Editing Third-order intercept point (section)

==Definitions==
Two different definitions for intercept points are in use:

* ''Based on [[harmonic]]s'': The device is tested using a single input tone. The nonlinear products caused by ''n''-th-order nonlinearity appear at ''n'' times the frequency of the input tone.
* ''Based on [[intermodulation product]]s'': The device is fed with two sine tones one at <math>f_1</math> and one at <math>f_2</math>.  When you cube the sum of these sine waves you will get sine waves at various frequencies including <math>(2f_2-f_1)</math> and <math>(2f_1-f_2)</math>.  If <math>f_1</math> and <math>f_2</math> are large but very close together then <math>(2f_2-f_1)</math> and <math>(2f_1-f_2)</math> will be very close to <math>f_1</math> and <math>f_2</math>. This two-tone approach has the advantage that it is not restricted to broadband devices and is commonly used for radio receivers.

[[Image:interceptpoint.png|thumb|right|250px|Intercept point definition]]
The intercept point is obtained graphically by plotting the output power versus the input power both on [[logarithmic scale]]s (e.g., [[decibel]]s). Two curves are drawn; one for the linearly amplified signal at an input tone frequency, one for a nonlinear product.
On a logarithmic scale, the function ''x<sup>n</sup>'' translates into a straight line with slope of ''n''. Therefore, the linearly amplified signal will exhibit a slope of 1. A third-order nonlinear product will increase by 3&nbsp;dB in power when the input power is raised by 1&nbsp;dB.

Both curves are extended with straight lines of slope 1 and ''n'' (3 for a third-order intercept point). The point where the curves intersect is the intercept point. It can be read off from the input or output power axis, leading to input (IIP3) or output (OIP3) intercept point respectively.

Input and output intercept point differ by the small-signal gain of the device.

{{anchor|IP5||IP7|IP9}}
[[File:3rd order intermod animation (thumbnail).png|frame|link=File:Two_carrier_3rd_order_intermod_measurement.png#/media/File:Two_carrier_3rd_order_intermod_measurement.png|Third-order intermodulation products (D3 and D4) are the result of nonlinear behavior of an amplifier. The input power level into the amplifier is increased by 1&nbsp;dB in each successive frame. The output power of the two carriers (M1 and M2) increases by about 1&nbsp;dB in each frame, while the third-order intermodulation products (D3 and D4) grow by 3&nbsp;dB in each frame. Higher-order intermodulation products (5th order, 7th order, 9th order) are visible at very high input power levels as the amplifier is driven past saturation. Near saturation, each additional dB of input power results in proportionally less output power going into the amplified carriers and proportionally more output power going into the unwanted intermodulation products. At and above saturation, additional input power results in a ''decrease'' in output power, with most of that additional input power getting dissipated as heat and increasing the level of the non-linear intermodulation products with respect to the two carriers.]]