Chapter 2 67
Making Measurements
Example 4: Harmonic Distortion
Example 4: Harmonic Distortion
What Is Harmonic Distortion?
Most transmitting devices and signal sources contain harmonics.
Measuring the harmonic content of such sources is frequently required.
In fact, measuring harmonic distortion is one of the most common uses
of a spectrum analyzer. Harmonic distortion can be checked very
quickly by using the measurement routine described below. This
measurement routine measures harmonic amplitudes relative to the
source frequency.
Spectrum Analyzer Functions Used
The following harmonic distortion measurement applies an important
group of spectrum analyzer operating skills: setting the frequency span
using start and stop frequencies; setting the video bandwidth; and
making relative measurements using two markers. It also
demonstrates setting a signal to center frequency using a marker, and
setting the frequency step size to the value of the center frequency.
There are two common ways to measure harmonic distortion using a
spectrum analyzer. Procedure A illustrates the faster method, which
permits a simultaneous display of the fundamental and its harmonics.
Procedure B takes longer to perform but provides a better
measurement of harmonics closer to the noise floor.
Stepping through the Fast Harmonic Measurements:
Procedure A
This example measures the harmonic content of a 1 MHz signal
provided by a signal generator. You may use the 300 MHz calibrator
signal of the spectrum analyzer, but be sure to adjust the spectrum
analyzer start and stop frequencies to accommodate the source
frequency and its harmonics.
1. Connect the signal source to the spectrum analyzer INPUT 50 Ω.
Press
PRESET to start the procedure from a preset state.
Measure the 1 MHz fundamental and its first two harmonics
1. Set the start frequency to 450 kHz.
2. Set the stop frequency to 3.5 MHz.
This displays the fundamental frequency and the second and third
harmonics as shown in Figure 2-15 on page 68.
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