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.