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Voyager
Voyager
527 Views
Registered: ‎06-26-2015

complex mult

question for DSP expert, why, when I do complex mult. the result only show 1 freq.  but when doing a real * real cos wave, my freq show all 4, sum and difference of freq.

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Mentor jmcclusk
Mentor
467 Views
Registered: ‎02-24-2014

Re: complex mult

consider the case of multiplying 2 complex frequencies:

 

Product =  exp( i * F1)  *  exp( i * F2)  =  exp( i * (F1 + F2))

 

In the complex case, the frequencies simply add together,  When you have only real data, then you see the sum and difference frequencies...  i.e.

 

 

2 * Cosine( F1 ) * Cosine (F2) =  Cosine( F1 + F2) +  Cosine(F1 - F2)

 

 

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