# Multiplication of complex signals in GRC

**URL:** <https://www.ruby-forum.com/t/multiplication-of-complex-signals-in-grc/240964>\
**Category:** GNU Radio\
**Created:** [May 8, 2014, 9:00am UTC](https://www.ruby-forum.com/t/multiplication-of-complex-signals-in-grc/240964 "2014-05-08T09:00:41Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\
**Post date:** [May 8, 2014, 9:00am UTC](https://www.ruby-forum.com/t/multiplication-of-complex-signals-in-grc/240964/1 "2014-05-08T09:00:41Z")

</div>

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??

---

<div class="post-metadata">

**Author:** ![ali\_hanif](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ac91a4/32.png) [@ali\_hanif](https://www.ruby-forum.com/u/ali_hanif)\
**Post date:** [May 8, 2014, 10:20am UTC](https://www.ruby-forum.com/t/multiplication-of-complex-signals-in-grc/240964/2 "2014-05-08T10:20:38Z")

</div>

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

---

<div class="post-metadata">

**Author:** ![ali\_hanif](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ac91a4/32.png) [@ali\_hanif](https://www.ruby-forum.com/u/ali_hanif)\
**Post date:** [May 8, 2014, 10:42am UTC](https://www.ruby-forum.com/t/multiplication-of-complex-signals-in-grc/240964/3 "2014-05-08T10:42:09Z")

</div>

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

---

<div class="post-metadata">

**Author:** ![ali\_hanif](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ac91a4/32.png) [@ali\_hanif](https://www.ruby-forum.com/u/ali_hanif)\
**Post date:** [May 8, 2014, 10:54am UTC](https://www.ruby-forum.com/t/multiplication-of-complex-signals-in-grc/240964/4 "2014-05-08T10:54:29Z")

</div>

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