Intermodulation distortion (IM) consists of the spurious signals which
result from the mixing of nth order frequencies in the non-linear
elements of a component. Third order intermodulation distortion is of
particular interest because third order products typically represent the
highest level distortion appearing close to the desired signal, and as
such the highest level non-filterable distortion. Third order IM level
(IM3) is tested by injecting two pure tones of equal magnitude (f1 and
f2) into the component to be tested. The third order IM products will
appear in the output spectrum at the frequencies 2f1-f2 and 2f2-f1.
These products are characterized by defining their level (in dBc)
relative to the fundamental output tones at either f1 or f2.|
For a mixer, two-tone third-order intermodulation distortion products
are described by (2fR2-fR1) + fLO and (2fR1-fR2) + fLO. The higher
the third-order intercept point and conversion compression points of a
mixer, the lower will be the intermodulation for given input signal levels.
See also Third Order Intercept Point