In telecommunications, a third-order intercept point (IP3 or TOI) is a measure for weakly nonlinear systems and devices, for example receivers, linear amplifiers and mixers. It is based on the idea that the device nonlinearity can be modeled using a low-order polynomial, derived by means of Taylor series expansion. The third-order intercept point relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal, in contrast to the second-order intercept point that uses second order terms.
The intercept point is a purely mathematical concept, and does not correspond to a practically occurring physical power level. In many cases, it lies far beyond the damage threshold of the device.
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... The third-order intercept point (TOI) is a property of the device transfer function O (see diagram) ... plus a new harmonic term, cos(3ωt), the third-order ... intersect), we find that this happens when which is the Third-Order Intercept Point (TOI) ...
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