Third-order intercept point

The intercept point, shortly IP ( dt intersection ) comes from the electronic measurement technology and provides a non- directly measurable quantity for characterizing the nonlinear properties of transmission elements (eg enhancers ) in communications engineering dar.

Emergence Basics

The approach is based on the approximation of the non-linear transfer characteristic (e.g., the amplifier ) by the mathematical method of the Taylor series. Upon excitation of the system by several additively superimposed sinusoidal oscillations at the input new additional vibration frequencies result from the higher orders of ( Taylor ) power series at the output corresponding to the addition theorems. This process is known also as a mixture.

The newly generated frequencies result from the equation

Where the sum of and equal to the order of each term of the power series and thus the order of the intermodulation is.

For example, the quadratic term ( second order ) is responsible for the generation of the frequencies

Definition

The intercept point is on the power of the excitation oscillation, wherein the oscillation on the output would be generated artificially achieve the same performance. However, this intersection does not exist actually, but is determined by extrapolation of the two curves. The results from the same effects saturation, limiting or compression of the transmission member leaves the curves before reaching the intercept point bend.

And an output IP3

The intercept point can be related to the input or the output of the transmission link:

• IIP3 is the designation for input IP3 and thus refers to the input power.
• The name stands for OIP3 Output IP3 and thus refers to the output power.

Furthermore:

Mathematically, the input IP3 is (neglecting higher orders ) 9.63 dB above the 1 dB compression point. The name comes from the English name TOI third-order intercept point.

Intercept points of higher order

In addition, various IP can be specified depending on the considered order. These can be separated by measurement, because the frequencies are different artificially generated.

In practice, usually following IP are given

• IP2 ( second order ) characterizes the demodulation to baseband
• IP3 ( third order ) characterizes the superficial intermodulation properties
• IP5: (fifth order ) refines based on the IP3 information

Metrological provision of IP3

To determine the IP3 (Device Under Test DUT, engl. ) Are next to the component to be tested two signal generators and a spectrum analyzer or signal analyzer is required. The adjusted frequencies and signal strength (both test signals have the same amplitude) is dependent on the particular DUT. It must be ensured that the DUT is not already gone into compression. The sinusoidal signals of the signal generators are brought together on the DUT. At the output of the DUT, the amplitudes of the intermodulation products are then measured with the signal analyzer.

In logarithmic measurements ( dB) of the OIP3 is calculated as follows:

With P1 = output level of the useful signal in dBm and P3 = output level on the 3rd order intermodulation product in dBm. IMA13 is the intermodulation distance between P3 and P1 in dB.

The IIP3 is calculated as

In which

P1out with the power of the excitation signal at the output and P1in the signal strength of the same signal at the input of the DUT.