EC6651 COMMUNICATION ENGINEERING

UNIT 1

Dr Gnanasekaran Thangavel

Professor and Head

Electronics and Instrumentation Engineering

R M K Engineering College

1

UNIT I ANALOG COMMUNICATION

AM – Frequency spectrum – vector representation – power

relations – generation of AM – DSB, DSB/SC, SSB, VSB

AM Transmitter & Receiver; FM and PM – frequency

spectrum – power relations : NBFM & WBFM, Generation

of FM and DM, Armstrong method & Reactance

modulations : FM & PM frequency.

2Dr Gnanasekaran Thangavel12/12/2017

YouTube Video Presentation

1. https://www.youtube.com/watch?v=_JMV4ywAJug

2. https://www.youtube.com/watch?v=QEubAxBfqKU

Communication systems

3

The purpose of a communication system is to transmit information signals (baseband

signals) through a communication channel

The term baseband is used to designate the band of frequencies representing the original

signal as delivered by the input transducer

Digital

Analog

• Input transducer: The device that converts a physical signal from

source to an electrical, mechanical or electromagnetic signal more

suitable for communicating

• Transmitter: The device that sends the transduced signal

• Transmission channel: The physical medium on which the signal

is carried

• Receiver: The device that recovers the transmitted signal from the

channel

• Output transducer: The device that converts the received signal

back into a useful quantity

4

What is Modulation?

5

Since this baseband signal must be transmitted through a communication channel

such as air using electromagnetic waves, an appropriate procedure is needed to shift

the range of baseband frequencies to other frequency ranges suitable for transmission,

and a corresponding shift back to the original frequency range after reception. This is

called the process of modulation and demodulation

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

more properties of a periodic waveform, called the carrier signal, with a modulating

signal that typically contains information to be transmitted.

Modulation is the process of putting information onto a high frequency carrier for

transmission (frequency translation).

This process is accomplished by a device called a modulator

The transmitter block in any communications system contains the modulator device

What are the reasons for modulation?

Long range transmission: To transmit audio signal (have a relatively short

range of transmission) over a longer distance it is necessary to modulate the

signal before transmission. When the frequency is increased energy is

increased thus long range transmission is possible.

Frequency division multiplexing: To support multiple transmissions via a

single channel - To avoid interference

Practicality of Antennas - Transmitting very low frequencies require antennas

with miles in wavelength.

Reduction of noise and interference: The noise effect can not be completely

eliminated but with the help of several modulation schemes, the noise and

interference effect can be minimized.

6

• Frequency of audio signal (f) =10000 Hz. For efficient radiation of

signal, the length of the antenna should be about one quarter f its

wavelength.

• λ= (3*10^8)/10000=300 km i.e. λ/4=75 km (practically impossible)

• h=λ/4, for efficient transmission. For f=30 Hz => h= 2500 km

f=3kHz => h= 25 km f=3MHz => h= 25 m Thus as Frequency

increases height of the antenna decreases

7

Amplitude Modulation

8

Amplitude Modulation is a process where the amplitude of a carrier signal is altered according to

information in a message signal.

The frequency of the carrier signal is usually much greater than the highest frequency of the input

message signal.

9

Once this information is received, the low frequency information must be removed from the high

frequency carrier. This process is known as “ Demodulation”.

The receiver block in any communications system contains the demodulator device

Types of Analog Modulation

Amplitude Modulation (AM)

Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant

Frequency Modulation (FM)

Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant

Phase Modulation (PM)

Another form of analog modulation technique which we will not discuss

Amplitude Modulation

Carrier wave

Baseband signal

Modulated wave

Amplitude varying-frequency

constant

Frequency Modulation

Carrier wave

Baseband signal

Modulated waveFrequency varying-amplitude

constant

Large amplitude: high

frequencySmall amplitude: low

frequency

AM vs. FM

AM requires a simple circuit, and is very easy to generate.

It is simple to tune, and is used in almost all short wave broadcasting.

The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies) are utilized-remember property of HF waves?)

However, it is quite inefficient, and is susceptible to static and other forms of electrical noise.

The main advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and an FM receiver will not respond to AM signals.

The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation.

The main disadvantage of FM is the larger bandwidth it requires

Digital Modulation

The analog communication systems that transmit information in analog

form using Amplitude or Frequency modulation

Digital communication systems also employ modulation techniques, some

of which include:

Amplitude Shift Keying

Frequency Shift Keying

Phase Shift Keying

Advantages of Amplitude Modulation

There are several advantages of amplitude modulation, and

some of these reasons have meant that it is still in

widespread use today:

It is simple to implement

It can be demodulated using a circuit consisting of very few

components

AM receivers are very cheap as no specialised components

are needed.

15

Disadvantages of amplitude modulation

Amplitude modulation is a very basic form of modulation, and although its

simplicity is one of its major advantages, other more sophisticated

systems provide a number of advantages. Accordingly it is worth looking

at some of the disadvantages of amplitude modulation.:

It is not efficient in terms of its power usage

It is not efficient in terms of its use of bandwidth, requiring a bandwidth

equal to twice that of the highest audio frequency

It is prone to high levels of noise because most noise is amplitude based

and obviously AM detectors are sensitive to it.

16

Amplitude Modulation: Bandwidth,

Spectrum, Sidebands

The bandwidth of an amplitude modulated signal is of importance

for many reasons.

The amplitude modulation, AM bandwidth is important when

designing filters to receive the signals, determining the channel

spacing, and for a number of other reasons.

The spectrum and bandwidth of a amplitude modulated signal are

determined by the sidebands that are generated when amplitude

modulation is applied to the carrier.

17

Amplitude modulation sidebands

When a carrier is modulated in any way, further signals are created either

side of the steady carrier. These sidebands carry the actual modulation

information.

The amplitude modulation sidebands are generated above and below the

main carrier. To see how this happens, take the example of a carrier on a

frequency of 1 MHz which is modulated by a steady tone of 1 kHz.

The process of modulating a carrier is exactly the same as mixing two

signals together, and as a result both sum and difference frequencies are

produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz,

a "sum" frequency is produced at 1 MHz + 1 kHz, and a difference

frequency is produced at 1 MHz - 1 kHz, i.e. 1 kHz above and below the

carrier.

18

• When modulated onto the carrier, these spectra are seen

above and below the carrier.

19

Spectrum arising from carrier modulated

by 1 kHz tone Amplitude Modulation Spectrum

• It can be seen that if the top frequency that is modulated onto the carrier is 6 kHz, then the top spectra will

extend to 6 kHz above and below the signal.

• In other words the bandwidth occupied by the AM signal is twice the maximum frequency of the signal

that is used to modulated the carrier, i.e. it is twice the bandwidth of the audio signal to be carried.

20

Amplitude Modulation Spectrum & Signal Bandwidth

• Carrier : c(t) = Vc cos (2πfct + φ)

• modulating signal v(t) = Vm cos (2πfmt).

• Modulated signal: v(t) = Vc cos (2πfct) {1 + m cos (2πfm t)}

• Vc = un modulated peak carrier amplitude ,fc= carrier frequency ,fm = modulation frequency =

modulation index (“degree” of modulation) m must be between 0 and 1 If m > 1 get over modulation (bad

…distortion)

• MODULATION INDEX m =Vmax – Vmin / Vmax + Vmin

• WHERE DO THE SIDEBANDS COME FROM

• Expand v(t) = Vc cos (2π fct) {1 + m cos (2π fm t)}

• Using trig identities to get: v(t) = Vc cos (2π fct)

+0.5m Vc cos (2π [fc- fm ]t)

+0.5m Vc cos (2π [fc+ fm ]t)

• This expression consists of 3 sine waves at frequencies of carrier (fc), lower sideband (fc-fm) and upper

sideband (fc+fm). fc= carrier frequency

• EFFICIENCY For a fully modulated carrier (m=1), 2/3 of the power is in the carrier, the rest in the

sidebands (33.33% efficient ) Total power Pt = Pc (1 + m2 /2) Carrier Power (Pc) = Vc 2 / 2

• Side band Power =Plsb=Pusb= m2 Pc / 4 Information in side band : Power gets wasted in carrier

AM is bandwidth inefficient (2 fm) 21

• The AM spectrum ranges from fc - fm(max) to fc + fm(max).

• Parameters :

• Lower sideband (LSB) = band of frequencies between fc -

• fm(max) and fc

• Lower side frequency (LSF) = any frequency within LSB

• Upper sideband (USB) = band of frequencies between fc and fc +

• fm(max)

• Upper side frequency (USF) = any frequencies within USB

• Bandwidth : twice the highest modulating signal frequency , B = 2 fm(max)

22

Amplitude Modulation, AM: Depth; Modulation Index

• Modulation Index and Modulation Depth are key issues for the effectiveness of amplitude modulated, AM

signals.

• It is possible to vary the level of modulation applied to an amplitude modulated signal.

• If little modulation is applied then the audio (assuming it us an audio transmission) will be difficult to hear. However if

too much is applied, distortion can result and signals will not be easy to listen to and interference will increase and

could affect users on nearby frequencies or channels.

• The term, Modulation Index, is used for a number of forms of modulation, including AM.

• For amplitude modulation, the modulation index is defined as the measure of extent of amplitude variation about an

un-modulated carrier. In other words it describes the amount by which the modulated carrier envelope varies about

the static level.

• Modulation Index, m=M / A ,

• Where:

A = the carrier amplitude.

M = the modulation amplitude and is the peak change in the RF amplitude from its un-modulated value.

• Using the equation above it can be seen that a modulation index of 0.75 means that the signal will increase by a

factor of 0.75 and decrease to 0.25 of its original level.

23

AM modulation index examples

• It can be seen from the diagram of the AM signal

with a modulation index of 100% that the signal level

falls to zero and rises to twice the value with no

modulation.

• In this case the voltage rises to a maximum of twice

the normal level – this means that the power will be

four times that of the quiescent value,

• i.e. 22 the value of the no modulation level.

24

AM Demodulation: Amplitude Modulation Detection

• One essential element of using amplitude

modulated signals is the process of demodulation

or detection.

• The terms detection and demodulation are often

used when referring to the overall demodulation

process. Essentially the terms describe the same

process, and the same circuits.

• Terms like diode detector, synchronous detector

and product detector are widely used. But the

term demodulation tends to be used more widely

when referring to the process of extracting the

modulation from the signal.

• There are a number of techniques that can be

used to demodulate AM signals. Different types

are used in different applications to suit their

performance and cost.

I. Diode rectifier envelope detector:

II. Product detector:

III. Synchronous detection: 25

AM amplitude modulation demodulation principle

Double-sideband suppressed-carrier transmission

• Double-sideband suppressed-carrier transmission (DSB-

SC) is transmission in which frequencies produced by

amplitude modulation (AM) are symmetrically spaced

above and below the carrier frequency and the carrier

level is reduced to the lowest practical level, ideally being

completely suppressed.

• In the DSB-SC modulation, unlike in AM, the wave

carrier is not transmitted; thus, much of the power is

distributed between the side bands, which implies an

increase of the cover in DSB-SC, compared to AM, for

the same power used.

• therefore reducing power waste, giving it a 100%

efficiency. This is an increase compared to normal AM

transmission (DSB) that has a maximum efficiency of

33.333%, since 2/3 of the power is in the carrier which

conveys no useful information and both sidebands

containing identical copies of the same information.

Single Side Band (SSB) Suppressed Carrier is 100%

efficient.

26

Spectrum plot of a DSB-SC signal

DSB-SC Generation

• DSB-SC is generated by a mixer.

This consists of a message signal

multiplied by a carrier signal. The

mathematical representation of this

process is shown below, where the

product-to-sum trigonometric identity

is used.

27

DSB-SC Demodulation

28

• Demodulation is done by

multiplying the DSB-SC signal with

the carrier signal just like the

modulation process.

• This resultant signal is then passed

through a low pass filter to produce

a scaled version of original

message signal.

• DSB-SC can be demodulated by a

simple envelope detector, like AM,

if the modulation index is less than

unity. Full depth modulation

requires carrier re-insertion.

n.

The equation above shows that by multiplying the

modulated signal by the carrier signal, the result is a

scaled version of the original message signal plus a

second term. Since ω c ≫ ω m , this second term is much

higher in frequency than the original message.

Once this signal passes through a low pass filter, the

higher frequency component is removed, leaving just the

original message.

Single Sideband, SSB Modulation

• A single sideband signal therefore consists of a

single sideband, and often no carrier, although the

various variants of single sideband are detailed

below.

• Single sideband modulation, SSB, provides a

considerably more efficient form of communication

when compared to ordinary amplitude modulation. It

is far more efficient in terms of the radio spectrum

used, and also the power used to transmit the signal.

• There is a number of different formats of single

sideband modulation that are used:

1. Single sideband suppressed carrier, SSBSC

2. Single sideband reduced carrier

3. Single sideband full carrier

4. Single sideband vestigial carrier

5. Independent sideband, ISB:

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Single sideband modulation

showing upper and lower sideband signals

30

SSB-SC - Implementation

• Frequency discrimination

MultiplierMessage

m(t)

Local oscillator

c(t) = cos ct

DSB-SC

tME

tME

ttEtc

mcc

mcc

cmm

)(cos2

)(cos2

coscos)(

Band passfilterc+ c

Band passfilterc- c

tME

tc mcc )(cos

2)(

tME

tc mcc )(cos

2)(

Upper sideband

Lower sideband

31

SSB-SC - Waveforms

B = 2m

USB

Bandwidth B = m

B = m

32

SSB-SC - Implementation cont.

• Phase discrimination (Hartley modulator)

X

SSB-SC

signal

XEm sin mt sin ct

sin ct

cos ctCarrier

90o

phase shift

Messagem(t)

90o

phase shift

+

-

Em cos mt cos ct

Em sin mt

Em cos mt

v(t) =Em cos mt cos ct + Em sin mt sin ct

= Em cos (m - c)t LSB

v(t) =Em cos mt cos ct - Em sin mt sin ct

= Em cos (m + c)t USB

VESTIGIAL SIDE BAND (VSB) MODULATION

• The following are the drawbacks of SSB signal

generation:

• 1. Generation of an SSB signal is difficult.

• 2. Selective filtering is to be done to get the original

signal back.

• 3. Phase shifter should be exactly tuned to 90°.

• To overcome these drawbacks, VSB modulation is

used. It can view as a compromise between SSB

and DSB-SC

• In VSB modulation, one pass band is passed almost

completely whereas only a residual portion of the

other sideband is retained in such a way that the

demodulation process can still reproduce the

original signal

• VSB signals are easier to generate because some

roll-off in filter edges is allowed. This results in

system simplification. And their bandwidth is only

slightly greater than that of SSB signals (-25 %).

33

Spectrum of VSB Signal

VSB Demodulator

Advantages:

• VSB is a form of amplitude modulation intended to save bandwidth over

regular AM. Portions of one of the redundant sidebands are removed to form

a vestigial side band signal.

• The actual information is transmitted in the sidebands, rather than the carrier;

both sidebands carry the same information. Because LSB and USB are

essentially mirror images of each other, one can be discarded or used for a

second channel or for diagnostic purposes.

Disadvantages:

• VSB transmission is similar to (SSB) transmission, in which one of the

sidebands is completely removed. In VSB transmission, however, the

second sideband is not completely removed, but is filtered to remove all but

the desired range of frequencies.

34

AM Transmitter

• The two types of AM transmitters that are used based on their transmitting powers are:

I. High Level

II. Low Level

• High level transmitters use high level modulation, and low level transmitters use low level

modulation.

• In broadcast transmitters, where the transmitting power may be of the order of kilowatts,

high level modulation is employed.

• In low power transmitters, where only a few watts of transmitting power are required , low

level modulation is used.

• The choice between the two modulation schemes depends on the transmitting power of

the AM transmitter.

• The basic difference between the two transmitters is the power amplification of the

carrier and modulating signals.

35

High-Level Transmitters

• Figure (a) shows the block diagram

of high-level AM transmitter.

• In high-level transmission, the

powers of the carrier and

modulating signals are amplified

before applying them to the

modulator stage

• The various sections of the figure

(a) are:

I. Carrier oscillator

II. Buffer amplifier

III. Frequency multiplier

IV. Power amplifier

V. Audio chain

VI. Modulated class C power

amplifier

36

Low-level AM transmitter

• The low-level AM transmitter

shown in the figure (b) is similar

to a high-level transmitter, except

that the powers of the carrier and

audio signals are not amplified.

These two signals are directly

applied to the modulated class C

power amplifier.

• Modulation takes place at the

stage, and the power of the

modulated signal is amplified to

the required transmitting power

level. The transmitting antenna

then transmits the signal.37

AM Receiver

• How the super heterodyne receiver works

• In order to look at how a superhet or super heterodyne radio

works, it is necessary to follow the signal through it. In this way the

processes it undergoes can be viewed more closely.

• The signal that is picked up by the antenna passes into the

receiver and enters a mixer. Another locally generated signal, often

called the local oscillator, is fed into the other port on the mixer and

the two signals are mixed. As a result new signal are generated at

the sum and difference frequencies.

• The output from the mixer is passed into what is termed the

intermediate frequency or IF stages where the signal is amplified

and filtered. Any of the converted signals that fall within the pass

band of the IF filter will be able to pass through the filter and they

will also be amplified by the amplifier stages. Any signals that fall

outside the pass band of the filter will be rejected.

• Tuning the receiver is simply accomplished by changing the

frequency of the local oscillator. This changes the incoming signal

frequency for which signals are be converted down and able to

pass through the filter.

38

The advantage of the super heterodyne radio

process is that very selective fixed frequency

filters can be used and these far out perform any

variable frequency ones.

Super heterodyne receiver

• Signals enter the receiver from the antenna and are applied to the RF amplifier where they are tuned to

remove the image signal and also reduce the general level of unwanted signals on other frequencies that

are not required.39

• The signals are then applied to the mixer along with the local oscillator where the wanted

signal is converted down to the intermediate frequency.

• Here significant levels of amplification are applied and the signals are filtered. This filtering

selects signals on one channel against those on the next. It is much larger than that employed

in the front end.

• The advantage of the IF filter as opposed to RF filtering is that the filter can be designed for a

fixed frequency.

• This allows for much better tuning. Variable filters are never able to provide the same level of

selectivity that can be provided by fixed frequency ones.

• Once filtered the next block in the super heterodyne receiver is the demodulator.

• This could be for amplitude modulation, single sideband, frequency modulation, or indeed any

form of modulation.

• It is also possible to switch different demodulators in according to the mode being received.

• The final element in the super heterodyne receiver block diagram is shown as an audio

amplifier, although this could be any form of circuit block that is used to process or amplified

the demodulated signal.

40

Frequency Modulation - FM

• While changing the amplitude of a radio signal is

the most obvious method to modulate it, it is by

no means the only way.

• It is also possible to change the frequency of a

signal to give frequency modulation or FM.

• Frequency modulation is widely used on

frequencies above 30 MHz, and it is particularly

well known for its use for VHF FM broadcasting.

• These transmissions could offer high fidelity

audio, and for this reason, frequency modulation

is far more popular than the older transmissions

on the long, medium and short wave bands.

• To generate a frequency modulated signal, the

frequency of the radio carrier is changed in line

with the amplitude of the incoming audio signal. 41

• When the audio signal is modulated onto the radio frequency carrier, the new radio

frequency signal moves up and down in frequency.

• The amount by which the signal moves up and down is important. It is known as the

deviation and is normally quoted as the number of kilohertz deviation. As an example

the signal may have a deviation of plus and minus 3 kHz, i.e. ±3 kHz. In this case the

carrier is made to move up and down by 3 kHz.

• Broadcast stations in the VHF portion of the frequency spectrum between 88.5 and

108 MHz use large values of deviation, typically ±75 kHz. This is known as wide-band

FM (WBFM). These signals are capable of supporting high quality transmissions, but

occupy a large amount of bandwidth. Usually 200 kHz is allowed for each wide-band

FM transmission.

• For communications purposes less bandwidth is used. Narrow band FM (NBFM) often

uses deviation figures of around ±3 kHz. It is narrow band FM that is typically used for

two-way radio communication applications. Having a narrower band it is not able to

provide the high quality of the wideband transmissions, but this is not needed for

applications such as mobile radio communication.

42

Frequency modulation advantages & disadvantages

Advantages of frequency modulation

• Resilience to noise: frequency modulation is its resilience to signal level variations.

• Easy to apply modulation at a low power stage of the transmitter: It is possible to apply the

modulation to a low power stage

• It is possible to use efficient RF amplifiers with frequency modulated signals: It is possible to use

non-linear RF amplifiers to amplify FM signals

Disadvantages of frequency modulation

• FM has poorer spectral efficiency than some other modulation formats:

• Requires more complicated demodulator: more complicated, and slightly more expensive

• Some other modes have higher data spectral efficiency:

• Sidebands extend to infinity either side:

43

FM Modulation Index & Deviation Ratio

• Two key parameters of any frequency modulated signal are the modulation index and the deviation ratio. These two

parameters describe some of the basic characteristics of a given FM signal - the modulation index providing a measure

of what is effectively the level of modulation and the deviation ratio a measure of the deviation relative to the

modulating frequency.

• Frequency modulation index: The FM modulation index is equal to the ratio of the frequency deviation to the

modulating frequency.

• To give an example of the FM modulation index, take the example where a signal has a deviation of ±5kHz, and the

modulating frequency is 1kHz, then the modulation index for this particular instance is 5 / 1 = 5.

• FM deviation ratio : FM deviation ratio can be defined as: the ratio of the maximum carrier frequency deviation to the

highest audio modulating frequency.

ne common example of the FM deviation ratio can be seen by taking the figures for a typical FM broadcast station. Fir

these stations the maximum frequency deviation is ±75 kHz, and the maximum audio frequency fort he modulation is

15 kHz.

Using the formula above, this means that the deviation ratio is 75 / 15 = 5. 44

FM bandwidth & modulation index

• Frequency modulation is used in a variety of applications. Different levels of deviation are used in

different applications.

• For broadcast FM transmissions the aim is to be able to transmit high quality audio and to achieve this

high levels of deviation are used and the bandwidth is wide.

• For communications purposes, quality is not the issue, but bandwidth is more important.

• Accordingly deviation levels are less and the bandwidth is much smaller.

• This has given rise to classifications of narrow band FM and wide band FM. These can be related to the

modulation index and deviation ratio.

• Wideband FM: Wideband FM is typical used for signals where the FM modulation index is above 0.5.

For these signals the sidebands beyond the first two terms are not insignificant. Broadcast FM stations

use wideband FM which enables them to transmit high quality audio, as well as other facilities like

stereo, and other facilities like RDS, etc..

• Narrowband FM: Narrow band FM, NBFM, is used for signals where the deviation is small enough that

the terms in the Bessel function is small and the sidebands are negligible. For this the FM modulation

index must be less than 0.5, although a figure of 0.2 is often used. For NBFM the audio or data

bandwidth is small, but this is acceptable for this type of communication.

45

Frequency modulation sidebands

• The FM sidebands are dependent on both the level of deviation and the frequency of the modulation.

In fact the total frequency modulation spectrum consists of the carrier plus an infinite number of

sidebands spreading out on either side of the carrier at integral multiples of the modulating frequency.

• The values for the levels of the sidebands can be seen to rise and fall with varying values of deviation

and modulating frequency as seen in the diagram below.

• The parameters for the FM sidebands are determined by a formula using Bessel functions of the first

kind.

46

Carson's Rule for FM bandwidth

• A very useful rule of thumb used by many engineers to determine the bandwidth of an FM signal

is known as Carson's Rule. This rule states that 98% of the signal power is contained within a

bandwidth equal to the deviation frequency, plus the modulation frequency doubled. Carson's

Rule can be expressed simply as a formula:

• BT=2(Δf+fm)

• Where:

Δf = deviation

BT = total bandwidth (for 98% power)

fm = modulating frequency

• To take the example of a typical broadcast FM signal that has a deviation of ±75kHz and a

maximum modulation frequency of 15 kHz, the bandwidth of 98% of the power approximates to 2

(75 + 15) = 180kHz. To provide conveniently spaced channels 200 kHz is allowed for each

station.

47

FM demodulation

• In any radio that is designed to receive frequency

modulated signals there is some form of FM

demodulator or detector.

• This circuit takes in frequency modulated RF

signals and takes the modulation from the signal

to output only the modulation that had been

applied at the transmitter.

• In order to be able to demodulate FM it is

necessary for the radio receiver to convert the

frequency variations into voltage variations.

• It is necessary to have a response that is as

linear as possible over the required bandwidth.

• The response that is normally seen for an FM

demodulator / FM detector is known as an "S"

curve for obvious reasons. There is a linear

portion at the centre of the response curve and

towards the edge the response becomes very

distorted.48

FM demodulation principle

Frequency demodulator S response curve

Types of FM demodulator

• There are several types of FM detector / demodulator that can be used. Some types were more popular in

the days when radios were made from discrete devices, but nowadays the PLL based detector and

quadrature / coincidence detectors are the most widely used as they lend themselves to being incorporated

into integrated circuits very easily and they do not require many, if any adjustments.

• Slope detection: This is a very simple form of FM demodulation and it relies on the selectivity of the

receiver itself to provide the demodulation. It is not particularly effective and is not used except when the

receiver does not have an FM capability.

• Ratio detector: This type of detector was one that was widely used when discrete components were used

in transistor radios.

• Foster Seeley FM : In the days when radio used discrete components, this was the other main contender

for the FM demodulator in radios.

• Phase locked loop demodulator: It is possible to use a phase locked loop to demodulate FM. The PLL

FM detector provides excellent performance and does not require many, if any adjustments in manufacture.

• Quadrature detector: The quadrature FM detector is now widely used in FM radio ICs. It is easy to

implement and provides excellent levels of performance.

• These FM demodulators are used in different applications. Although the PLL FM detector and the quadrature

detectors are most widely used, the Foster Seeley and ratio FM detectors are still used on some occasions.

49

FM Slope Detector Demodulator

• The very simplest form of FM demodulation is known as

slope detection or demodulation. It consists of a tuned circuit

that is tuned to a frequency slightly offset from the carrier of

the signal.

• As the frequency of the signals varies up and down in

frequency according to its modulation, so the signal moves

up and down the slope of the tuned circuit. This causes the

amplitude of the signal to vary in line with the frequency

variations. In fact at this point the signal has both frequency

and amplitude variations.

• It can be seen from the diagram that changes in the slope of

the filter, reflect into the linearity of the demodulation

process. The linearity is very dependent not only on the filter

slope as it falls away, but also the tuning of the receiver - it is

necessary to tune the receiver off frequency and to a pint

where the filter characteristic is relatively linear.

• The final stage in the process is to demodulate the amplitude

modulation and this can be achieved using a simple diode

circuit.50

• A variety of FM slope detector circuits may

be used, but the one below shows one

possible circuit with the applicable

waveforms.

• The input signal is a frequency modulated

signal. It is applied to the tuned

transformer (T1, C1, C2 combination)

which is offset from the centre carrier

frequency. This converts the incoming

signal from just FM to one that has

amplitude modulation superimposed upon

the signal.

• This amplitude signal is applied to a simple

diode detector circuit, D1. Here the diode

provides the rectification, while C3

removes any unwanted high frequency

components, and R1 provides a load.

51

FM slope detector circuit showing waveforms

FM slope detection advantages & disadvantages

52

Advantages Disadvantages

Simple - can be used to provide FM

demodulation when only an AM

detector is present.

Enables FM to be detected without any

additional circuitry

Not linear as the output is dependent

upon the curve of a filter.

Not particularly effective as it relies on

centering the signal part of the way

down the filter curve where signal

strengths are less.

Both frequency and amplitude

variations are accepted and therefore

much higher levels of noise and

interference are experienced.

Ratio Discriminator / FM Detector Demodulator

• The operation of the ratio detector centers around a frequency

sensitive phase shift network with a transformer and the diodes that

are effectively in series with one another. When a steady carrier is

applied to the circuit the diodes act to produce a steady voltage

across the resistors R1 and R2, and the capacitor C3 charges up as

a result.

• The transformer enables the circuit to detect changes in the

frequency of the incoming signal. It has three windings. The primary

and secondary act in the normal way to produce a signal at the

output. The third winding is un-tuned and the coupling between the

primary and the third winding is very tight, and this means that the

phasing between signals in these two windings is the same.

• The primary and secondary windings are tuned and lightly coupled.

This means that there is a phase difference of 90 degrees between

the signals in these windings at the centre frequency. If the signal

moves away from the centre frequency the phase difference will

change. In turn the phase difference between the secondary and third

windings also varies. When this occurs the voltage will subtract from

one side of the secondary and add to the other causing an imbalance

across the resistors R1 and R2. As a result this causes a current to

flow in the third winding and the modulation to appear at the output.

• The capacitors C1 and C2 filter any remaining RF signal which may

appear across the resistors. The capacitor C4 and R3 also act as

filters ensuring no RF reaches the audio section of the receiver.

53

Ratio detector advantages & disadvantages

Advantages Disadvantages

Simple to construct using discrete

components

Offers good level of performance and

reasonable linearity

High cost of transformer

Typically lends itself to use in only

circuits using discrete components and

not integrated within an IC

54

As a result of its advantages and disadvantages the ratio detector is not widely used these days.

Techniques that do not require the use of a transformer with its associated costs and those that can be

more easily incorporated within an IC tend to be used.

Foster Seeley Discriminator or FM Detector

• The Foster Seeley detector or as it is sometimes described the Foster Seeley

discriminator has many similarities to the ratio detector. The circuit topology looks

very similar, having a transformer and a pair of diodes, but there is no third

winding and instead a choke is used.

Like the ratio detector, the Foster-Seeley circuit operates using a phase

difference between signals. To obtain the different phased signals a connection is

made to the primary side of the transformer using a capacitor, and this is taken to

the centre tap of the transformer. This gives a signal that is 90 degrees out of

phase.

When an un-modulated carrier is applied at the centre frequency, both diodes

conduct, to produce equal and opposite voltages across their respective load

resistors. These voltages cancel each one another out at the output so that no

voltage is present. As the carrier moves off to one side of the centre frequency

the balance condition is destroyed, and one diode conducts more than the other.

This results in the voltage across one of the resistors being larger than the other,

and a resulting voltage at the output corresponding to the modulation on the

incoming signal.

The choke is required in the circuit to ensure that no RF signals appear at the

output. The capacitors C1 and C2 provide a similar filtering function.

Both the ratio and Foster-Seeley detectors are expensive to manufacture. Wound

components like coils are not easy to produce to the required specification and

therefore they are comparatively costly. Accordingly these circuits are rarely used

in modern equipment.

55

The Foster-Seeley discriminator / detector

Foster-Seeley detector advantages & disadvantages

Advantages Disadvantages

Offers good level of performance

and reasonable linearity.

Simple to construct using discrete

components.

Does not easily lend itself to being

incorporated within an integrated

circuit.

High cost of transformer.

56

As a result of its advantages and disadvantages the Foster Seeley detector or discriminator is not

widely used these days. Its main use was within radios constructed using discrete components.

PLL FM demodulator / detector

• The way in which a phase locked loop, PLL FM

demodulator works is relatively straightforward. It requires

no changes to the basic phase locked loop, itself, utilizing

the basic operation of the loop to provide the required

output.

• When used as an FM demodulator, the basic phase

locked loop can be used without any changes. With no

modulation applied and the carrier in the centre position

of the pass-band the voltage on the tune line to the VCO

is set to the mid position. However if the carrier deviates

in frequency, the loop will try to keep the loop in lock. For

this to happen the VCO frequency must follow the

incoming signal, and in turn for this to occur the tune line

voltage must vary. Monitoring the tune line shows that the

variations in voltage correspond to the modulation applied

to the signal. By amplifying the variations in voltage on

the tune line it is possible to generate the demodulated

signal.

57

Phase locked loop PLL FM demodulator

PLL FM demodulator performance

• The PLL FM demodulator is normally considered a relatively high performance form of

FM demodulator or detector. Accordingly they are used in many FM receiver

applications.

• The PLL FM demodulator has a number of key advantages:

• Linearity: The linearity of the PLL FM demodulator is governed by the voltage to

frequency characteristic of the VCO within the PLL. As the frequency deviation of the

incoming signal normally only swings over a small portion of the PLL bandwidth, and

the characteristic of the VCO can be made relatively linear, the distortion levels from

phase locked loop demodulators are normally very low. Distortion levels are typically a

tenth of a percent.

• Manufacturing costs: The PLL FM demodulator lends itself to integrated circuit

technology. Only a few external components are required, and in some instances it

may not be necessary to use an inductor as part of the resonant circuit for the VCO.

These facts make the PLL FM demodulator particularly attractive for modern

applications.

58

Quadrature FM Demodulator / Detector

• It can be seen that the signal is split into two

components. One of these passes through a network

that provides a basic 90° phase shift, plus an element

of phase shift dependent upon the deviation.

• The original signal and the phase shifted signal are

then passed into a multiplier or mixer.

• If the operation of the system is designed to ensure

that the deviation remains well away from the ±90°

points, then the linearity remains very good.

• In terms of performance, the quadrature detector is

able to operate with relatively low input levels,

typically down to levels of around 100 microvolt's and

it is very easy to set up requiring only the phase shift

network to be tuned to the centre frequency of the

expected signal. It also provides good linearity and

this results in low levels of distortion.

59

Quadrature FM demodulator circuit

Mixer phase response for

quadrature FM detector

Quadrature detector advantages & disadvantages

60

Advantages Disadvantages

Offers good level of performance

and including linearity.

Can be incorporated into an

integrated circuit.

Requires the use of a coil.

Some designs may require setting

during manufacture.

Despite the disadvantages, the quadrature FM detector is the circuit of choice for many radio

receivers these days.

Phase Modulation

• As the name implies, phase modulation, PM uses variations in phase for carrying the modulation. Phase modulation, PM is

sometimes used for analogue transmission, but it has become the basis for modulation schemes used for carrying data.

Phase shift keying, PSK is widely used for data communication.

• Phase modulation is also the basis of a form of modulation known as quadrature amplitude modulation, where both phase

and amplitude are varied to provide additional capabilities.

• Before looking at phase modulation it is first necessary to look at phase itself. A radio frequency signal consists of an

oscillating carrier in the form of a sine wave is the basis of the signal. The instantaneous amplitude follows this curve moving

positive and then negative, returning to the start point after one complete cycle - it follows the curve of the sine wave.

61

• The sine wave can also be represented by the movement of a point around a circle, the phase at any given point

being the angle between the start point and the point on the waveform as shown.

62

Phase angle of points on a sine wave

• Also the phase advances as time

progresses so points on the waveform

can be said to have a phase difference

between them.

63

• Phase modulation works by modulating the phase of

the signal, i.e. changing the rate at which the point

moves around the circle. This changes the phase of

the signal from what it would have been if no

modulation was applied. In other words the speed of

rotation around the circle is modulated about the

mean value.

To achieve this it is necessary to change the

frequency of the signal for a short time. In other

words when phase modulation is applied to a signal

there are frequency changes and vice versa. Phase

and frequency are inseparably linked as phase is the

integral of frequency.

Frequency modulation can be changed to phase

modulation by simply adding a CR network to the

modulating signal that integrates the modulating

signal. As such the information regarding sidebands,

bandwidth and the like also hold true for phase

modulation as they do for frequency modulation,

bearing in mind their relationship.64

Forms of phase modulation

Although phase modulation is used for some analogue transmissions, it is far more widely used as a

digital form of modulation where it switches between different phases. This is known as phase shift

keying, PSK, and there are many flavours of this. It is even possible to combine phase shift keying and

amplitude keying in a form of modulation known as quadrature amplitude modulation, QAM.

The list below gives some of the forms of phase shift keying that are used:

PM - Phase Modulation

PSK - Phase Shift Keying

BPSK - Binary Phase Shift Keying

QPSK - Quadrature Phase Shift Keying

8 PSK - 8 Point Phase Shift Keying

16 PSK - 16 Point Phase Shift Keying

OPSK - Offset Phase Shift Keying

These are just some of the major forms of phase modulation that are widely used in radio

communications applications today. With today's highly software adaptable radio communications

systems, it is possible to change between the different types of modulation to best meet the prevailing

conditions.

65

FM TRANSMITTER

• The part of the Armstrong FM transmitter (Armstrong phase

modulator) which is expressed in dotted lines describes the

principle of operation of an Armstrong phase modulator. It

should be noted, first that the output signal from the carrier

oscillator is supplied to circuits that perform the task of

modulating the carrier signal. The oscillator does not change

frequency, as is the case of direct FM. These points out the

major advantage of phase modulation (PM), or indirect FM,

over direct FM. That is the phase modulator is crystal

controlled for frequency.

• The crystal-controlled carrier oscillator signal is directed to two

circuits in parallel. This signal (usually a sine wave) is

established as the reference past carrier signal and is

assigned a value 0°.The balanced modulator is an amplitude

modulator used to form an envelope of double side-bands and

to suppress the carrier signal (DSSC). This requires two input

signals, the carrier signal and the modulating message signal.

The output of the modulator is connected to the adder circuit;

here the 90° phase-delayed carriers signal will be added back

to replace the suppressed carrier. The act of delaying the

carrier phase by 90° does not change the carrier frequency or

its wave-shape. This signal identified as the 90° carrier signal. 66

• The carrier frequency change at the adder output is a function of

the output phase shift and is found by. fc = ∆θfs (in hertz)

•

• When θ is the phase change in radians and fs is the lowest audio

modulating frequency. In most FM radio bands, the lowest audio

frequency is 50Hz. Therefore, the carrier frequency change at

the adder output is 0.6125 x 50Hz = ± 30Hz since 10% AM

represents the upper limit of carrier voltage change, then ± 30Hz

is the maximum deviation from the modulator for PM.

•

• The 90° phase shift network does not change the signal

frequency because the components and resulting phase change

are constant with time. However, the phase of the adder output

voltage is in a continual state of change brought about by the

cyclical variations of the message signal, and during the time of

a phase change, there will also be a frequency change.

• In figure. (c). during time (a), the signal has a frequency f1, and

is at the zero reference phase. During time (c), the signal has a

frequency f1 but has changed phase to θ. During time (b) when

the phase is in the process of changing, from 0 to θ. the

frequency is less than f1.67

Reactance modulator direct method

• The FM transmitter has three basic sections.

• 1. The exciter section contains the carrier oscillator,

reactance modulator and the buffer amplifier.

• 2. The frequency multiplier section, which features several

frequency multipliers.

• 3. The power output section, which includes a low- level

power amplifier, the final power amplifier, and the impedance

matching network to properly load the power section with the

antenna impedance.

• The essential function of each circuit in the FM transmitter may

be described as follows.

• The reactance modulator takes its name from the fact that the

impedance of the circuit acts as a reactance (capacitive or

inductive) that is connected in parallel with the resonant circuit

of the Oscillator. The varicap can only appear as a capacitance

that becomes part of the frequency determining branch of the

oscillator circuit. However, other discrete devices can appear as

a capacitor or as an inductor to the oscillator, depending on how

the circuit is arranged. A colpitts oscillator uses a capacitive

voltage divider as the phase-reversing feedback path and would

most likely tapped coil as the phase-reversing element in the

feedback loop and most commonly uses a modulator that

appears inductive.68

Comparisons of Various Modulations

69

Comparisons of Narrowband and Wideband FM

70

References

Book:

1. Taub & Schiling “Principles of Communication Systems” Tata McGraw hill 2007.

2. Kennedy and Davis “Electronic Communication Systems” Tata McGraw hill, 4th Edition, 1993.

3. Sklar “Digital Communication Fundamentals and Applications“ Pearson Education, 2001.

4. TG Thomas and S Chandra Sekhar, “Communication Theory” Tata McGraw hill 2006.

Web:

http://www.doc.ic.ac.uk/~nd/surprise_96/journal/vol4/vk5/report.html

http://www.ni.com/white-paper/14940/en/

https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/8909

https://www.electronics-notes.com/articles/radio/modulation/amplitude-modulation-am.php

https://en.wikipedia.org/wiki/Double-sideband_suppressed-carrier_transmission

http://www.radio-electronics.com/info/rf-technology-design/am-amplitude-modulation/single-sideband-ssb-modulation.php

http://cpassignments.blogspot.in/2015/04/block-diagram-of-am-transmitter-and.html

http://www.radio-electronics.com/info/rf-technology-design/fm-reception/fm-slope-detector-discriminator.php

PPT:

https://cnx.org/resources/.../Amplitude%20Modulation.ppt

mason.gmu.edu/~abaranie/it101/lecture15.ppt

soe.northumbria.ac.uk/ocr/teaching/ppp/SSBSC/SSBSC.ppt

71

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72Dr Gnanasekaran Thangavel12/12/2017

73

Thank You

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