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

On Mar 27, 2014, at 15:10, Winderson Luiz Franzoi Speranzini <soules_nt@hotmail.com> 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/>


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