# Multiplication of complex signals in GRC

When we multiply two complex signals in GNU Radio companion and see the
result using WX FFT block then there is no frequency difference
component(f1-f2) but only the addition component(f1+f2)…why is that??

On 08.05.2014 08:59, ali hanif wrote:

When we multiply two complex signals in GNU Radio companion and see the
result using WX FFT block then there is no frequency difference
component(f1-f2) but only the addition component(f1+f2)…why is that??

That is actually the mathematically correct thing.

M

Can you plz explain Martin or give me a link atleast…thnx in advance

I think you are confused because you’re doing the math on paper with
real sin/cos functions. The complex signal sources produce complex
oscillations ($s_f(t)=e^{j2\pi f t}$), which do not have symmetrical
spectrum.
Do the math with the exponential function; it get’s easier than what you
have to do to prove the real intermodulation signals, and you clearly
see that $s_a s_b = s_{a+b}$.

Greetings,
Marcus