How to use a Spectrum Analyzer

- key essentials and tutorial about using a spectrum analyzer: how to use it to make radio frequency tests and measurements.

Spectrum 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 using spectrum analysers it is possible to make measurements of the bandwidth of modulated signals can be checked to discover whether they fall within the required mask. Another way of using 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 analyser display

A key element of using a spectrum analyser is in understand in 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.

Images showing a spectrum analyzer being used

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.

An image of a spectrum analyser display when it is being used to display a signal spectrum
Typical spectrum analyser display

Setting the spectrum analyzer frequency

When using a spectrum analyser, one of the first settings is that of the frequency.

Dependent upon the spectrum analyser in use, there are various ways in which this can be done:

  1. Using centre frequency:   Using this method, 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 display.

  2. Using upper and lower frequencies:   Another option that is available on most spectrum analysers 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

In order to maintain the correct signal levels when using a spectrum analyser, there are two main gain controls available. Their use needs to be balanced to ensure the optimum performance is obtained.

  • RF Attenuator:   as the name implies this control provides RF attenuation in the RF section. It is actually placed before the RF mixer and serves to control the signal level entering the mixer.
  • IF Gain:   The IF Gain control controls the level of the gain within the IF stages of the spectrum analyser after the mixer. It enables the level of gain to be controlled to allow the signal to be positioned correctly on the vertical scale on the display.

The two level controls must be used together. If the signal level at the mixer is too high, then this stage and further stages can become overloaded. However if the attenuation is set too high and additional IF gain is required, then noise at the input is amplified more and noise levels on the display become higher. If these background noise levels are increased too much, they can mask out lower level signals that may need to be seen. Thus a careful choice of the relevant gain levels within the spectrum analyzer is needed to obtain the optimum performance

Filter bandwidths

Other controls on the spectrum analyzer determine the bandwidth of the unit. There are two main controls that are used:

  • IF bandwidth:   The IF filter, sometimes labelled as the resolution bandwidth adjusts the resolution of the spectrum analyzer in terms of the frequency. Using a narrow resolution bandwidth is the same as using a narrow filter on a radio receiver. Choosing a narrow filter bandwidth or resolution on the spectrum analyzer will enable signals to be seen that are close together. It will also reduce the noise level and enable smaller signals to be seen.
  • Video bandwidth:   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.

Adjustment of the IF or resolution bandwidth and the video filter bandwidths on the spectrum analyzer has an effect on the rate at which the analyzer is able to scan. The controls should be adjusted together to provide a scan that is as accurate as possible as detailed below.

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. The slower the scan, obviously the longer it takes for the measurements to be made. As a result, there is always the need to ensure that the scans are made as fast as reasonably possible.

However the rate of scan of the spectrum analyzer is limited by a number of factors:

  • IF filter bandwidth:   The IF bandwidth or resolution bandwidth has an effect on the rate at which the analyzer can scan. The narrower the bandwidth, the slower the filter will respond to any changes, and accordingly the slower the spectrum analyzer must scan to ensure all the required signals are seen.
  • Video filter bandwidth:   Similarly the video filter which is used for averaging the signal as explained above. Again the narrower the filter, the slower it will respond and the slower the scan must be.
  • Scan bandwidth:   The bandwidth to be scanned has a directly proportional effect on the scan time. If the filters within the spectrum analyzer determine the maximum scan rate in terms of Hertz per second, it follows that the wider the bandwidth to be scanned, the longer the actual scan will take.

Normally the processor in the spectrum analyzer will warn if the scan rate is too high for the filter settings. This is particularly useful as it enables the scan rate to be checked without undertaking any calculations.

Also if the scan appears to be particularly long, an initial wide scan can be undertaken, and this can be followed by narrower scans on identified problem areas.

Hints and tips

There are several hints and tips for using a spectrum analyser to its best effect.

  • Beware input level:   IIn order to ensure the optimum performance of the system, the input is normally connected to the primary mixer with only the input attenuator control, often labelled RF level, between them. Accordingly RF can be applied directly to the mixer with no protection. It is therefore very important to ensure that the input is not overloaded and damaged. One major and expensive cause of damage on spectrum analysers is the input mixer being blown when the analyser is measuring high power circuits.
  • Determine if spurs are real:   One aspect of using a spectrum analyser that will often be encountered is the spurious signals are often viewed. Sometimes these may be generated by the item under test, but it is also possible that they can be generated by the analyser. To check if they are generated by the item under test, reduce the input sensitivity of the analyser by 10dB for example. If the spurious signals fall by 10dB then they are generated by the unit under test, if they fall by more than 10dB then they are generated by the analyser and possibly as a result of overloading the input.
  • Wait for self alignment:   When a spectrum analyser is first switched on, not only does it go through its software boot-up procedure, but most also undertake a number of self-test and calibration routines. In addition to this, elements such as the reference oven controlled crystal oscillator oven need come up to temperature and stabilise. Often manufacturers suggest that fifteen to thirty minutes before it can be used reliably. The crystal oscillator may take a little longer to completely stabilise, but refer to the manufacturers handbook for full details.
  • Power measurement:   While the accuracy of a spectrum analyser making power measurements is not as accurate as power meter in terms of absolute accuracy. However it should be remembered that both test instruments make slightly different power measurements. A power meter will make a measurement of the total power within the bandwidth of the sensor head - essentially it will measure the power regardless of the frequency. A spectrum analyser will make a measurement of the power level at a specific frequency. In other words it can make a measurement of the carrier power level, for example, without the addition of any spurious signals, noise, etc. While the absolute accuracy of a spectrum analyser is not quite as good as that of a power meter, they are improving all the time and the difference in accuracy is generally small.

By Ian Poole

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