What is FM? Frequency Modulation and its Applications

Frequency modulation, FM is used most commonly in radio communications. Radio, FM broadcasts are very much in demand due to the capability of the VHF bands to deliver absolutely stunning quality of audio. FM transmitters are also used for a wide range of two way radio information exchange. One of the major uses of FM radio in real life is the mobile radio communications used in taxis and other vehicles. During the recent technological spurt, Frequency modulation has truly gained a foothold. FM is now considered the gold standard in high fidelity broadcast transmission.

frequency modulation

What is FM?

Frequency modulation involves superimposing on the high frequency carrier wave, baseband signal ie, if the amplitude of the baseband increases, there is an increase in the wavelength meaning a drop in the frequency of the carrier wave and vice versa. The amount by which the signal frequency varies is a very important factor since this is what decides the quality of the signal sent by the FM transmitter. This variation is known as deviation and is normally quoted in terms of kilohertz.

In order for the frequency modulated signal to be meaningful to the receiver, the signal first has to interpret and then the frequency variation should be smoothened to obtain just the baseband signal. This is done by an equipment called the Demodulator. The output at the demodulator provides the information carried and transmitted by the FM wave.

History

Frequency modulation, FM, has been around for almost as long as Amplitude Modulation but it had a few issues. Well, frequency modulation itself did not have an issue but we couldn’t understand the potential of FM transmitters. In the early days of wireless communications, it was considered that a narrower bandwidth was required to reduce noise and interference.  Under such a criterion, FM suffered while AM flourished. It was a long time after that, that an American Engineer named Edwin Armstrong made the conscious effort to explore the depths of FM transmitters. He introduced the idea of using FM for broadcast which was against the trend at that time.

Narrowband FM and Wideband FM

The deviation decides whether a frequency modulated wave is narrowband or wideband. Broadast stations that transmit in the VHF range of the spectrum between 88.5 and 108 MHz use a typical deviation of ±75 kHz. This is known as Wideband FM. These signals though good at high quality transmissions occupy a large amount of bandwidth. For radio communications, a lesser bandwidth turns out to be the most feasible. These FM waves have deviation of less than ±3 kHz and are called Narrowband FM. Though not the best in terms of quality, Narrowband FM makes up for it in terms of radio spectrum efficiency.

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Two lenses with powers; 2D & -4D are kept together. What is the effective focal length of the combination?