Understanding Spectrum Analyzer Specifications

- key spectrum analyzer specifications or specs - what the different parameters mean and how to interpret them for buying new or as used test equipment.

Spectrum analyser specifications are used to determine whether a particular test instrument will be able to meet the requirements placed upon it.

There are several different specifications, each detailing different aspects of the performance of the instrument.

When choosing a spectrum analyser, it is necessary to use the data sheet detailing all of the specifications to determine whether the instrument will be able provide the required results.

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.

Normally the frequency range must be such that harmonics of the fundamental and other important spurious signals can be viewed. To achieve this the analyser frequency range must extend well beyond the fundamental frequency of the signal. Often the figure used is ten times that of the fundamental, although often it is necessary to settle for top frequencies of less than this, especially for RF applications where the very high frequencies required may place a significant cost penalty on achieving this specification.

The lowest frequency specification may also be important. As spectrum analysers are often AC coupled, there will be a lower cut-off point. This should be checked.

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:

Noise Figure   =   Sensitivity (dBm/Hz)   -   Noise floor at room temp (-174 dBm/Hz)

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.

By Ian Poole

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