The Process of FM Radio

8/2/2019 The Process of FM Radio

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From Airways to Automobiles:

The process of FM Radio

Christopher Payne

English 202c

Section 14

3/16/2012


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Scope and Audience

This document will cover the basic concepts and principles behind the process of the

transmission and reception of FM radio programs. The intended audience is an age group

between high school graduates and middle-aged adults that have graduated with a high school

degree. The scope limitation for this audience is the requirement of a high school degree under

the assumption that the audience has taken a basic science course. Concepts such as frequency,

filters, and waves will be reviewed, but without a prior understanding to these concepts the

content within this document may need supplemental material.

Introduction

Have you ever wondered about what really happens

when you turn on your car stereo and switch through

different radio programs? Have you ever thought abouthow radio stations magically appear on certain

frequencies and why a white noise-like sound is found

between those frequencies? This document will provide

a thorough explanation into how radio stations send

their radio broadcasts and how an FM stereo is able to

understand, amplify, and switch between them.

Properties of Sound Waves

Before we begin on the explanation of how sound travels, we must introduce the basic

concepts and characteristics of waves. Sound is usually characterized as a type of wave. It is an

electromagnetic wave, which involves the movement of particles that carry energy. For our

purposes, the particles that carry this energy consist of the air around us. Sound is also a

transverse wave, which means that

the movement of the wave is

perpendicular to the direction that

the energy travels. Figure 1 shows an

example of an electromagnetic

transverse wave and should provide

a useful example of how sound

relates to us. There are three

additional traits that define a wave:

Image 1 Radio Broadcast Booth

Figure 1 Sound Waves


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1. AmplitudeThe amplitude of a wave is defined as a waves maximum displacement of energy. In

other words, it is measured as the distance between the peak of a wave and the point at

which the wave would lie at rest. As the amplitude increases, the energy also increases

and vice versa.

2. WavelengthWavelength is defined as the

distance between two amplitudes of

a wave. The higher the wavelength

of a wave is, the more time it takes

for energy to go from minimum

displacement to maximum

displacement.

3. FrequencyThe frequency of a wave represents

how many oscillations occur within one second. An oscillation can be defined as the

time it takes for one wavelength of a traveling wave to pass a non-moving point of

measurement. For example, a wave at a frequency of 5 Hz will pass a non-moving

location at 5 wavelengths per second. The frequency is also related to a waves phase,

which is defined as the time it takes for one oscillation to occur. Sound has an audible

frequency between 20 Hz and 20 kHz, and anything above and below this range cannot

be heard by a human ear.

Modulation of Sound Waves

In order to send waves between distant locations for accurate transmission, the sound waves

need to undergo a process called modulation. AM stations are sent from the radio stations to

radio receivers using amplitude modulation and FM stations similarly use frequency modulation

to send information. There are different styles of modulation for different purposes but, for the

purpose and scope of this document, we will focus on Frequency Modulation.

Modulation is the process of using convolution, which could be defined as the multiplication of

waves, to place a message signal inside a carrier signal. A message signal consists of valuable

information, which in our case is radio programs and advertisements, and a carrier signal

consists of an empty wave of a particular radio stations frequency. Figure 3 shows a carrier

signal, a message signal (shown as the modulated wave), and the resulting transmission signal

(shown as the modulated result).

Figure 2 Wave Characteristics


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The Carrier Signal

There are two main functions of the carrier signal, and the first is to raise the frequency of the

message signal. A lower frequency signal will have a higher energy loss per distance when

compared to the minimal energy losses of a high frequency signal. For instance, when you play

music from your speakers and you walk slowly away, you will notice that the higher frequencies

such as the singer and guitars will still be

audible and the lower frequencies such as

the bass guitar or bass drum are harder to

hear. That is because the lower

frequencies have lost more energy than

the higher frequencies at that distance.

The second main function of the carrier

signal is to establish a frequency to

broadcast from. For instance, a radio

station that broadcasts itself as The Jams

of 99.1 has a carrier frequency of 99.1

kHz. What the frequency modulator does

is create a transmission signal that has

constant amplitude but has a frequency

that changes depending on the message

signal. We will use the 99.1 kHz broadcasting company as our example. When the message

signal has zero amplitude, the frequency of the transmission signal is 99.1 kHz. But when the

message signal has maximum amplitude, the frequency of the transmission signal raises to a

value such as 99.25 kHz. As the message signals information raises and lowers in amplitude,

the transmission signals frequency raises and lowers.

Transmission of Radio Waves

FM radio stations send their audio broadcasts in the audible range (from 20 Hz to 20 kHz) to a

frequency convolution circuit system that creates a transmission signal at the carrier frequency

that they were designated by the Federal Communications Commission. The upper and lower

frequency limits have also been designated by the FCC to make sure that no two radio stations

overlap. Most FM stations are usually separated by 0.2 kHz, so a carrier frequency at 99.1 kHz

has a maximal range from 99.0 kHz to 99.2 kHz to avoid conflicting stations and maintain FCC

Figure 3 Frequency Modulation


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regulations. The actual range is usually less than the maximal amount to absolutely avoid the

possibility of conflicting stations in the case that a station

accidently goes over their regulated amount.

The transmission signal is then sent to a large antenna that

broadcasts their program. It is able to accomplish this by

sending electrical charges up and down the broadcast antenna

at the same rate of the carrier frequency. The movement of

charges creates an electromagnetic field which sends out waves

in all directions. Because the carrier frequency is very high, the

transmission signal is able to travel hundreds of feet without

losing much of its energy. Eventually, the signal is picked up by a

receiver which is connected to an FM radio.

Reception of Radio Waves

An FM radio antenna is able to select specific frequency bands in order to focus in on a

particular radio station. An antenna by itself cannot discriminate between frequencies, so the

input from the receiver contains information from every frequency that the antenna is able to

pick up. In order to chose a particular radio station and block out all of the others, a filter must

be implemented to the incoming signal.

An adjustable band-pass filter is implemented within every FM radio to obtain a desired radio

station and reject the undesired ones. A band-pass filter is a frequency filter that allows a

certain range of frequencies in and rejects all other frequencies out. The tuner on a radio

station moves the band-pass filter to center

itself on the desired radio station. In order

to tune in to 99.1 kHz, the band-pass filter

must center itself at 99.1 kHz and must

reject frequencies below 99 kHz and above

99.2 kHz so that only the desired radio

station frequencies remain.

After the desired frequency band iscollected and the other frequencies are

rejected, the transmission frequency is then

demodulated back into the audible range. Just as convolution worked to multiply the carrier

signal with the message signal, deconvolution works as a divider to separate and obtain the

message signal from the carrier signal in a process called demodulation. Once the message

Image 2 Radio Antenna Tower

Figure 4 Band-pass FIlter


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signal is obtained within its audible range, the output is then sent to speakers or an audio jack

that can play the electromagnetic waves as sound waves that are based within hearing range.

Conclusion

The process of how an FM radio receives broadcasts can be summarized into 6 steps:

a. The original broadcast is frequency modulated with radio stations designatedcarrier frequency to create the transmission frequency used for broadcasting

b. The transmission signal is sent up and down an antenna at the transmissionfrequency which creates an electromagnetic field

c. A receiver antenna picks up the electromagnetic waves from all frequencies,including the radio station, and sends it to the FM radio

d. The radio uses a band-pass filter to select the desired radio station and reject allof the others.

e. The radio demodulates the transmission signal to obtain the original messagesignal which is played through external speakers or an audio jack.

FM radios provide a reliable and efficient way to transport data. In fact, AM radio and satellite

radio both use their own types of modulation to transport data using the exact same steps. The

process of the FM radio is a similar concept to how cell phones, ipads, and even wifi routers

perform daily tasks and communicate information. Understanding how an FM radio works will

provide a foundation of basic knowledge for wireless communication devices of all types. As

engineering and science evolves, more and more future technologies will be developed using

modulation and transmission concepts to create a world that is based on wireless

communication.

References

Text References

1. Neamen, Donald A. Microelectronics: Circuit Analysis and Design. New York: McGraw-Hill, 2010. Print.

2. Barrett, Thomas E., and David Halliday. Fundamentals of Physics (9th), Condensed: AStudy Guide to Accompany Fundamentals of Physics, Ninth Edition, David Halliday,

Robert Resnick, Jearl Walker. 9th ed. Vol. 2. Hoboken, NJ: Wiley, 2011. Print.

3. Lathi, Bhagwandas Pannalal, and Zhi Ding. Modern Digital and Analog CommunicationSystems. New York: Oxford UP, 2010. Print.


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Image References

All images have been provided by Flickr.com under a creative commons license.

Figure References

1. http://www.mediacollege.com/audio/01/sound-waves.html2. http://www.webanswers.com/education/define-fm-06578d3. http://encyclopedia2.thefreedictionary.com/Frequency-modulation4. http://en.wikipedia.org/wiki/Band-pass_filter
http://www.mediacollege.com/audio/01/sound-waves.htmlhttp://www.mediacollege.com/audio/01/sound-waves.htmlhttp://www.webanswers.com/education/define-fm-06578dhttp://www.webanswers.com/education/define-fm-06578dhttp://encyclopedia2.thefreedictionary.com/Frequency-modulationhttp://encyclopedia2.thefreedictionary.com/Frequency-modulationhttp://en.wikipedia.org/wiki/Band-pass_filterhttp://en.wikipedia.org/wiki/Band-pass_filterhttp://en.wikipedia.org/wiki/Band-pass_filterhttp://encyclopedia2.thefreedictionary.com/Frequency-modulationhttp://www.webanswers.com/education/define-fm-06578dhttp://www.mediacollege.com/audio/01/sound-waves.html

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