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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. Since relative
uncertainties affect the accuracy of both relative and
Sources of uncertainty absolute measurements, let's look first at those fac-
The simplified block diagram in Figure 3 shows tors affecting relative measurement uncertainty.
the elements of a typical spectrum analyzer. Some
of these elements can contribute uncertainty to Table 1. Amplitude Uncertainty Factors
amplitude measurements. These contributions are Relative