Radio Frequency Modulation FM radio, of course, employs frequency modulation. FM radio operates on a frequency range of 88 to 108 MHz. Music and speech are examples of information transmissions that are part of the auditory spectrum. The complete audio spectrum spans 20 to 20,000 Hz, however FM radio restricts the maximum modulating frequency to 15 kHz (cf. Nyquist rate).
This means that you can transmit only one signal per channel, as opposed to stereo music which uses two frequencies (one for each earpiece). Stereo broadcasting allows for greater volume than monaural broadcasting, but it requires more bandwidth than FM radio can use by itself.
Audio modulation is also called amplitude modulation, as the sound intensity is modulated by changing its average value. This works in opposition to frequency modulation where the frequency changes instead.
The audio signal is a wave whose height represents the intensity of the sound. By lowering this height we can modulate it between on and off states. When the intensity is low, it will be at an off state. As it gets louder, it will go into an on state. This is what happens with voice signals when you talk on the phone or type on a computer keyboard. As you speak or type the intensity rises and falls so your voice or keys rise and fall on the radio receiver or microphone.
With music signals, the intensity generally stays constant, so they don't require audio modulation.
Frequency bands for FM. In radio transmissions in the 88-108 MHz VHF range, frequency modulation is employed. This bandwidth range is denoted as FM on radio receiver band scales, and equipment capable of receiving such broadcasts are known as FM receivers. In UHF television transmissions, between 218 and 862 MHz, frequency modulation is also used.
The term "frequency modulated" refers to the fact that the amplitude of the signal's carrier wave is varied instead of being fixed as with AM. The amount by which the carrier wave is lowered or raised is called the "modulation index". A value of 100% means that the carrier wave is at its highest point while a value of 0% means that it is at its lowest point. Values in between these two extremes represent different degrees of modulation. For example, an FM signal with a modulation index of 75% would be vibrating at half the height of the carrier wave compared to when it is fully modulated. Other modulation indices can be used depending on the type of sound recording that is being made - 50% would be considered low-quality music, but could be acceptable for voice transmission.
FM was originally designed for use with recorded music, where a constant tone is needed at each frequency to identify the song that is being played. This is not necessary for live broadcasts, so other modulation techniques have been developed over time.
FM radio broadcasting, like AM, has been around for a long time. FM radio transmission in the United States takes place in the frequency range of 88 to 108 MHz. The carrier frequency fluctuates in proportion to the amplitude of the modulating signal in FM modulation. For example, if the modulating signal increases in amplitude, the carrier frequency will increase; if it decreases in amplitude, the carrier frequency will decrease.
The average carrier frequency of an American FM radio is 95.5 MHz. But the actual carrier waveform is a sinusoid with a certain peak-to-peak voltage, so the average value does not reflect the true nature of an FM signal. In fact, the term "carrier frequency" is somewhat of a misnomer for FM. There is no such thing as a carrier frequency for FM transmission.
For most radios, the carrier frequency is fixed when you buy the radio. However, some tuners have a variable carrier frequency that can be adjusted by the user. This is useful if you want to tune in stations that are near the border of the AM and FM ranges but which are not exactly on one side or the other.
In general, the carrier frequency of an FM radio is between 90 and 100 Megahertz (MHz). Higher frequencies allow for higher transmission rates but also require more power from the transmitter.
To broadcast program material, FM transmitters change the frequency of the carrier wave. The carrier frequency fluctuates in proportion to the loudness of the modulated audio stream. The carrier frequency deviates at a larger rate as the information signal frequency increases. Thus, the carrier frequency of an FM broadcast transmitter varies between 88 and 107 kHz.
The deviation of the carrier frequency from its nominal value is called "carrier drift." Carrier drift can be positive or negative. If it is positive, the frequency of the transmitted signal is higher than that of the carrier signal. If it is negative, the frequency of the transmitted signal is lower than that of the carrier signal.
Carrier drift is caused by several factors: physical movement of the antenna, temperature changes, etc. The drift rate depends on how much each factor affects the frequency of the carrier signal. For example, if the antenna is moved away from the tower, this would cause the frequency of the transmitted signal to drop; if the antenna is moved toward the tower, the frequency of the transmitted signal would have to rise in order to maintain the same distance from the station site.
Generally, the drift rate is very small (about 1 Hz/day), but it can be greater for high-power stations. At these power levels, carrier frequencies may need to be adjusted more frequently in order to keep the radio waves focused on the receiving station.