Third-Order Intermodulation Distortion - Keysight X Series Measurement Manual

Third-Order Intermodulation Distortion

Two-tone, third-order intermodulation distortion is a common test in communication
systems. When two signals are present in a non-linear system, they can interact and
create third-order intermodulation distortion products that are located close to the
original signals. These distortion products are generated by system components such
as amplifiers and mixers.
This procedure tests a device for third-order intermodulation using markers. Two
sources are used, one set to 300 MHz and the other to 301 MHz. This combination of
signal generators and a directional coupler (used as a combiner) results in a two-tone
source with very low intermodulation distortion. Although the distortion from this
setup may be better than the specified performance of the analyzer, it is useful for
determining the TOI performance of the source/analyzer combination. After the
performance of the source/analyzer combination has been verified, the
device-under-test (DUT) (for example, an amplifier) would be inserted between the
directional coupler output and the analyzer input.
Step
1 Connect two sources to the
analyzer RF INPUT as
shown.
2 Set up the signal sources.
Action
a. Set the frequency of signal
generator #1 to 300 MHz.
b. Set the frequency of signal
generator #2 to 300.1 MHz.
c. Set signal generator #1 amplitude
to −5 dBm.
d. Set signal generator #2 amplitude
to −5 dBm.
67
Making Distortion Measurements
Third-Order Intermodulation Distortion
Notes
The coupler should have a high
degree of isolation between the
two input ports so the sources
do not intermodulate.
This produces a frequency
separation of 1 MHz.
Sets the sources equal in
amplitude as measured by the
analyzer.