36
/
78
In the change of input attenuation, the absolute amplitude of
△
Mkr1 is about ≥1dB, it can be
assumed that at least part of this harmonic distortion component is generated by the spectrum
analyzer (as shown in Figure 3-17). The input attenuation should be adjusted up in this case.
Figure
3-17 Component of Harmonic Distortion
The reading of amplitude difference of
△
Mkr1 is affected by the following factors:
1. Increasing the input attenuation will deteriorate the signal-to-noise ratio, and that causes the reading
of
△
Mkr1 to positive.
2. Losses in the internal circuitry of the spectrum analyzer for harmonics will cause
△
Mkr1 to read
negative.
A large reading of
△
Mkr1 indicates a significant test error. It can be minimized by input the
attenuation.
Third-Order Intercept Distortion
Measurement of two-tone third-order intermodulation distortion is common in communication systems.
When two signals are input to a nonlinear system, they may interact with each other to produce a third
intermodulation component (TOI) adjacent to the original signal in the spectrum. These distortions are
generated by system component such as the amplifier and mixer.
To quickly set up TOI measurement, see “Third-Order Intercept” for more details.
This example describes how to use a marker to measure the third-order intermodulation distortion of an
instrument. The frequency of the two signal sources used are 299.95 MHz and 300.05MHz.
1. As shown in Figure 3-18 to connect an instrument. Connect the input port of the spectrum
analyzer via low pass filter and directional coupler, the output of the directional coupler is
two-tone signal source with low intermodulation distortion.
Although the distortion performance with this connection is better than the spectrum analyzer,
but the TOI for measuring the signal source/spectrum analyzer combination is still useful.
After the TOI performance of the signal source/spectrum analyzer combination has been
calibrated, the instrument under test (DUT) (e.g., amplifier) is inserted between the output of
the directional coupler and the input of the spectrum analyzer.
