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Assignment Report

Modulation

  Amplitude Modulation (AM)

  Frequency Modulation (FM)

  Phase Modulation (PM)

  Pulse-Amplitude Modulation (PAM)

  Pulse-Code Modulation (PCM)

Arshdeep Singh

100905012

EIC-1

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  MODULATION

Modulation is the addition of information (or the signal) to an electronic or opticalsignal carrier. Modulation can be applied to direct current (mainly by turning it on

and off), to alternating current, and to optical signals. One can think of blanket

waving as a form of modulation used in smoke signal transmission (the carrier

being a steady stream of smoke). Morse code, invented for telegraphy and still

used in amateur radio, uses a binary (two-state) digital code similar to the code

used by modern computers. For most of radio and telecommunication today, the

carrier is alternating current (AC) in a given range of frequencies. Commonmodulation methods include:

  Amplitude modulation (AM), in which the voltage applied to the carrier is

varied over time

  Frequency modulation (FM), in which the frequency of the carrier waveform

is varied in small but meaningful amounts

  Phase modulation (PM), in which the natural flow of the alternating currentwaveform is delayed temporarily

In electronics and telecommunications, modulation is the process of varying one

or more properties of a high-frequency periodic waveform, called the carrier 

signal, with a modulating signal which typically contains information to be

transmitted. This is done in a similar fashion to a musician modulating 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"). Any of these

properties 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 be used.

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 into a passband signal, for example low-frequency

audio signal into a radio-frequency signal (RF signal). In radio communications,

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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 thepassband.

A device that performs modulation is known as a modulator and a device that

performs the inverse operation of modulation is known as a demodulator

(sometimes detector or demod ). A device that can do both operations is a modem

(from "modulator – demodulator").

Digital & Analog Modulation

The aim of digital modulation is to transfer a digital bit stream over an analog

bandpass channel, for example over the public switched telephone network (where

a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a

limited radio frequency band.

The aim of analog modulation is to transfer an analog baseband (or lowpass)

signal, for example an audio signal or TV signal, over an analog bandpass channel

at a different frequency, for example over a limited radio frequency band or a cable

TV network channel.

Analog and digital modulation facilitate frequency division multiplexing (FDM),

where several low pass information signals are transferred simultaneously over the

same shared physical medium, using separate passband channels (several different

carrier frequencies).

The aim of digital baseband modulation methods, also known as line coding, is

to transfer a digital bit stream over a baseband channel, typically a non-filteredcopper wire such as a serial bus or a wired local area network. 

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The aim of pulse modulation methods is to transfer a narrowband analog signal,

for example a phone call over a wideband baseband channel or, in some of the

schemes, as a bit stream over another digital transmission system.

In music synthesizers, modulation may be used to synthesize waveforms with an

extensive overtone spectrum using a small number of oscillators. In this case the

carrier frequency is typically in the same order or much lower than the modulating

waveform. See for example frequency modulation synthesis or ring modulation

synthesis. 

Amplitude Modulation (AM)

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 televisionpixels. (Contrast this with frequency modulation, also commonly used for sound

transmissions, in which the frequency is varied; and phase modulation, often usedin remote controls, in which the phase is varied)

In the mid-1870s, a form of amplitude modulation — initially 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 (see AM radio).

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An audio signal (top) may be carried by an AM or FM radio wave.

Forms of amplitude modulation

In radio communication, a continuous wave radio-frequency signal (a sinusoidal

carrier wave) has its amplitude modulated by an audio waveform before being

transmitted.

In the frequency domain, amplitude modulation produces a signal with power

concentrated at the carrier frequency and in two adjacent sidebands. Each sideband

is equal in bandwidth to that of the modulating signal and is a mirror image of the

other. Amplitude modulation that results in two sidebands and a carrier is oftencalled double-sideband amplitude modulation (DSB-AM). Amplitude modulation

is inefficient in terms of power usage. At least two-thirds of the power is

concentrated in the carrier signal, which carries no useful information (beyond the

fact that a signal is present).

To increase transmitter efficiency, the carrier can be removed (suppressed) from

the AM signal. This produces a reduced-carrier transmission or double-sideband 

suppressed-carrier (DSBSC) signal. A suppressed-carrier amplitude modulation

scheme is three times more power-efficient than traditional DSB-AM. If the carrier

is only partially suppressed, a double-sideband reduced-carrier (DSBRC) signalresults. DSBSC and DSBRC signals need their carrier to be regenerated (by a beat

frequency oscillator, for instance) to be demodulated using conventional

techniques. 

Improved bandwidth efficiency is achieved — at the expense of increased

transmitter and receiver complexity — by completely suppressing both the carrier

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and one of the sidebands. This is single-sideband modulation, widely used inamateur radio due to its efficient use of both power and bandwidth.

A simple form of AM often used for digital communications is on-off keying, a

type of  amplitude-shift keying by which binary data is represented as the presence

or absence of a carrier wave. This is commonly used at radio frequencies to

transmit Morse code, referred to as continuous wave (CW) operation.

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.

So if h = 0.5, the carrier amplitude varies by 50% above and below its

unmodulated level, and for h = 1.0 it varies by 100%. To avoid distortion in the

A3E transmission mode, modulation depth greater than 100% must be avoided.

Practical transmitter systems will usually incorporate some kind of limiter circuit,

such as a VOGAD, to ensure this. However, AM demodulators can be designed todetect the inversion (or 180 degree phase reversal) that occurs when modulationexceeds 100% and automatically correct for this effect.

[citation needed ] 

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Variations of modulated signal with percentage modulation are shown below. In

each image, the maximum amplitude is higher than in the previous image. Note

that the scale changes from one image to the next.

Modulation depth 

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Amplitude Modulation Block Diagram

Angle Modulation

Variation of the angle of carrier signal with time results in angle modulation. It

is of two types;

a) Frequency Modulation b) Phase Modulation

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Frequency Modulation (FM)

The type of modulation in which the instantaneous frequency of the carrier is

varied according to amplitude of modulating signal is called frequencymodulation.

Frequency modulation is widely used in VHF communication systems e.g. FM

broadcasting, transmission of sound signal in TV, Satellite Communication etc.

A signal modifies the frequency of a carrier in FM

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Frequency Modulated wave

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Theory

Suppose the baseband data signal (the message) to be transmitted is xm(t ) and the

sinusoidal carrier is , where f c is the carrier's base

frequency and Ac is the carrier's amplitude. The modulator combines the carrier

with the baseband data signal to get the transmitted signal:

In this equation, is the instantaneous frequency of the oscillator and is

the  frequency deviation, which represents the maximum shift away from f c in

one direction, assuming xm(t ) is limited to the range ±1.

Although it may seem that this limits the frequencies in use to f c ± f Δ, this

neglects the distinction between instantaneous frequency and spectral

 frequency. The frequency spectrum of an actual FM signal has components

extending out to infinite frequency, although they become negligibly small

beyond a point.

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Modulation index

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

modulated variable varies around its unmodulated level. It relates to the variationsin the frequency of the carrier signal:

where is the highest frequency component present in the modulating signal

 xm(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 FM and its bandwidth is

approximately . While wideband FM uses more bandwidth, it can improve

signal-to-noise ratio significantly. For example, doubling the value of while

keeping f m constant, results in an eight-fold improvement in the signal to noise

ratio.[1]

 Compare with Chirp spread spectrum, which uses extremely wide

frequency deviations to achieve processing gains comparable to more traditional,

better-known spread spectrum modes.

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

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components decrease in strength as others increase. If the frequency deviation is

held constant and the modulation frequency increased, the spacing between spectra

increases.

Frequency modulation can be classified as narrow band if the change in the carrier

frequency is about the same as the signal frequency, or as wide-band if the change

in the carrier frequency is much higher (modulation index >1) than the signal

frequency. [2]

 For example, narrowband FM is used for two way radio systems such

as Family Radio Service where the carrier is allowed to deviate only 2.5 kHz above

and below the center frequency, carrying speech signals of no more than 3.5 kHz

bandwidth. Wide-band FM is used for FM broadcasting where music and speech istransmitted with up to 75 kHz deviation from the center frequency, carrying audio

with up to 20 kHz bandwidth

Frequency Modulation Block Diagram

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Phase Modulation (PM)

Phase modulation (PM) is a form of modulation that represents information asvariations 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 tends to 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 (both modulation

types sound very similar, but PM is usually easier to implement in this area).

Theory

An example of phase modulation. The top diagram shows the modulating signal

superimposed on the carrier wave. The bottom diagram shows the resulting phase-

modulated signal.

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PM changes the phase angle of the complex envelope in direct proportion to the

message signal.

Suppose that the signal to be sent (called the modulating or message signal) is m(t )

and the carrier onto which the signal is to be modulated is

Annotated:

carrier(time) = (carrier amplitude)*sin(carrier frequency*time + phase shift)

This makes the modulated signal

This shows how m(t ) modulates the phase - the greater m(t) is at a point

in time, the greater the phase shift of the modulated signal at that point.

It can also be viewed as a change of the frequency of the carrier signal,

and phase modulation can thus be considered a special case of FM in

which the carrier frequency modulation is given by the time derivative of the phase modulation.

The mathematics of the spectral behavior reveals that there are two

regions of particular interest:

  For small amplitude signals, PM is similar to amplitude modulation

(AM) and exhibits its unfortunate doubling of baseband bandwidth

and poor efficiency.

  For a single large sinusoidal signal, PM is similar to FM, and its

bandwidth is approximately

,

where f M = ωm / 2π and h is the modulation index defined below. This is also

known as Carson's Rule for PM.

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Modulation index

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

modulated variable varies around its unmodulated level. It relates to the variationsin the phase of the carrier signal:

,

where Δθ is the peak phase deviation. 

Phase Modulation Block Diagram

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Pulse-amplitude modulation (PAM)

Pulse-amplitude modulation, acronym PAM, is a form of signal modulation

where the message information is encoded in the amplitude of a series of signal

pulses.

Example: A two-bit modulator (PAM-4) will take two bits at a time and will map

the signal amplitude to one of four possible levels, for example −3 volts, −1 volt, 1volt, and 3 volts.

Demodulation is performed by detecting the amplitude level of the carrier at every

symbol period.

Pulse-amplitude modulation is widely used in baseband transmission of digital

data, with non-baseband applications having been largely replaced by pulse-codemodulation, and, more recently, by pulse-position modulation. 

In particular, all telephone modems faster than 300 bit/s use quadrature amplitude

modulation (QAM). (QAM uses a two-dimensional constellation).

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The disadvantage of PAM is that any noise "riding" on the signal changes the pulse

height, thereby introducing distortion. One solution to this problem is to use pulse-duration modulation or PDM, in which the PAM signal goes to a keyer which

produces new pulses of uniform height but varying length. The information is then

carried by the pulse length, or duration, rather than by the pulse height as in PAM.

PDM is somewhat less susceptible to noise than PAM. However, any distortion of 

the pulse shape may change the apparent pulse duration, thereby producing a

distorted output signal.

Pulse Code Modulation or PCM offers a method of overcoming some of thedisadvantages of other types of pulse modulation. In PCM, the instantaneous

amplitude of the sampled signal is represented by a coded arrangement of binary

digits or bits resulting in a series of pulses and spaces. All pulses are the same

height and the same shape. Therefore, it is only necessary for the receiving

equipment to detect the presence or absence of a pulse. A distorted pulse does not

degrade the signal as long as the pulse can still be recognized. Thus, PCM is less

sensitive to noise than either PAM or PDM and is easily implemented using

modern electronic technology.

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Pulse Code Modulation (PCM)

The Basics of PCM

Considerable immunity to noise and other transmission difficulties can be achieved

if Pulse Code Modulation or PCM techniques are used in telemetry systems. The

multiplexed signal is coded as a series of identical pulses and spaces. The receiving

equipment need only make a simple "yes or no" decision as to the presence orabsence of a pulse at a particular time.

Before it is coded for transmission, the analog signal is sampled just as in other

forms of pulse modulation. The range of possible pulse heights, from zero to full

scale, is then divided into discrete steps so that each step can be represented by a

particular arrangement of binary pulses and spaces, as shown in the figure

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"Converting an Analog Signal to a Digital Signal". This coded arrangement of 

binary pulses is the PCM signal.

A Basic PCM Encoder

This figure shows a simplified block diagram of a PCM encoder. A number of 

transducer signals are applied to the input of a multiplexer switch. Signals are

sampled in any order and at any rate as defined by the user of the system. Manysystems provide programmability in order to select channel sampling conditions.

The output of the multiplexer switch is a PAM signal which carries time-division

multiplexed samples of each input channel. The ADC samples this PAM signal and

executes an analog-to-digital conversion on each sample. The output from the

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ADC is serialized and formatted into a PCM wavetrain in accordance with the

applicable telemetry standards (IRIG-106).