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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, 6 Jul 2020 10:02:46 +0200

Hello Barry,

Thanks for your links at 2)

Regarding 1) I have been able to adapt it to my case, I now have an output signal at 48 kS/s at the output of the rational resampler with my data bit sequence.

For 3), I indeed try to translate these vectors into FM : in fact, I have the bit sequence formatted with PCM format and after it's been modulated with the signal source, the next step is to pass it to FM modulation so on two frequencies in either L or S band. That's now what I am willing to achieve with several blocks you also mentioned :)



Le sam. 4 juil. 2020 à 00:05, Barry Duggan <barry@dcsmail.net> a écrit :
Hi Anthony,

To answer your questions, here are some references:
* https://wiki.gnuradio.org/index.php/Tutorials
* https://github.com/duggabe/gr-morse-code-gen

1) Morse Code is an on-off keying method. The vectors represent each dot
bit time as a 1 and each space bit time as a 0. To transmit this data,
the RF carrier is turned on and off with these vectors. In the example,
the Signal Source block simulates the RF carrier and the Multiply block
keys it on and off. Note that in my github link I have revised the
filter to create a better keying wave shape.

2) Study the Tutorials and the Sample Rate Tutorial in particular to get
a better understanding of setting sample rates.

3) Now to address your desire to translate vectors into FM, I am
guessing that you are looking at RadioTeletype (RTTY). To do that, or a
similar application, you must key two frequencies with the vectors. For
RTTY, the standard tones are 2295 for Mark (1) and 2125 for Space (0).
There are various modulator blocks to use, or you could use a "brute
force" method with two signal sources.

GNU Radio allows you to experiment with various arrangements easily. So
study the Tutorials and try things out!

Barry Duggan KV4FV

On Fri, 3 Jul 2020 17:56:26 +0200, Anthony wrote:

Hello Kevin,

Thanks, your link had useful information about the principles to
consider with a vector source.

Considering the following generic flowgraph : Vector Source > Repeat >
Multiply (with a sine wave) > Resampling / Filtering > Output (?)

I have seen that most of the variables are related to the Morse code
transmission with the words per minute. However, I was wondering about
how to tune the interpolation rate of the Repeat block after the Vector
source for a known bit sequence, but out of the Morse standard. I would
also expect some kind of periodic signal, if not an audio signal, at the
input of the FM transmitter (I guess). What would you expect as a clean
signal at the input of the FM transmitter (output here), if not an audio
signal ?

I am also wondering about how to set the cut off frequency and
transition width of the low pass filter in these flowgraphs. Is it by
hand, or is there a relationship with the flowgraph variables ?

Thanks for your feedback!


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