Re: [Discuss-gnuradio] Simple GMSK encoder and decoder with very large bandwidth

[Marcus Müller]

Hey Fred,

nice to have you around!

Could you maybe post a screenshot of the frequency plot so that we can evaluate what you mean with "huge"?

I think you might not be realizing that DSP like GNU Radio does it doesn't care about the number you set as sampling rate – i.e. a GMSK symbol in the configuration you're using of the GMSK mod will always be 2 samples long. And that is what defines the shape in the spectrum. So, the frequency axis in your frequency plot is really just -bandwidth;+bandwidth _labeled_.

Best regards,

Marcus

On 04/20/2017 07:19 PM, Fred Castello wrote:

Hi there,
My goal here is to ultimately develop a telemetry system but I am starting very simply by building a GMSK encoder using a file sync fed directly to a decoder.  It seems to be working as I can see the constellation diagram (which eventually goes away-dont know why) and on the scope sync I can see the phase transitions but in the FFT sync it appears that the bandwidth is huge.  From what I understand, I would not want to filter the results going from the modulator to the demodulator, but I may not understand that correctly.  I know I have a long way to go, but I am just trying to learn as I go and try to figure out the theory and see how it applies as I build using GNU radio.  Thank you anyone for your help!!  I am including the Flow Graph as well as the test file that I am using for the file sync. Much appreciated.
Fred

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