Agilent Technologies E8267C PSG Bedienungsanleitung PDF herunterladen (Seite 4)

How is IMD Characterized?

Overview

IMD is measured by examining the output of a device under test (DUT) with a spectrum

analyzer while the DUT is being stimulated with a multitone test signal, Figure 2. For

example, two-tone intermodulation distortion is a common measurement performed

when characterizing the nonlinear behavior of an amplifier. Two discrete tones with

equal power, that fall within the passband of the DUT, are applied to the input of the

DUT. The resultant harmonic and intermodulation distortion products are then measured

using a spectrum analyzer. A spectrum analyzer with wide dynamic range is desirable to

measure low-level distortion products, like 5th and 7th order IMD and harmonics.

Based

on these measurements, figures of merit like third order intercept point (TOI) can be

determined.

Figure 2. Traditional setup to measure IMD products using a two-tone test stimulus.

As the bandwidths of devices and systems increase, multitone signals are commonly used

to characterize nonlinear distortion. Placing multiple tones across the bandwidth of the

DUT enables more efficient characterization of the passband and also stresses the DUT

with a higher crest factor signal.

Dependence on phase relationships

IMD measured at a specific frequency varies widely depending on the phase relationships

of the tones that comprise the test signal. You might expect this when you consider that

the phase distribution of the tones determines how they add up at each point in time,

Figure 3. The time domain profile of the composite signal has a different peak to average

ratio depending on whether the individual tones add constructively or destructively.

1. The Agilent PSA Performance Spectrum

Analyzer Series was used for all measure-

ments made in this document. The PSA was

chosen because it provides a leading edge

combination of dynamic range, speed and

accuracy. For more information on the PSA

series of spectrum analyzers, including how

to optimize the PSA distortion measurement

capability, refer to the related literature

section at the end of this document.

DUT Spectrum analyzer

Signal generator #1

Signal generator #2

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Agilent Technologies E8267C PSG Bedienungsanleitung Seite 4