Ceyear 4041 Series, User Manual

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Chapter IV Spectrum Analyzer Mode
34
Fig 4-8 Relative measurement of internal modulation distortion
g) Calculation method of third-order intercept point TOI:
The ratio of the distortion component level to the signal level (in dB) does not specify the distortion of system
significantly, unless the signal level is also specified. The concept of the intercept point may be used to specify
and pre-estimate the distortion level of the system, and the difference between the third-order distortion
component level and the fundamental wave signal level is twice the difference between the fundamental wave
signal level and the third-order intercept point. Thus, the third-order intercept point TOI can be calculated as per
the formula shown below:
2
dB
L TOI
3 im
in

 
Where:
in
L
: represents the two input signal levels, dBm.
3 im
dB

: represents the difference between the third-order intermodulation product and the input signal
level, dB.
As shown in Fig 4-8, when the differential marker reading is -59.37 dB and the signal level is -20.0 dBm, the
input third-order intercept point TOI is:
TOI= -20- (-59.37/2)= 9.685(dBm)
4.1.5 Measurement of drift signal
1) Definition of drift signal
If the signal to be detected is a drift signal, it can be observed only when the center frequency is changed
continuously at different time when a spectrum analyzer is applied for measurement. If the signal tracking
function of the spectrum analyzer is applied, the marker peak will be always at the center frequency of the
spectrum analyzer, which is convenient for measurement.
This section will introduce how to measure the drift signal. The signal tracking, marker function and maximum