What is IP3?

RF Basics 
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Anjali lahoti - Keysight Technologies

Dec 8, 2017

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). 

So, Third order intercept point is an ideal point as once the device reaches to 1 dB compression point the two curves will become parallel to  each other and they will never cut. Which shows that the power for fundamental and third order component will not be same. but this parameter is very important in terms of characterizing a device. 

Fig 1 : Third order intercept point for Nonlinear devices

If the results will be analyzed on spectrum analyzer, the frequency components will look as shown below:

Fig 2 : Frequency components versus power

This Concept can be better understood by an example:

If one have following two frequency signals:

f1 = 100 MHz
f2 = 110 MHz

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

2*f1-f2 = 90 MHz 
2*f2-f1 = 120 MHz

So, third order intercept point will be the point where all these four frequency component will be at same power level