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Multitone Intermodulation Distortion Analysis

Active devices such as amplifiers and mixers cause distortion when high power is applied to them. The distortion is a critical factor when two or more signals are injected into the unit, which will cause multiples of unwanted product appearing at higher power levels. This is called intermodulation. This product will appear at higher power levels. 3rd order intercept point (2 · F_{1} ± F_{2}, F_{1} ± 2 · F_{2}) is the most critical parameter. It falls in closely to fundamental signals (see Figure A1). 2nd order intercept (F_{1} ± F_{2}) sometimes is also important, especially for wideband application.

The intercept point is a theoretical value (cannot be measured but can be calculated) which determines the distortion of the unit at specific power levels. Output power is specified for amplifiers and Input power is specified for mixers (see Figure A2).

Practical Evaluation of Intermodulation Distortion

Often 2nd or 3rd order product can be measured with respect to the fundamental signal. Then the equation of intercept point becomes:

For example, what is third order intercept point (IP_{3}) for an amplifier with two equal output power levels of +10 dBm (P_{0} = +10)?

IM_{3} can be measured with a spectrum analyzer. Assuming IM_{3} is measured to be -25 dBm:

The simplest way to analyze IP_{3} is to set two equal signals at 0 dBm

The same analogy applies to mixers except that two equal signals are applied at the input and IM_{3} is measured at the output.