How to use a Spectrum Analyzer Tutorial

- a tutorial, article information about how to use a spectrum analyzer to make radio frequency tests and measurements.

Spectrum analysers or analyzers are an invaluable item of electronic test equipment used in the design, test and maintenance of radio frequency circuitry and equipment. Spectrum analysers, like oscilloscopes are a basic tool used for observing signals. However, where oscilloscopes look at signals in the time domain, spectrum analyzers look at signals in the frequency domain. Thus a spectrum analyser will display the amplitude of signals on the vertical scale, and the frequency of the signals on the horizontal scale.

In view of the way in which a spectrum analyzer displays its output, it is widely used for looking at the spectrum being generated by a source. In this way the levels of spurious signals including harmonics, intermodulation products, noise and other signals can be monitored to discover whether they conform to their required levels. Additionally spectrum analysers can make measurements of the bandwidth of modulated signals can be checked to discover whether they fall within the required mask. Another application of a spectrum analyzer is in checking and testing the response of filters and networks. By using a tracking generator - a signal generator that tracks the instantaneous frequency being monitored by the spectrum analyser, it is possible to see the loss at any given frequency. In this way the spectrum analyser makes a plot of the frequency response of the network.

Spectrum analyzer

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. Although there is normally the possibility of selecting a linear or logarithmic scale, for most applications a logarithmic scale is chosen. 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 (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. Similarly when using a linear scale is used, this is often calibrated in volts to enable absolute measurements to be made using the spectrum analyzer.

Setting the frequency

To set the frequency of a spectrum analyser, there are two selections that can be made. These are independent of each other. The first selection is the centre frequency. As the name suggests, this 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 and the regions either side can be monitored. The second selection that can be made on the analyzer is the span, or the extent of the region either side of the centre frequency that is to be viewed or monitored. The span may be give as a given frequency per division, 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.

Adjusting the gain

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 give. 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 filter scan rate, and the filter bandwidths are linked to ensure the optimum combination is chosen.

Filter bandwidths

The other controls concern the filter bandwidths within the instrument. There are generally two types, namely the IF and video filters.

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.

Summary

Spectrum analyzers are very useful tools for anyone looking at radio frequency signals. They are somewhat more complicated than many other instruments to use, but with a little familiarity they can be used to display a wide variety of useful information.