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TERM PAPER:

COMPARISON BETWEEN AM, FM &

PM

ELE-102

ELECTRICAL SCIENCES

SUBMITTED TO: MR. ASHISH SHARMA

SUBMITTED BY: AKSHAY PATHANIA

SECTION-E4911


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ROLL NO.: B50

CONTENTS:

INTRODUCTION

MODULATION

TYPES OF MODULATION

AMPLITUDE MODULATION(AM)

FREQUENCY MOADULATION(FM)

PHASE MODULATION(PM)

ADVANTAGES AND DISADVANTAGES


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COMPARISON BETWEEN AM,FM AND PM

ACKNOWLEDGEMENT

If practical knowledge carves and sharps the carrier of a person, practical

experience polishes it and adds luster and brilliance to it. Here, we found this golden chance

to acknowledge all those people who had blessed, encouraged and supported us technically

and morally through all the phases of our term paper. We take this opportunity to express our

profound sense of gratitude. We thank all mighty God for giving us this valuable opportunity

to express to all those who helped in successful completion of this term paper.

Before we get into thick of the things I would like to add a few heartfelt words for the

people who were part of this term paper in numerous ways. We reserve heartiest gratitude to

who has been very supportive and encouraging throughout this term paper. He guides us for

having given us an opportunity to undertake the term paper and providing us with feedback

and influenced the development of this term paper. We gratefully acknowledge invaluable

note of our term paper guide MR. ASHISH SHARMA and to all teachers who besides

helping us in this term paper, guided and encouraged us along each step.

We express heartfelt thanks to our friends for their morale and support and kind

corporation during this course of formulation of this project work who directly or indirectly

helps us to complete this term paper. Last but not least, my sincere regards are reserved for


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our family and friends who have always encouraged and blessed us with their best. Specially

thanks to my elder brothers who always encourage me to do your best.

INTRODUCTION

In electronics, modulation is the process of varying one or more properties of a high

frequency periodic waveform, called the carrier signal, with respect to a modulating signal.

This is done in a similar fashion as a musician may modulate a tone (a periodic waveform)

from a musical instrument by varying its volume, timing and pitch. The three key parameters

of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency

("pitch"), all of which can be modified in accordance with a low frequency signal to obtain

the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal,but a square wave pulse train may also occur.

In telecommunications, modulation is the process of conveying a message signal, for

example a digital bit stream or an analog audio signal, inside another signal that can be

physically transmitted. Modulation of a sine waveform is used to transform a baseband

message signal to a passband signal, for example a radio-frequency signal (RF signal). In

radio communications, cable TV systems or the public switched telephone network for

instance, electrical signals can only be transferred over a limited passband frequency

spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine

wave carrier makes it possible to keep the frequency content of the transferred signal as close

as possible to the centre frequency (typically the carrier frequency) of the passband. When

coupled with demodulation, this technique can be used to, among other things, transmit a

signal through a channel which may be opaque to the baseband frequency range (for instance,

when sending a telephone signal through a fiber-optic strand).

In music synthesizers, modulation may be used to synthesise waveforms with a desired

overtone spectrum. In this case the carrier frequency is typically in the same order or much


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lower than the modulating waveform. See for example frequency modulation synthesis or

ring modulation.

Types of modulation:

Analog modulation

Digital modulation

Pulse modulation


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Modulation Techniques

Modulation techniques are methods that are used to encode digital information in an analog

world. The 3 basic modulation techniques are as follows:

o AM (amplitude modulation)

o FM (frequency modulation)

o PM (phase modulation)

All 3 modulation techniques employ a carrier signal. A carrier signal is a single frequency

that is used to carry the intelligence (data). For digital, the intelligence is either a 1 or 0.

When we modulate the carrier, we are changing its characteristics to correspond to either a 1

or 0.

AM - Amplitude Modulation

Amplitude modulation (AM) is a technique used in electronic communication, most

commonly for transmitting information via a radio carrier wave. AM works by varying the

strength of the transmitted signal in relation to the information being sent. For example,

changes in the signal strength can be used to specify the sounds to be reproduced by a

loudspeaker, or the light intensity of television pixels. (Contrast this with frequency

modulation, also commonly used for sound transmissions, in which the frequency is varied;

and phase modulation, often used in remote controls, in which the phase is varied)

In the mid-1870s, a form of amplitude modulationinitially called "undulatory currents"

was the first method to successfully produce quality audio over telephone lines. Beginning

with Reginald Fessenden's audio demonstrations in 1906, it was also the original method

used for audio radio transmissions, and remains in use today by many forms of

communication"AM" is often used to refer to the medium wave broadcast band. Amplitude

Modulation modifies the amplitude of the carrier to represent 1s or 0s.In the above example,


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a 1 is represented by the presence of the carrier for a predefined period of 3 cycles of carrier.

Absence--or no carrier--indicates a 0. It is imposing a signal on the amplitude of a frequency

- has the effect of modifying the frequency a bit as well, and is much more susceptible to

environmental noise such as lightning. Works well across much of the radio spectra.

Modulation index

It can be defined as the measure of extent of amplitude variation about an unmodulated

maximum carrier. As with other modulation indices, in AM, this quantity, also

called modulation depth, indicates by how much the modulated variable varies around its

'original' level. For AM, it relates to the variations in the carrier amplitude and is defined as:

where and were introduced above.

Advantages:


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Simple to design .

Disadvantages:

Noise spikes on transmission medium interfere with the carrier signal.

Loss of connection is read as 0s.

FM - Frequency Modulation

frequency modulation (FM) conveys information over a carrier wave by varying its

instantaneous frequency (contrast this with amplitude modulation, in which the amplitude of

the carrier is varied while its frequency remains constant). In analog applications, thedifference between the instantaneous and the base frequency of the carrier is directly

proportional to the instantaneous value of the input signal amplitude. Digital data can be sent

by shifting the carrier's frequency among a set of discrete values, a technique known as

frequency-shift keying. Frequency Modulation modifies the frequency of the carrier to

represent the 1s or 0s. In the above example, a 0 is represented by the original carrier

frequency, and a 1 by a much higher frequency (the cycles are spaced closer together). It

imposes a signal by altering the frequency - it takes a very high frequency to modulate either

audio or video signals - e.g. megahertz, so it eats up bandwidth.

Modulation index

As with other modulation indices, this quantity indicates by how much the modulated

variable varies around its un-modulated level. It relates to the variations in the frequency of

the carrier signal:


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where is the highest frequency component present in the modulating signalxm(t),

and is the Peak frequency-deviation, i.e. the maximum deviation of

the Instantaneous frequency from the carrier frequency. If , the modulation is

called narrowband FM, and its bandwidth is approximately . If , the

modulation is called wideband FMand its bandwidth is approximately . While

wideband FM uses more bandwidth, it can improve signal-to-noise ratio significantly.

With a tone-modulated FM wave, if the modulation frequency is held constant and the

modulation index is increased, the (non-negligible) bandwidth of the FM signal

increases, but the spacing between spectra stays the same; some spectral components

decrease in strength as others increase. If the frequency deviation is held constant and

the modulation frequency increased, the spacing between spectra increases.

Advantages:

Immunity to noise on transmission medium.

Always a signal present. Loss of signal easily detected

Disadvantages:

Requires 2 frequencies

Detection circuit needs to recognize both frequencies when signal is lost.

PM - Phase Modulation
http://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://en.wikipedia.org/wiki/Signal-to-noise_ratio


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Phase modulation (PM) is a form of modulation that represents information as variations in

the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency

modulation (FM), PM is not very widely used for radio transmissions. This is because it tendsto require more complex receiving hardware and there can be ambiguity problems in

determining whether, for example, the signal has changed phase by +180 or -180. PM is

used, however, in digital music synthesizers such as the Yamaha DX7, even though these

instruments are usually referred to as "FM" synthesizers. Phase Modulation modifies the

phase of the carrier to represent a 1 or 0. The carrier phase is switched at every occurrence of

a 1 bit, but remains unaffected for a 0 bit. The phase of the signal is measured relative to the

phase of the preceding bit. The bits are timed to coincide with a specific number of carrier

cycles (3 in this example = 1 bit). by altering the phase with a desired signal - works when

the bandwidth of the desired signal is very small - e.g. digital communications - but

reflections of phase modulated signals can be easily corrupted. Radio frequency has a work to

do in this field so an equipment is a must.

Modulation index


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As with other modulation indices, this quantity indicates by how much the modulated

variable varies around its unmodulated level. It relates to the variations in the phase of the

carrier signal:

,

where is the peak phase deviation

Advantage:

Only 1 frequency used

Easy to detect loss of carrier

Disadvantages:

Complex circuitry that is required to generate and detect phase changes.

COMPARISON BETWEEN AM, FM AND PM

AM (Amplitude Modulation) and FM (Frequency Modulation) are two popular ways

to get information onto a radio wave. The bandwidth requirements are similar for

standard data formats (PPM, PCM), so both AM and FM sets are available on most

RC frequency bands.

AM is the simplest to implement, but is susceptible to RF interference. It is generally

used in the cheapest radios where cost is more important than performance, eg. for

cars, boats and toys.

FM requires more complex circuitry, which is a bit more expensive. However the

extra cost is well worth it for increased range and reduced glitching, particularly in

electric powered models. FM is used in almost all currently produced airplane radios.

Both AM and FM radio signal carriers are output at a single frequency (FM is usually

much higher than AM).

AM (amplitude modulation) carriers alter in amplitude to mimic the audio being sent.

The carrier signal frequency doesnt alter.


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FM (frequency modulation) carriers alter in frequency to mimic the audio being sent.

Receivers of either type are designed to detect those changes which are applied to the

carrier signal to retrieve the audio signal.

DIFFERENCE BETWEEN AM AND FM

The differeces between frequency and amplitude modulation does not effect the range.Fundamentally the amplitude modulation system is less efficient in that a carrier is generated,

which is modulated. The modulation power is half of the carrier power. The bandwidth

required is twice the highest modulating fequency. Because of the small bandwidth required,

the Amplitude Modulated band on radios is from 550 Khz to 1500 Khz. Channels are

separated by 10 Khz, with actual users usually separated by 2 or three channels minimum.

Frequency modulation varys the frequency of the carrier by the audio modulating component.

While the frequency deviation can be as low as the audio frequency modulating the carrier,

better noise performance is achieved by deviating the carrier by as much as possible. In

commercial broadcast operations, the 15 Khz audio signal deviated the carrier by 75 Khz.

This presents exceptionally good audio reproduction. It also causes the FM (Frequency

Modulated) signal to occupy a band of about 240 Khz for a 15 Khz audio channel. This

compares to 30 Khz for an equivalent AM (Amplitude modulated) signal. Because of the

larger bandwidth required, the FM broadcast band is moved higher in frequency. This makes

the other comments about range etc come into effect. AM signals can be received even when

an interfering signal is present. Weak signals can be recieved in the presence of strong


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signals. Because of this property, Aviation signals, (Airplane to control tower and tower to

plane) are in AM. FM signals have an effect called "capture effect" in which a stronger signal

will capture the channel and eliminate interference from the weaker signal. Basically you are

trading bandwidth for interference rejection. Because of this, signals from satellites, where

the signal strength is extremenly small, used to be sent in FM. Present technology uses digital

broadcast techniques.

Talking about digital, because of the ability of digital receivers to process signals of

extremely small size, digital signals are being sent along with both AM and FM broadcast

signals for either better quality, or supplementary services. Cell phones are all being switched

over to digital technology for better reliability and better channel usage.

Differences between AM, FM and PM

FM and PM are similar to each other, but fundamentally different from AM. All three have a

carrier which is modulated in some way, and that modulation causes sidebands, but how

those sidebands interact with the carrier is very different.

Because noise basically affects amplitude, we can remove it from an FM or PM signal by

clipping (limiting) the signal to a fixed amplitude; this doesnt affect the modulation.

However , we cannot remove this noise from an AM signal without affecting the amplitude,

and therefore the modulation itself. FM and PM can therefore provide the better signal-to-

noise ratio that listeners want for good music.

But to get the best signal-to-noise ratio requires also a wide deviation; hence FM signals used

for music also have fairly much wider bandwidth than most AM signals.

In those communications applications where real hi-fi sound is not required, narrow-band FM

(NBFM) with fairly small deviation and a small modulation index can produce bandwidths

not much larger than AM; this is the mode that is used for most mobile communications,

business users, and cellular phones.


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FM and PM receivers also experience a capture effect ,which affects how interfering

signals affect each other.

For example, suppose that within reach of a receiver there are two transmitters, both of which

transmit on the same frequency, but A is close to you, while B is farther away and weaker.

If two transmitters are AM, then their signals will add to each other, and interfere with each

other. If the two carrier frequencies are exactly the same , then the receiver might hear the

audio from both; more likely, the frequencies would be just a bit different, in which case the

receiver would get a combination of both signals, plus a beat note- an audio tone equal to the

difference between the two carrier frequencies. The result would be most unpleasant, and

possibly useless.

If, on the other hand, the two signals are FM or PM, the stronger signal captures the

receiver, and the weaker signal doesnt interfere at all.

In the typical receiver, the stronger signal must be about 2 to 3 db stronger than the weaker

one to completely capture the receiver; this number is called the capture ratio, and depends on

the receiver design. If we have an FM receiver in our car, we may have noticed the capture

effect when you drive in a region somewhere between two FM stations (call them A and B)

which are on the same frequency. Since signals vary in strength in different places, the

relative strengths of A and B also vary. In the spot A may be stronger and thus capture the

receiver; move over a small distance and suddenly B is stronger and thus captures the

receiver. Thus our receiver rapidly switches back ans fourth between stations A and B as we

drive.