In one of my classes I have learned that if we look at transmitting a binary signal with ASK, we can get the bandwidth (i.e. the width of the main lobe in the frequency domain) with BW = 2 * Fb, where Fb is the bit-rate (i.e. 1/T, where T = 'length' of one bit).
I am a little confused as to how this BW <-> Fb relation comes to be. Intuitively I understand that if we assume a carrier frequency of 2GHz and transmit a signal with a bitrate of 5 bits/s, we simply modulate the amplitude of our 2GHz carrier signal, without any frequency changes. Why, then, do we have a lobe in the frequency domain with a width that is greater than 1? As I understand it, there is only one frequency, the 2GHz signal?.. I know my understanding is wrong, but intuitively I do not understand how we have a variance in frequency, independently of what our bitrate is. How is modulating the amplitude of a 2GHz signal 5 times a second different from doing it 20 times a second when viewed in the frequency domain?