Its wide dynamic range makes the spectrum analyzer the test instrument of choice for measuring

harmonic distortion, intermodulation distortion, adjacent channel power ratio, spurious-free dynamic

range, gain compression, etc. Distortion measurements such as these are bounded on one side by

the noise floor of the spectrum analyzer and on the other side by the signal power strength at which

the spectrum analyzer’s internally generated distortion masks the distortion being measured. The

simultaneous low noise floor and low internally generated distortion products uniquely qualify the

spectrum analyzer for making distortion measurements.

Having wide dynamic range and accessing this dynamic range are two different things. Unless

the user is given enough information on how to optimize the spectrum analyzer to make distortion

measurements, its dynamic range performance cannot fully be exploited. Many distortion measure-

ments are very straightforward: measure the fundamental tone power, measure the distortion product

power, and compute the difference. Problems arise when the device under test has distortion product

levels that approach the internally generated distortion product levels of the spectrum analyzer.

Further complications arise when trying to maximize speed and minimize measurement uncertainty. In

these cases more care in the measurement technique is required.

The search for information on making distortion measurements begins with the spectrum analyzer

data sheet. The data sheet is most useful for comparing one spectrum analyzer against another in its

dynamic range capability and the relevant measurement uncertainties in the distortion measurement.

What the data sheet fails to convey is how to configure the spectrum analyzer to achieve the specified

dynamic range performance.

Primers are another source of information. Two excellent references are [1] and [2] listed on page 39

of this document. Primers such as these provide the necessary fundamental knowledge for making

distortion measurements. Yet, primers treat spectrum analyzers as a general class of test instru-

mentation. In order to make truly demanding distortion measurements accurately or less demanding

measurements more quickly, the user needs product specific information.

This product note bridges the gap between primers and data sheets, focusing on distortion mea-

surements using the Keysight Technologies, Inc. performance spectrum analyzer (PSA) series (model

E4440A). Part I is a self-contained section for making the less demanding distortion measurement

quickly using the auto-coupled settings found in the PSA. Part II guides the user in setting the

appropriate power at the input mixer in order to maximize the dynamic range for carrier wave or

continuous wave (CW) measurements. Part III explains the measurement of distortion measurements

on digitally modulated signals. Part IV details some of the internal architecture of the PSA as it relates

to distortion measurements. Finally, Part V describes some measurement techniques, both internal

and external to the PSA, that yield more accuracy in certain kinds of distortion measurements.

Introduction