the calibrations on the graticule. Another option that is often available is to set the start and stop frequencies of the scan. This is another way of expressing the span as the difference between the start and stop frequencies is equal to the span.




• Gain and attenuation adjustments   There are many other controls on a spectrum analyser. Most of these fall into one of two categories. The first is associated with the gain or attenuation of sections within the spectrum analyzer. If sections are overloaded, then spurious signals may be generated within the instrument. If this occurs then false readings will be given. To prevent this happening it is necessary to ensure that the input stages in particular are not overloaded and an RF attenuator is used. However if too much attenuation is inserted, additional gain is required in the later stages (IF gain) and the background noise level is increased and this can sometimes mask lower level signals. Thus a careful choice of the relevant gain levels within the spectrum analyzer is needed to obtain the optimum performance.
  
  
• Scan rate   The spectrum analyser operates by scanning the required frequency span from the low to the high end of the required range. The speed at which it does this is important. Obviously the faster it scans the range the faster the measurement can be made. However the rate of scan of the spectrum analyzer is limited by two other elements within the instrument. These elements are the filter that is used in the IF, and the video filter that may also be used to average the reading. These filters must have time to respond otherwise signals will be missed and the measurements rendered useless. Nevertheless it is still essential to keep the scan rate as high as is reasonably feasible to ensure that measurements are made as quickly as possible. Often the scan rate, and the filter bandwidths are linked within the instrument to ensure the optimum combination is chosen.




• Filter bandwidths  The other controls concern the filter bandwidths within the instrument. There are generally two types:
  
  
  • IF filter
  
  
  • Video filter
  
  
  
  The IF filter basic provides the resolution of the spectrum analyzer in terms of the frequency. Choosing a narrow filter bandwidth will enable signals to be seen that are close together. However by the very fact that they are narrow band these filters do not respond to changes as quickly as wider band ones. Accordingly a slower scan rate must be chosen when using them.
  
  The video filters enable a form of averaging to be applied to the signal. This has the effect of reducing the variations caused by noise and this can help average the signal and thereby reveal signals that may not otherwise be seen. Using video filtering also limits the speed at which the spectrum analyser can scan.
  
  When having to use narrow bandwidths and slow scan rates, the time that a measurement can be made by reducing the span that needs to be scanned. Even though a slow scan rate must be used, the range over which the scan must be made can be reduced, thereby reducing the scan time for the analyzer.

Although different spectrum analysers may have further controls, the ones mentioned above are the main ones and will enable a start to be made when using a spectrum analyzer.

Principle of a spectrum analyzer

In essence a spectrum analyzer is a form of radio receiver with a display at the output indicating the output level. The receiver is tuned or scanned over the required range and the filters are chosen to accept the required signal bandwidth.

The spectrum analyser uses the superhet principle used in many radio receivers as the underlying principle on which its operation depends. The superhet principle uses a mixer and in addition to a locally generated or local oscillator to translate the frequency.

The mixing principle used in the spectrum analyzer operates in exactly the same manner as it does for a superhet radio. The signal entering the front end is translated to another frequency, typically lower in frequency. Using a fixed frequency filter in the intermediate frequency section of the equipment enables high performance filters to be used, and the analyzer or receiver input frequency can be changed by altering the frequency of the local oscillator signal entering the mixer.

Summary

The spectrum analyzer is a particularly useful piece of test equipment for any radio frequency engineer. Although spectrum analyzers are normally expensive instruments to buy, they can occasionally be picked up on the second hand market and this would make them accessible to enthusiastic RF experimenters and radio hams. Although an essential item of equipment in many RF development labs where they are sued extensively, they are not normally found outside these environments.