Use the "Frequency Mod" block as a frequency modulator to make things simpler.
The GUI Sink picture you show could be right. You're multiplying a 360 kHz square wave by another slower square wave and looking at the first harmonic. On Mon, Aug 10, 2020 at 7:54 AM Anthony B. <[email protected]> wrote: > Hello, > > Thanks for your answer. I've taken a break on this issue and now going > back to it. > > As you describe it, I created a Python class that generates my binary > sequence (which is a PCM Major Frame based on IRIG 106). Then I set the > data of the Vector Source to my binary sequence so that it moves it to > the downstream blocks. I don't really know if it's currently the right > way to go, as I am trying to get a feeling of the bitrate that goes over > the flowgraph and it just outputs as fast as it can. I therefore use a > Repeat Block to slow down the samples stream and knowing the sample rate > of the blocks downstream, I can get an idea of how much bits are > processed (that's what I am trying to apply). > > In order to transmit it to a USRP device using FM modulation, I mostly > got inspiration from the RTTY transmitter that is based on the same > idea. It is described as an example of the VCO use here : > > https://wiki.gnuradio.org/index.php/VCO > > In my case, I settled the coefficients before the VCO in order to obtain > a 350 kHz deviation from the NRZ-L sequence I generated (-1 or +1). I > can visualize this deviation in the Spectrum plot after the VCO. > However, I am quite far from the theoretical spectrum from the IRIG 106 > standard I'd get in a PCM/FM transmission. It is said that the optimal > deviation should be 0.35 * bitrate, however knowing the current bitrate > is kinda challenging in GNU Radio and in the theoretical spectrum it is > based on 1Mbps. > > Currently I am trying to set the parameters in order to have a better > spectrum shape at the output, but I don't succeed to get something > workable. In my tests I am using a Vector Source feeding the downstream > blocks either with -1 or +1 as my binary sequence have. > > Would you have suggestions on how to proceed ? > > I've attached my current flowgraph, the Spectrum plot and the result I > am trying to converge to. > > Thanks for your answers ! > > Anthony > > > >
