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Modulation specific question

Kim Mortensen
 

Hi,

First time on these groups, but find it my only outlet for a very Modulation specific question - But I have no clue where else to seek for help.

The question is regarding demodulation of DQPSK used in TETRA. I want to build a demodulator plugin, and have to start somewhere. I have made a C# application that can plot the I/Q vector as it whirls around in the constellation chart. However it looks sort of distorted, and i have used a simple low pass filter to remove unwanted frequencies. Question is do someone know what filter would be appropriate to filter a DQPSK properly, for better constellation.

Best Regards
Kim Mortesnen

Greg Ella
 

I'm guessing you are already familiar with the Osmocom project and their work with TETRA?

Greg Ella
N0EMP


On Fri, Feb 14, 2020 at 2:45 AM <kimfmx@...> wrote:
Hi,

First time on these groups, but find it my only outlet for a very Modulation specific question - But I have no clue where else to seek for help.

The question is regarding demodulation of DQPSK used in TETRA. I want to build a demodulator plugin, and have to start somewhere. I have made a C# application that can plot the I/Q vector as it whirls around in the constellation chart. However it looks sort of distorted, and i have used a simple low pass filter to remove unwanted frequencies. Question is do someone know what filter would be appropriate to filter a DQPSK properly, for better constellation.

Best Regards
Kim Mortesnen

Kim Mortensen
 

I am now (-:

It looks good, but it will not help me understand it fully, and I want to make the program to decode a wav file. I have done filtering and plotting on screen, but its not like i expected. the I and Q does not play well with eachother, so the plot is all over the place. im questioning if my wav file is ok for this test, as im not sure if the quality if good enough. but it was sampled when another program received tetra on that freq, so the information should be in there.

Best Regards
Kim Mortensen

Den fre. 14. feb. 2020 kl. 16.23 skrev Greg Ella <gregella53@...>:

I'm guessing you are already familiar with the Osmocom project and their work with TETRA?

Greg Ella
N0EMP

On Fri, Feb 14, 2020 at 2:45 AM <kimfmx@...> wrote:
Hi,

First time on these groups, but find it my only outlet for a very Modulation specific question - But I have no clue where else to seek for help.

The question is regarding demodulation of DQPSK used in TETRA. I want to build a demodulator plugin, and have to start somewhere. I have made a C# application that can plot the I/Q vector as it whirls around in the constellation chart. However it looks sort of distorted, and i have used a simple low pass filter to remove unwanted frequencies. Question is do someone know what filter would be appropriate to filter a DQPSK properly, for better constellation.

Best Regards
Kim Mortesnen