Ceyear 4041 Series, User Manual

Page: 43 / 203

Chapter IV Spectrum Analyzer Mode
31
Press [Limit Test Off On] to enable the limit test function. If the adjacent channel power ratio goes above the set
limit, its background on the screen will turn red.
4.1.4 Third-order intermodulation distortion measurement
1) Definition of third-order intermodulation distortion
In the crowded working environment of communication system, mutual interference of devices is an ubiquitous
problem. For example, second-order intermodulation distortion and third-order intermodulation distortion
commonly occur in narrow-band systems. When there are two signals (F
1
and F
2
) in one system, the two signals
will be mixed with the generated second harmonic distortion signals (2F
1
and 2F
2
), generating the third-order
intermodulation products (2F
2
-F
1
and 2F
1
-F
2
) fairly close to the original signal. High-order intermodulation
distortion also occurs. Most of these distortion products are generated by devices such as amplifiers and mixers in
the system.
The following section introduces the measurement method of third-order intermodulation distortion. This section
illustrates the way to display two signals on the screen of spectrum analyzer simultaneously and introduces the
way to set the resolution bandwidth, mixer level and reference level, as well as the use of some marker functions.
2) Measurement procedure
a) Connect the instrument under test (as shown in Fig 4-5) to the spectrum analyzer:
A 6 dB directional coupler and two signal generators set to 1 GHz and 1.001 GHz respectively are used in this
example. Certainly, the frequency of signal generator may also be set to other values, but the frequency interval
must be kept at 1 MHz approximately in this example.
Spectrum Analyzer
1GHz
1.001GHz
50 Ω RF input
Directional
coupler
Signal generator 1
Signal generator 2
Fig 4-5 Connection of third-order intermodulation distortion measurement system
Set the output frequency of one signal generator to 1 GHz and set the output frequency of the other signal
generator to 1.001 GHz, such that the interval of two signal frequencies inputted to the spectrum analyzer is 1
MHz.
Set the output amplitudes of the two signal generators to the same value (-20 dBm in this example).
b) Set the spectrum analyzer to allow the two signals to be displayed on the screen of spectrum analyzer
simultaneously;
Press
【
Preset
】
.
Press
【
Freq
】
, [Center Freq] and 1.0005 [GHz] successively.