The carrier wave

The heart of most of the communications signals is a sine wave, normally of a frequency well above range of human hearing. This is known as carrier or carrier wave. The lowest carrier frequency used for the radio communications is 9 kHz. The highest frequency is in hundreds of gigahertz.

For the modulation to work effectively, carrier must should a frequency many times the largest frequency of the modulating signal. For instance, if you want to modulate the radio wave with hi-fi music, that has a frequency range from a few hertz to 20 kHz or so, the carrier wave should have a frequency above 20 kHz. A good rule is that carrier should have a frequency of at least 10 times the largest modulating frequency.  So for the good hi-fi music transmission, a radio carrier must be at 200 kHz or above.

This rule holds for all the types of modulation, whether it be of amplitude, phase, or frequency. If rule is violated, efficiency of transmission will be degraded, resulting below optimum data transfer.