I have been enjoying an older Elements of Radio Servicing publication. Free download at:
http://www.antiqueradios.com/archive.shtml
Top is AM signal and in time watching max voltage setting numbers on bar below wave forms you will see a slow rise and fall of blue modulating wave. ABOVE and BELOW the steady 1 volt 600 kHz carrier frequency.
Bottom circuit is FM signal. Click on green 78 kHz AC source (modulator). Watch frequency changes rise ABOVE and BELOW the 80 kHz carrier frequency. (I made the carrier and modulator waves close in frequency to show frequency changes in simulator. If this was 400 hz. modulation the FM signal would have to rise above and fall below 80 kHz. 400 times a second.
