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Fig. 4-8 Relative Measurement of Internal Modulation Distortion
g) Calculation of third-order interception (TOI) point:
The ratio (in dB) of the distortion component level to signal level has no great significance for system distortion,
unless the signal level is specified. The interception point can be applied to specify and evaluate the system
distortion level. The difference between the third-order distortion component level and fundamental signal level is
twice of that between the fundamental signal level and third-order interception point. The third-order interception
(TOI) point can be calculated by the following formula:
2
dB
LTOI
3im
in


Where:
in
L
: Refer to two input signal levels, in dBm.
3im
dB
: means the difference between the third-order IM product and input signal level, in dB.
As shown in Fig. 4-7, if the differential marker reading is -47.30dB and the signal level is -23.0dBm, the
third-order interception (TOI) point is:
TOI= -22.2- (-51.11/2)=0.65(dBm)