Understanding Spectrum Analyzer Specifications

- key facts about spectrum analyzer specifications or specs - what they mean and how to interpret them for buying new or as used test equipment or obtaining one from a test equipment rental company.

This spectrum analyzer tutorial is split into several pages each of which address different aspects of spectrum analyser operation and technology:

Spectrum analyzers are an essential item of test equipment for any RF design or test laboratory. They enable a view of the spectrum of an RF signal to be seen and in this way they provide a unique insight into the way an RF circuit or piece of equipment is working. However choosing the correct spectrum analyzer to purchase, either as new equipment or as used test equipment or possibly to hire from a test equipment rental company is not always easy. A knowledge of spectrum analyzer specifications is needed to be able to make the right choice for any given application.

There is a wide variety of different types of spectrum analyzer ranging from an audio spectrum analyzer for low audio frequencies right through to RF spectrum analyzers capable of measuring frequencies up to many GHz. For each category there are many different types for a variety of different manufacturers. In addition to the manufacturers offering a wide variety of instruments, used test equipment suppliers as well as test equipment rental companies will hold stocks of a wide variety of instruments. Accordingly it is necessary to be able to judge which might be most suitable by consulting the spectrum analyzer specifications.

Frequency coverage

The frequency coverage of the spectrum analyser is one of the basic specifications or parameters. When determining whether a particular spectrum analyzer is suitable for the application, it is necessary to consider the maximum frequencies that will need to be viewed. It is worth remembering that the maximum frequency to be viewed should include the harmonics and intermodulation products of the wanted signals.

Amplitude accuracy

The amplitude accuracy is a major spectrum analyzer specification. While the accuracy of a spectrum analyzer itself will not match that of a dedicated power meter for example, the accuracy of the individual level measurements need to be accurate to enable useful measurements to be made.

The amplitude accuracy specification of a spectrum analyzer is determined by a number of factors, including the basic accuracy of the instrument as well as its frequency response. This means that the frequency elements should also be taken into consideration. Often accuracy levels of the order of ± 0.4 dB are achievable.

For microwave spectrum analyzers a YIG oscillator is normally used. As YIGs are highly non-linear devices, and as a result the amplitude accuracy specification figures will be less (typically ± 1 dB) when the YIG oscillator is used.

Some spectrum analyzers incorporate a power meter which operates with the analyzer to provide a very accurate measurement specification. For this, the spectrum analyser has a special power sensor that calibrates the input level at a number of absolute level points, then uses the very good linearity of the analyser to very accurately measure levels over the full range which may be in excess of 100dB.

Frequency accuracy specification

Most spectrum analyzers today employ frequency synthesized sources. This means that the accuracy of the frequency measurement is governed by that of the peak detection circuitry, detecting where the centre of a signal is, and also the accuracy of the reference source within the frequency synthesizer.

Spectrum analyzers can be used as extremely accurate frequency counters with relatively high specifications. They locate a signal and track it, simultaneously with measuring it's absolute frequency. This can be particularly advantageous in many applications.

Spectrum analyzer sensitivity specification

In order to determine the low signal performance of spectrum analyzer a sensitivity specification is normally given. This is normally specified in terms of dBm / Hz at a given frequency.

If a noise figure specification is required, then this can be calculated:

If a further improvement in the sensitivity or noise figure specification is required, then it is possible to add a low noise pre-amplifier.

Phase noise specification

There are many instances when the phase noise of a signal source, e.g. a transmitter, receiver local oscillator, etc needs to be measured. When this is the case, the phase noise specification of the spectrum analyzer is of particular importance. It should be better than the signal source being measured, typically by at least 10 dB for it not to affect the readings being made. For these applications, the spectrum analyser specification for phase noise needs to be carefully considered.

Techniques apart from a straight measurement can be sued to improve the operation of the spectrum analyzer. These techniques include a noise correction process, where the noise of the spectrum analyzer is subtracted from the measurement. For higher performance it is possible to utilise cross-correlated phase noise measurements where the spectrum analyzer is effectively able to remove the phase noise of its internal local oscillators from the measurement. This process allows phase noise measurements to be made below the physical thermal limit, i.e. better than -174dBm/Hz.

Spectrum analyzer dynamic range

Dynamic range is a particularly important parameter for any spectrum analyzer. This type of test equipment is normally used on a logarithmic scale and is required to look at signals with enormously wide level ranges. Therefore the ability of the spectrum analyzer to accurately look at small signals in the presence of relatively close strong signals is particularly important.

Summary

Spectrum analyzer specifications need to be understood when buying an analyser. It is necessary to ensure that the spectrum analyser will be able to meet its requirements and fulfil the role expected for it. While most spectrum analyzer specifications mean the instruments perform to a high level, it is necessary to ensure that it is right for the job, and not over-specified, costing more than it should.

Further pages from this tutorial
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