Mathematical Principle
Heterodyning is based on the trigonometric identity:
The product on the left hand side represents the multiplication ("mixing") of a sine wave with another sine wave. The right hand side shows that the resulting signal is the difference of two sinusoidal terms, one at the sum of the two original frequencies, and one at the difference, which can be considered to be separate signals.
Using this trigonometric identity, the result of multiplying two sine wave signals, and can be calculated:
The result is the sum of two sinusoidal signals, one at the sum f1 + f2 and one at the difference f1 - f2 of the original frequencies
Read more about this topic: Heterodyne
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