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.

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).