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Example 5: Third-Order Intermodulation
Distortion
What Is Intermodulation Distortion?
In crowded communication systems, signal interference of one device
with another is a common problem. For example, two-tone, third-order
intermodulation often is a problem in narrow-band systems. When two
signals (F
and F
) are present in a system, they can mix with the
1
2
second-order harmonics that are generated (2F
third-order intermodulation distortion products, which are located close
to the original signals at 2F
intermodulation distortion can also occur. These distortion products are
generated by such system components as amplifiers and mixers.
Spectrum Analyzer Functions Used
The following procedure describes how to measure third-order
intermodulation distortion. It shows how to tune two signals onto the
spectrum analyzer display and demonstrates setting the resolution
bandwidth, mixer level, and reference level. It also incorporates several
marker functions.
Stepping through the Measurement
Set up the test
1. To test a device for third-order intermodulation, connect the
equipment as shown in Figure 2-21 on page 76. This example uses a
6 dB directional coupler, and two sources set to 20 MHz and 21 MHz.
You may substitute other source frequencies, but for this example,
maintain a frequency separation of about 1 MHz. The device under
test is a 26 dB preamplifier. The low-pass filters are required to stop
the signal from the two sources from causing distortion due to
reflecting the signal. If you are performing this procedure only for
practice using the instrument, you don't need the low-pass filters.
Chapter 2
Example 5: Third-Order Intermodulation Distortion
1
−F
−F
and 2F
. Higher order
2
1
1
2
Making Measurements
and 2F
), creating
2
75
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