Re: FM Transmission with a vector of bits

[Anthony B.]

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

vco_irig.grc
Description: XML document

RF Spectrum from the flowgraph.png
Description: PNG image

expected RF spectrum.png
Description: PNG image