Re: [Discuss-gnuradio] Multiply block problem in GRC 3.6.5.1

[Kevin Reid]

On Mar 27, 2014, at 15:10, Winderson Luiz Franzoi Speranzini <address@hidden> 
wrote:

> Hello GRC community,
> I'm initiating with grc, trying to make a simple BASK. I searched a lot in 
> the mailing list and in the tracker (I don't know if it's called that way) 
> but I was unable to find anything about. It's a simple problem (maybe due my 
> GRC version) that does not ruin my projects at all.
> When I use the multiply block, inputing a cossine(12kHz) and a 
> rectangular(1kHz) wave from the signal generator block, I don't get the nice 
> BASK that we should (Viewed from a wx osciloscope). Instead, the cossine 
> change the amplitude in the middle 3 cycles and last longer 3 other cycles 
> than it should.

I wasn't sure myself what a complex square wave actually ought to be, so I took 
a look at the source code and found this statement:

        /* Implements a real square wave high from -PI to 0.
         * The imaginary square wave leads by 90 deg.
         */

This would explain your observation, I think: the complex square wave provided 
by GR is zero only 1/4 of the time. It probably isn't what you want.

> The fact is that it just happen when I use complex outputs and inputs. Both 
> the I and Q wave have the same style. When I use real inputs, or if I 
> separete the real and imag part, multiply they separately and then put 
> together in a complex signal, it works like it should be. Maybe it can be an 
> old and corrected bug

GR's multiply_cc block follows the rules of complex arithmetic as it should. 
Complex multiplication is not component-wise multiplication.

To get the result I think you had in mind, then you should multiply the complex 
sinusoid by a _real_ square wave. That is, set your square wave signal 
generator to have a float output, convert the output to complex (leaving the 
imaginary input unconnected, implicitly zero), then multiply it with the sine.

-- 
Kevin Reid                                  <http://switchb.org/kpreid/>