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Re: FM Transmission with a vector of bits
From: |
Anthony B. |
Subject: |
Re: FM Transmission with a vector of bits |
Date: |
Mon, 10 Aug 2020 13:51:19 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 |
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
- Re: FM Transmission with a vector of bits,
Anthony B. <=