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Re: FM Transmission with a vector of bits

From: Barry Duggan
Subject: Re: FM Transmission with a vector of bits
Date: Fri, 3 Jul 2020 17:05:05 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

Hi Anthony,

To answer your questions, here are some references:
* https://wiki.gnuradio.org/index.php/Tutorials
* https://wiki.gnuradio.org/index.php/Sample_Rate_Tutorial#Sink_hardware_example
* 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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