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Agilent AN 1316 Optimizing Spectrum Analyzer Amplitude Accuracy Application Note RF & Microwave Spectrum Analyzers Table of Contents 3 Introduction 3 Absolute versus relative measurements 4 Factors affecting measurement uncertainty 4 Sources of uncertainty 5 Relative amplitude uncertainty factors 7 Absolute amplitude uncertainty factors 8 Calculating total uncertainty 8 Examples 13 Errors 13 Overall amplitude accuracy 14 Reducing overall uncertainty 16 Summary 16 References Introduction Absolute versus relative measurements This application note covers the factors affecting Amplitude measurements are either absolute the accuracy (or uncertainty) of amplitude meas- (Figure 1) or relative (Figure 2). Absolute ampli- urements made with spectrum analyzers, and tude is the power or voltage level of a signal. For explains how to calculate worst-case uncertainty in example, measuring the power level of a carrier in general situations. It will help you analyze a meas- dBm is an absolute measurement. Relative ampli- urement and decide what procedure to follow to tude is the difference between two levels, typically minimize overall uncertainty. Also, you will learn expressed in dB. Relative measurements use one the most important accuracy-related features and signal or signal component as a reference. For specifications to consider when choosing a spec- example, a harmonic distortion measurement uses trum analyzer. This note does not cover specialized the fundamental as the reference, and the harmon- measurement procedures for certain types of sig- ic is usually measured in dBc (decibels relative to nals that introduce additional sources of uncer- the carrier level). Most spectrum analyzer applica- tainty, such as noise, pulsed RF, and EMI. Refer to tions involve relative amplitude measurements. the References for additional reading. Figure 1. Absolute measurement (one signal level Figure 2. Relative measurement (difference between measured with a single marker) two signal levels measured with the delta marker) 3 Factors affecting measurement uncertainty Let's look first at the factors affecting measure- Table 1 lists the amplitude uncertainty factors of a ment uncertainty. Later sections will show how to typical spectrum analyzer. The range of values for calculate the uncertainty of an amplitude measure- each factor covers a variety of spectrum analyzers. ment by considering each of these factors, and how Most spectrum analyzers have specifications for both to obtain the best accuracy for a measurement. absolute and relative uncertainties