I'm trying to understand way which Frequency Mod block works in gnuradio. According to my intuition there should be at least two inputs:

  • signal to encode
  • base frequency / carrier signal

Instead there are some mysterious input signal and sensitivity parameter.

It seems that I don't got the main point of it, can you fix my thinking please.


1 Answer 1


The FM modulator in GnuRadio (as well as other modulator blocks) work in baseband: the carrier frequency is 0 Hz.

Maybe you should recall that, when working with SDR, you work with complex-valued samples. This means that positive and negative frequencies are distinguishable, and having a carrier frecuency of 0 Hz is perfectly valid.

When you send this signal to an actual SDR hardware, you specify at which frequency will the baseband signal be modulated.

So, to sum up, the carrier frequency is specified in the SDR (source or sink) blocks. The FM modulator only needs the input samples (real-valued), and the sensitivity (which is proportional to $f_\Delta$, according to the documentation).

  • $\begingroup$ I see my mistake now. I wanted to put output to audio sing, but for it I have to convert complex to float (not by taking Re) how can i do it? $\endgroup$
    – Hieronim
    Dec 2, 2019 at 15:05
  • $\begingroup$ If you want to listen to FM as audio, multiply by a complex exponential to the carrier frequency, and then take the real value. $\endgroup$
    – Juancho
    Dec 2, 2019 at 15:10

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