Third-order intercept point

In telecommunications, a third-order intercept point (IP3 or TOI) is a specific figure of merit associated with the more general third-order intermodulation distortion (IMD3), which is a measure for weakly nonlinear systems and devices, for example receivers, linear amplifiers and mixers. It is based on the idea that the device nonlinearity can be modeled using a low-order polynomial, derived by means of Taylor series expansion. The third-order intercept point relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal, in contrast to the second-order intercept point that uses second-order terms.

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.

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 one at

f

1

{\displaystyle f_{1}}

and one at

f

2

{\displaystyle f_{2}}

. When you cube the sum of these sine waves you will get sine waves at various frequencies including

(

2

f

2

−

f

1

)

{\displaystyle (2f_{2}-f_{1})}

and

(

2

f

1

−

f

2

)

{\displaystyle (2f_{1}-f_{2})}

. If

f

1

{\displaystyle f_{1}}

and

f

2

{\displaystyle f_{2}}

are large but very close together then

(

2

f

2

−

f

1

)

{\displaystyle (2f_{2}-f_{1})}

and

(

2

f

1

−

f

2

)

{\displaystyle (2f_{1}-f_{2})}

will be very close to

f

1

{\displaystyle f_{1}}

and

f

2

{\displaystyle f_{2}}

.