**IMD3** refers to the intermodulation distortion from the third order 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
×
f
2
−
f
1
and
2
×
f
1
−
f
2
. If
f
1
and
f
2
are large but very close together then
2
×
f
2
−
f
1
and
2
×
f
1
−
f
2
will be very close to
f
1
and
f
2
.