Bill wrote:
*IMD is caused by the odd order terms of the polynomial ( output
voltage/input voltage) of a device. The harmonics of these can terms ( 3rd,
5th etc harmonics) can easily be filtered but the near by IMD products
cannot since they are KHz or less from each other. Many think IMD is due to
frequency mixing of harmonics but it is not, the additional frequencies are
created in the device with out the aid of harmonics.
......................................................................
Just to set the record straight: IMD products ARE the mixing terms of the the
"harmonics" with the "fundamental(s) (correct for the lowest order IMDs)".
For example for a two tone case, F1 and F2, the first close-in-band IMDs are:
3*F2squared-F1 and 3*F1squared-F2, (F2squared is the 2nd harmonic of F2, and so
on....). Ignoring the amplitude term, this leads to the 3rd order IMD products:
2F2-F1 and 2F1-F2.
The harmonics are part of the picture and are generated when the fundamentals
are raised to the higher power(s) of the binomial expansion.
The lowest order IMDs are created when the harmonics are mixed with the
fundament(s). For higher order IMD terms it is the mixing of also the harmonics
among themselves. These are the cross terms of the binomial expansion.
Only the cross term of the quadratic gives the desired beat products because
none of the terms are raised to a higher power (n=1) (no harmonics are
involved). All the other cross terms of the binomial are part of the IMD
baggage.
So harmonics are part of the picture in generating IMDs.
Wonder why we use a "switch" for mixing instead of a true multiplier?
Well that another whole story.
kb3bf, chris
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