Frequency modulation and demodulation using Matlab

Frequency modulation, commonly referred to as FM, is a common term that we hear in our daily lives. Today, Frequency modulation is used widely in radio communication and broadcasting. But have we wondered what it actually is or what is the technology and mechanism behind FM? We will understand what frequency modulation is in this lesson and also learn its mechanism as well as its applications.

Frequency modulation is a technique or a process of encoding information on a particular signal (analogue or digital) by varying the carrier wave frequency in accordance with the frequency of the modulating signal. As we know, a modulating signal is nothing but information or message that has to be transmitted after being converted into an electronic signal.

Much like amplitude modulation, frequency modulation also has a similar approach, where a carrier signal is modulated by the input signal. However, in the case of FM, the amplitude of the modulated signal is kept, or it remains constant.

The frequency modulation index is mostly over 1, and it usually requires a high bandwidth at a range of 200 kHz. FM operates in a very high-frequency range, normally between 88 to 108 Megahertz. There are complex circuits with an infinite number of sidebands that help in receiving high-quality signals with high sound quality.

Broadcast stations in the VHF portion of the frequency spectrum between 88.5 and 108 MHz often use large values of deviation (±75 kHz). This is known as wide-band FM (WBFM). Even though these signals support high-quality transmissions, they do occupy a large amount of bandwidth. Normally, 200 kHz is allowed for each wide-band FM transmission. On the other hand, communications use very little bandwidth. Whereas narrowband FM (NBFM) often uses deviation figures of around ±3 kHz. Besides, narrow-band FM is mostly used for two-way radio communication applications.

Applications of Frequency Modulation
If we talk about the applications of frequency modulation, it is mostly used in radio broadcasting. It offers a great advantage in radio transmission as it has a larger signal-to-noise ratio, which means that it results in low radio frequency interference. This is the main reason that many radio stations use FM to broadcast music over the radio.

Additionally, some of its uses are also found in radar, telemetry, seismic prospecting, and in EEG, different radio systems, music synthesis as well as in video-transmission instruments. In radio transmission, frequency modulation has a good advantage over other modulation. It has a larger signal-to-noise ratio, meaning it will reject radio frequency interferences much better than an equal power amplitude modulation (AM) signal. Due to this major reason, most music is broadcasted over FM radio.

FM Modulators
There are several methods that can be used to generate either direct or indirect frequency-modulated signals.

A voltage-controlled oscillator or varactor diode oscillator: A voltage-controlled oscillator can be used to form direct FM modulation by directly feeding the message into the input of the oscillator. In the case of the varactor diode, we place this device within the tuned circuit of an oscillator circuit.
Crystal oscillator circuit: A varactor diode can also be used within a crystal oscillator circuit wherein the signal needs to be multiplied in frequency, and only narrowband FM is attained.
Phase-locked loop: This is an excellent method to generate frequency modulation signals. However, the constraints within the loop should be checked carefully, and once everything is stable, it offers an excellent solution.