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Can I view every signal that has been transmitted using software defined radio as a amplitude modulated signal? For example I could make a frequency modulated baseband signal in GNU Radio and connect it to Adalm Pluto sink. Since analog part of Pluto SDR involves frequency mixing does it mean on top of FM I get AM?

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Every signal transmitted by an SDR is a quadrature signal: the sum of two amplitude-modulated signals, with carriers that are in quadrature (one is at 90 degrees from the other).

A bit of trigonometry shows that this is equivalent to an amplitude and phase modulated signal; in other words, a quadrature signal is equivalent to $A(t)\cos(2\pi f_c t + \phi(t))$.

As long as you can express your signal with one of these two (equivalent) expressions, you can do anything! You can certainly do FM on top of AM if you want to. For many of these "exotic" signals, the challenge will be in figuring out a demodulation process, but don't let that stop you from experimenting.

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