Understanding modulations with GNURadio

Our friend Prof. Dr. Hans-Hellmuth Cuno DL2CH once gave me an old diploma thesis of one of his students. In his thesis the student demonstrated with a transmitter setup that it was possible to transmit a signal in a way such that it could be received with both an SSB receiver and an FM receiver. This method is called Real Zero SSB.

After discussions around GNURadio we came to the conclusion that such a setup should be rather simple with SDR. I have not yet been able to do that, but I found out that using GNURadio it is very easy to understand modulations.

FM Simulator

Here is my FM simulator. You can play around with the AF volume and the AF frequency. Although GNURadio cannot display everything in real time it is extremely valuable. Go and use very slow AF frequencies of 0.5 Hz so that the signal becomes nicely visible in the FFT screen. Then play around with the volume. You will see the deviation change on the FFT display. Then choose higher frequencies. Then choose 480k instead of 48k samp_rate and repeat the procedure.

Note: When you reproduce everything in your GRC canvas, make sure to use samp_rate at all places where you can see 48k!