Understanding amplitude level accuracy in new generation Spectrum Analyzers

24 June 2013
Understanding amplitude level accuracy in new generation Spectrum Analyzers
When specifying the amplitude level performance of a spectrum analyzer, there are many factors which can affect the
performance and hence many parameters that must be specified and considered when selecting the right analyzer. In this
paper we will examine the fundamental sources of level accuracy errors, and then look at how they are specified and how
they affect real world measurements. This will include new hardware architectures from Anritsu that significantly improve
the amplitude level accuracy.
The importance of amplitude level accuracy is to enable the spectrum analyzer to measure the power level of a signal to
the best accuracy. This could be for the purpose of measuring:
• The power level of a single signal that is part of a more complex wide band modulation scheme (e.g. the power
of a single carrier in an OFDM signal),
• The power of a specific narrow band signal that is “isolated”, e.g. an oscillator or amplifier harmonic spur
• The total power of all signals within a given channel bandwidth (e.g. total power in a receiver)
These are all examples where a power meter would not normally be used to measure power level, as the frequency selectivity
of a spectrum analyzer is needed where a wide band power meter/sensor would not be suitable. For all of these examples,
it is important that the spectrum analyzer is correctly specified in terms of level accuracy to make an accurate and reliable
measurement. In particular the effects on level accuracy of changing frequency and bandwidth, or changing input signal
level, must be well understood. Measuring precisely the power of sub-carriers in a complex modulation scheme, or the
power level of harmonic spurs in a transmitter, requires many settings for the bandwidth and input attenuator on a spectrum
analyzer, and these all can affect the level accuracy. This in turn will affect the uncertainty in confirming the correct operation
and specification of a device or in the worst case could cause a non-compliant device to be measured as “OK” and pass a
quality inspection.
When specifying the level performance of a spectrum analyzer, the two areas most often considered are the amplitude level
sensitivity (the lowest level of signal that can be measured) and the amplitude level accuracy (the level of accuracy on the
actual measured value).
Understanding amplitude level accuracy in new generation Spectrum Analyzers #2

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