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Third order intercept point is an hypothetical point where the power of third order components will reach to the same level of fundamental component's power.

If one draws the power input versus power output, they will observe the different frequency components having different slopes (fig1).

This Concept can be better understood by an example:

If one have following two frequency signals:

then the two, third order component which will be closer to fundamental components are:

Third order intercept point (IP3) is a hypothetical point where the power level of third-order harmonics will reach the power level of the fundamental signal. 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.

When the device is tested for a single input tone, the nonlinear products caused by n-th-order nonlinearity appear at n times the frequency of the input tone. That means, on a logarithmic scale, the function xn plotted on the graph of an n-th-order non-linearity translates into a straight line with a slope of n.

When the power input vs power output graphs are drawn (see graph below), one can observe the different frequency components having different slopes. The two curves are drawn - one for the linearly amplified signal at an input tone frequency and one for a nonlinear product. The linear response curve will have a slope of 1 because it is made using first-order components. The graph for third-order distortions will have a slope of 3 making it a cubic response curve. Once the device reaches to 1 dB compression point the two curves become parallel to each other as the device enters saturation and then never intersects.

The extrapolated point where the curves of the fundamental signal and third-order distortion product signal meet is called the Third-order Intercept Point (IP3). Hence, the third-order intercept point is a hypothetical point as it can only be plotted on a graph by extending the linear and cubic response slope lines to the point they intersect. This is shown in the figure above. At this point, the input power level is known as IIP3 (input IP3), and the output power when this situation occurs is known as the OIP3 (output IP3) point. IIP3 mentioned above is the 3rd order intercept point at the input of the amplifier for OIP3 (Output Third Order Intercept point).

The IP3 value can be co-related with linearity. Higher the IP3 value, the more linear the device is. It basically tells users who large a signal a device can handle before Intermodulation Distortion (IMD) occurs. So when looking for amplifiers or mixers for high linearity applications, the IP3 value can be a good indicator.

The output power of the nonlinear device when fed with input power having a strength equal to IIP3 level is known as OIP3. Ideally, OIP3 is usually about 10 dB higher than the P1dB gain compression point (i.e. OP1dB).

When evaluating Intermodulation Distortion (IMD) in the frequency domain, it can be seen in the figure below that the Third-order signals are the ones to worry about, as they can be very close to the fundamental signals. So IP3 is the point at which the Third-order products equal the fundamental signals.

Let us look at an example where f1 = 100 MHz and f2 = 110 MHz. Then the third harmonics will be found at 90 MHz (2 x f1 – f2) and 120 MHz (2 x f2 – f1). So IP3 represents the point at which all these four signals are equal in amplitude.

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