| How to use a Spectrum Analyzer Tutorial |
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.
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