Sum and Difference Frequencies

When you superimpose two sine waves of different frequencies, you get components at the sum and difference of the two frequencies. This can be shown by using a sum rule from trigonometry. For equal amplitude sine waves

The first term gives the phenomenon of beats with a beat frequency equal to the difference between the frequencies mixed. The beat frequency is given by

since the first term above drives the output to zero (or a minimum for unequal amplitudes) at this beat frequency. Both the sum and difference frequencies are exploited in radio communication, forming the upper and lower sidebands and determining the transmitted bandwidth.

When you say that the beat frequency is f₁-f₂ rather than (f₁-f₂)/2, that requires some explanation. For the difference frequency you can just say that you get a minumum when the modulating term reaches zero, which it does twice per cycle, so that the number of minima per second is f₁-f₂.

Carriers and Sidebands

Radio transmission involves putting audio frequency information on a much higher frequency electromagnetic wave called a carrier wave. The process of superimposing the "electrical image" of the sound information on the carrier wave is called modulation, and there are two commonly used schemes: amplitude modulation (AM) and frequency modulation (FM). Either form of modulation produces frequencies which are the sum and the difference of the carrier and modulation frequencies - these frequencies are sometimes called sidebands.

Because of the existence of the sidebands, the frequency range or bandwidth necessary for radio transmission depends on the range of modulating frequencies.

Bandwidth for Communication

When you quote a frequency for a radio station, you generally quote the frequency of the carrier. But when you superimpose a signal on the carrier by AM or FM, you produce sidebands at the sum and difference of the carrier frequency fC and modulation frequency fM. This means that the transmitted signal is spread out in frequency over a bandwidth which is twice the highest frequency in the signal. 当引用广播电台的频率时，你通常引用载波频率。但当你通过AM或FM将信号叠加到载波上时，会产生侧带，其频率为载波频率f_C和调制频率f_M之和与之差。这意味着传输的信号在频率上被扩展，其带宽是信号中最高频率的两倍。

Bandwidths are assigned for all types of broadcast communication and this imposes a maximum signal frequency which may be transmitted. The bandwidths assigned to AM and FM radio are such as to limit the fidelity of music broadcasts in AM, but permit the luxury of stereo high-fidelity broadcasts by FM. The high signal frequencies associated with video broadcasting require higher bandwidths for channels assigned to television.

Radio Frequency Bands

Because of the division of the FM band for the transmission of FM stereo, the frequency limit for music transmission is at 15 kHz. This allows high fidelity signal transmission. The operational bandwidth is limited to 150 kHz, with 25 kHz on each side of that for gaurd bands. Actually FM stereo covers 106 kHz of that.

AM radio is limited to 5000 Hz maximum frequency by the width of the AM bands. Music transmission by AM radio is limited in fidelity, and adjacent stations tend to interfere with each other.