Ceyear 4041 Series, User Manual

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Chapter IV Spectrum Analyzer Mode
39
Press
【
Marker
】
.
At this time, the amplitude readings of noise marker are correct, because the average values of all points are based
on the same frequency, and will not be affected by the resolution bandwidth filter shape. The noise marker is
obtained based on the average calculation of interested frequency points. When it is required to measure the power
of discrete frequency points, the spectrum analyzer should be tuned to the interested frequency points first, and
then be measured at the zero span.
4.1.7 Distortion measurement
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 a system has two signals (F
1
and F
2
), they mix with the generated
second harmonic distortion signals (2F
1
and 2F
2
) to generate the third-order intermodulation products, 2F
2
-F
1
and
2F
1
-F
2
, which are close to the original signals. High-order intermodulation distortion also occurs. Most of these
distortion products are generated by devices such as amplifiers and mixers in the system. Most transmitting sets
and signal generators have harmonic waves, whose components should be measured usually.
1) Recognition of distortion generated by the spectrum analyzer
When a large signal is input in the spectrum analyzer, the spectrum analyzer will distort, affecting the distortion
measurement results of the true signal. Through the attenuator settings, you can determine which signal is the
distorted signal generated inside the spectrum analyzer. This instance will use the input signal generated by the
signal generator to explain if the spectrum analyzer generates the harmonic distortion.
a) Set the signal generator output signal:
Set the signal generator frequency at 200 MHz and signal power at 0 dBm, connect the signal generator output to
the input port of spectrum analyzer, as shown in Fig 4-1, and turn on the RF switch.
b) Set the center frequency and span of the spectrum analyzer:
Press
【
Preset
】
.
Press
【
Freq
】
, [Center Freq], 400 [MHz].
Press [Span], [Span] and 500 [MHz].
As known from traces on the spectrum analyzer, the harmonic distortion generated by the signal has a 200 MHz
frequency offset from the original 200 MHz signal, as shown in Fig 4-11.
c) Set the center frequency of the spectrum analyzer to the position of the first harmonic distortion.
Press
【
Peak
】
, [Next Pk].
Press
【
Peak
】
, [Marker→Center Frequency].
d) Set the span to 50 MHz, and reset the center frequency.
Press [Span], [Span] and 50 [MHz].
Press [Marker→], [Marker→ Center Frequency].