IMD3

IMD3 refers to the intermodulation distortion from the third harmonic of a circuit. It is of particular importance in narrow band signals because its distortion is often in the signal band. This can be seen by looking at a signal that is made up of two sine waves, one at $f\_1$ and one at $f\_2$. When you cube the sum of these sine waves you will get sine waves at various frequencies including $2\; imes\; f\_2-f\_1$ and $2\; imes\; f\_1-f\_2$. If $f\_1$ and $f\_2$ are large but very close together then $2\; imes\; f\_2-f\_1$ and $2\; imes\; f\_1-f\_2$ will be very close to $f\_1$ and $f\_2$.