EC6651 COMMUNICATION ENGINEERING UNIT 1

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 wave
Frequency varying-amplitude
constant
Large amplitude: high
frequency
Small 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:
29
Single sideband modulation
showing upper and lower sideband signals

30
SSB-SC - Implementation
• Frequency discrimination
Multiplier
Message
m(t)
Local oscillator
c(t) = cos ct
DSB-SC
t
ME
t
ME
ttEtc
mc
c
mc
c
cmm
)(cos
2
)(cos
2
coscos)(


Band pass
filter
c+ c
Band pass
filter
c- c
t
ME
tc mc
c
)(cos
2
)( 
t
ME
tc mc
c
)(cos
2
)( 
Upper sideband
Lower sideband

31
SSB-SC - Waveforms
B = 2m
USB
Bandwidth B = m
B = m

32
SSB-SC - Implementation cont.
• Phase discrimination (Hartley modulator)
X
SSB-SC
signal
X
Em sin mt sin ct
sin ct
cos ctCarrier
90o
phase shift
Message
m(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
 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
 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
 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

Other presentations
http://www.slideshare.net/drgst/presentations
72Dr Gnanasekaran Thangavel12/12/2017

73
Thank You
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EC6651 COMMUNICATION ENGINEERING UNIT 1

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