Definitions
Two different definitions for intercept points are in use:
- Based on harmonics: 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 products: The device is fed with two sine tones with a small frequency difference. The n-th order intermodulation products then appear at n times the frequency spacing of the input tones. This two-tone approach has the advantage that it is not restricted to broadband devices and is commonly used for radio receivers.
It is worth noticing that these definitions differ by 4.8 dB (10 log10 3), so care should be taken when using existing equations, models or measurement data.
The intercept point is obtained graphically by plotting the output power versus the input power both on logarithmic scales (e.g., decibels). 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 xn 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 dB in power when the input power is raised by 1 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 or output intercept point, respectively (IIP3/OIP3).
Input and output intercept point differ by the small-signal gain of the device.
Read more about this topic: Third-order Intercept Point
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