Design and implementation of test bench for frequency modulation and demodulation

1. Ministry of Higher Education
And Scientific Research
University of Technology
Electrical Engineering Department
Design And Implementation Of Test Bench
for frequency Modulation And Demodulation
A project
Submitted to the department of Electrical Engineering, University of
Technology, in partial fulfillment of the requirement for the degree of
Bachelor of Science in Communication Engineering
Team of work
karrar abdul hadi jafar sarmad kadhem abdul majid
SUPERVISED BY:
Assist. Prof. Dr.Ashwaq Q.Hameed
2016

3. Table of Contents
Contents page
Acknowledgements
Abstract
List of Contents
Chapter One
1-1 Introduction 2
1-2 Technical Background 3
1-3 Modulation Index 4
1-4 Bessel function 5
FM Power Distribution 8
Average Power 8
Chapter two
2-1 Frequency Modulation (FM) 10
2-2 Theory of Frequency Modulation (FM) 11
Principles of Electronics 13
2-3 Narrow Band FM 14
2-4 Narrow band FM Modulator 14
2-5 Wide Band FM Modulator 16
2-6 Generation of Wideband FM 19
2-6-1 Indirect Method 19
2-6-2 Direct Method 20
Chapter Three
3-1 Demodulation 24
3-2 Essentials in Demodulation 25
3-3 Demodulation of FM Signals 26
3-4 FM Demodulator Classification 27
3-5 frequency discrimination 28

4. Chapter four
4.1 FREQUENCY MODULATION CIRCUIT DIAGRAM 31
4.2 FM DEMODULATOR 32
4.3 AIM 33
4.4 THEORY 34
4.5 The results 36
4.5.1 Frequency Modulation 36
4.5.2 Frequency Demodulation 38
Reference

5. Acknowledgment
The success and outcome of this project required a lot of guidance and
assistance from many people and we are extremely fortunate to have
got this all along the completion of our project work. Whatever we have
done is only due to such guidance and assistance, and we would not
forget to thank them.
we thank God for providing us with everything that we required in
completing this project.
we are highly indebted to our supervisor Assist. Prof. Dr.Ashwaq
Q.Hameed for her guidance and constant supervision as well as for
providing necessary information regarding the project and also for
her support in completing it .
we would like to express our gratitude towards my parents for their kind
cooperation and encouragement, which helped us along our study
journey. They are our candle in all dark times.
Our thanks and appreciations also go to our project group and
classmates in developing the project and to the people who have
willingly helped us out with their abilities.
at the end thankful to and fortunate enough to get constant
encouragement, support and guidance from all Teaching staffs of
Department of electrical engineering which helped us in successfully
completing our project work. Also, I would like to extend our sincere
regards to all the non-teaching staff of department of computer science
for their timely support.

6. Abstract
The communication is an essential part of nowadays life.
The modulation and demodulation it the most important part of the
communication system. The process of changing some
characteristic (e.g. amplitude, frequency or phase) of a carrier wave
by the intensity of the signal is known as modulation. The process
of recovering the audio signal from the modulated wave is known
as demodulation or detection.
There are many of them, but the FM is the most popular one. Due
to:
 Resilient to noise.
 Resilient to signal strength variations.
 Does not require linear amplifiers in the transmitter
 Enables greater efficiency than many other modes
The FM modulation is applied as a laboratory test-bench. In order
to implement and deep understand the knowledge that has been
gotten along study journey. In addition, we aimed to produce an
experimental kit to support the communication laboratory. As a
practical project, it provides an optimal environment to practice the
theoretical information. It is fabulous to learn how selecting and
combining the electronic equipments to form an integral
communication system. The proposed FM kit is a useful tool for
communication department because it was made using local
Capabilities, which made it extremely cheaper than any
commercial product. Moreover, it can be maintained simply inside
the laboratory.

8. 1.1 Introduction
The comparatively low cost of equipment for an FM broadcasting
station, resulted in rapid growth in the years following World War II.
Within three years after the close of the war, 600 licensed FM stations
were broadcasting in the United States and by the end of the 1980s there
were over 4,000. Similar trends have occurred in Britain and other
countries. Because of crowding in the AM broadcast band and the
inability of standard AM receivers to eliminate noise, the tonal fidelity of
standard stations is purposely limited. FM does not have these drawbacks
and therefore can be used to transmit music, reproducing the original
performance with a degree of fidelity that cannot be reached on AM
bands. FM stereophonic broadcasting has drawn increasing numbers of
listeners to popular as well as classical music, so that commercial FM
stations draw higher audience ratings than AM stations.
Edwin H. Armstrong, known as one of the founding fathers of radio
technology, invented the superheterodyne radio receiver in 1918 and
frequency modulation (FM) in 1933 [1]. These two concepts, along with
his regenerative circuit technique developed in 1912, formed the basis of
radio frequency electronics as we know it today. In the United States,
FM radio stations broadcast between radio frequencies of 88 MHz to 108
MHz with a channel bandwidth of 200 kHz. FM radio was first deployed
in monaural in 1940; and in 1960, FM stereo was introduced. This article
presents a basic tutorial on FM with descriptions of multiplex (MPX)
signaling and noise improvement techniques such as stereo-mono
blending and soft mute.
Frequency modulation (FM) has a long history of its application and is
widely used in radio broadcast. To transmit stereo music, FM is
enhanced by stereo multiplexing which carries both L and R audio
channel content. With the digital age, Radio Data System (RDS) enables
FM to carry text information such as traffic, weather, and radio station
information which can be displayed on the end-user’s device interface.
Currently, growing number of mobile phones and consumer mobile
devices will have an integrated FM receiver feature. The FM transmitter
feature is also becoming popular for allowing users to transmit audio
content from their mobile devices through their car radio. To make sure
the FM-related functions work well, the FM mono, FM stereo, and FM
RDS functions need to be tested in production.

9. 1.2 Technical Background
The main frequencies of interest are from 88MHz to 108MHz with
wavelengths between 3.4 and 2.77 meters respectively.

10. 1.3 Modulation Index
As in other modulation systems,the modulation index indicates by
how much the modulated variable varies around its unmodulated
level. It relates to variations in the carrier frequency:
where is the highest frequency component present in the
modulating signal xm(t), and is the peak frequency-deviation—
i.e. the maximum deviation of the instantaneous frequency from
the carrier frequency. For a sine wave modulation, the modulation
index is seen to be the ratio of the peak frequency deviation of the
carrier wave to the frequency of the modulating sine wave.
If , the modulation is called narrowband FM, and its
bandwidth is approximately .Sometimes modulation index
h<0.3 rad is considered as Narrowband FM otherwise Wideband
FM.
For digital modulation systems, for example Binary Frequency
Shift Keying (BFSK), where a binary signal modulates the carrier,
the modulation index is given by:
where is the symbol period, and is used as the
highest frequency of the modulating binary waveform by
convention, even though it would be more accurate to say it is the
highest fundamental of the modulating binary waveform. In the
case of digital modulation, the carrier is never transmitted.
Rather, one of two frequencies is transmitted, either
or , depending on the binary state 0 or 1 of the
modulation signal.
If , the modulation is called wideband FM and its
bandwidth is approximately . While wideband FM uses more
bandwidth, it can improve the signal-to-noise ratio significantly;
for example, doubling the value of , while keeping
constant, results in an eight-fold improvement in the signal-to-
noise ratio. (Compare this with Chirp spread spectrum, which uses

11. extremely wide frequency deviations to achieve processing gains
comparable to traditional, better-known spread-spectrum modes).
With a tone-modulated FM wave, if the modulation frequency is
held constant and the modulation index is increased, the (non-
negligible) bandwidth of the FM signal increases but the spacing
between spectra remains the same; some spectral components
decrease in strength as others increase. If the frequency deviation
is held constant and the modulation frequency increased, the
spacing between spectra increases.
Frequency modulation can be classified as narrowband if the
change in the carrier frequency is about the same as the signal
frequency, or as wideband if the change in the carrier frequency is
much higher (modulation index >1) than the signal frequency.
For example, narrowband FM is used for two way radio systems
such as Family Radio Service, in which the carrier is allowed to
deviate only 2.5 kHz above and below the center frequency with
speech signals of no more than 3.5 kHz bandwidth. Wideband FM
is used for FM broadcasting, in which music and speech are
transmitted with up to 75 kHz deviation from the center frequency
and carry audio with up to a 20-kHz bandwidth.
1.4 Bessel function
 Thus, for general equation:
)coscos()( tmtVtv mfCCFM  





 
 

 2
n
ncos)m(J)cosmcos(
n
n








 

n
mcnC
2
n
tntcos)m(JV)t(m

12.  It is seen that each pair of side band is preceded by J
coefficients. The order of the coefficient is denoted by
subscript m. The Bessel function can be written as
 N = number of the side frequency
 Mf = modulation index
figure 1-2
  




 





 

2
t)(cos)m(J
2
t)(cos)m(Jtcos)m(J{Vtv mCf1mCf1Cf0CFM
    )...}m(J...t)2(cos)m(Jt)2(cos)m(J fnmCf2mCf2 
   
 
 
  

















 ....
!2!2
2/
!1!1
2/1
2
42
n
m
n
m
n
m
mJ
ff
n
f
fn

14. FM Power Distribution
 As seen in Bessel function table, it shows that as the
sideband relative amplitude increases, the carrier
amplitude,J0 decreases
 This is because, in FM, the total transmitted power is
always constant and the total average power is equal to
the unmodulated carrier power, that is the amplitude of
the FM remains constant whether or not it is modulated.
 In effect, in FM, the total power that is originally in the
carrier is redistributed between all components of the
spectrum, in an amount determined by the modulation
index, mf, and the corresponding Bessel functions.
 At certain value of modulation index, the carrier
component goes to zero, where in this condition, the
power is carried by the sidebands only.
Average Power
 The average power in unmodulated carrier
 The total instantaneous power in the angle modulated
carrier.
 The total modulated power
R
V
P c
c
2
2

R
V
tt
R
V
P
tt
R
V
R
tm
P
c
c
c
t
c
c
t
2
)](22cos[
2
1
2
1
)]([cos
)(
22
2
22











R
V
R
V
R
V
R
V
PPPPP nc
nt
2
)(2
..
2
)(2
2
)(2
2
..
22
2
2
1
2
210 

16. 2.1 Frequency Modulation (FM)
When the frequency of carrier wave is changed in accordance with
the intensity of the signal, it is called frequency modulation (FM).
FM was invented and commercialized after AM. Its main
advantage is that it is more resistant to additive noise than AM.
In frequency modulation, only the frequency of the carrier wave is
changed in accordance with the signal. However, the amplitude of
the modulated wave remains the same i.e. carrier wave amplitude.
The frequency variations of carrier wave depend upon the
instantaneous amplitude of the signal as shown in Fig. 2-1. When
the signal voltage is zero as at A, C, E and G, the carrier frequency
is unchanged. When the signal approaches its positive peaks as at B
and F, the carrier frequency is increased to maximum as shown by
the closely spaced cycles. However, during the negative peaks of
signal as at D, the carrier frequency is reduced to minimum as
shown by the widely spaced cycles.
.
figure 2-1 figure 2- 2

17. Illustration. The process of frequency modulation (FM) can be
made more illustrative if we consider numerical values. Fig. 2-2
shows the FM signal having carrier frequency fc = 100 kHz.
Note that FM signal has constant amplitude but varying
frequencies above and below the carrier frequency of 100 kHz (=
fc). For this reason, fc (= 100 kHz) is called centre frequency. The
changes in the carrier frequency are produced by the audio-
modulating signal. The amount of change in frequency
from fc (= 100 kHz) or frequency deviation depends upon the
amplitude of the audio-modulating signal. The frequency deviation
increases with the increase in the modulating signal and viceversa.
Thus the peak audio voltage will produce maximum frequency
deviation. Referring to Fig.2-2, the centre frequency is 100 kHz
and the maximum frequency deviation is 30 kHz. The following
points about frequency modulation (FM) may be noted carefully :
(a) The frequency deviation of FM signal depends on the amplitude
of the modulating signal.
(b) The centre frequency is the frequency without modulation or
when the modulating voltage is zero.
(c) The audio frequency (i.e. frequency of modulating signal) does
not determine frequency deviation.
Advantages : The following are the advantages of FM over AM :
(i) It gives noiseless reception. As discussed before, noise is a form
of amplitude variations and a FM receiver will reject such signals.
(ii) The operating range is quite large.
(iii) It gives high-fidelity reception.
(iv) The efficiency of transmission is very high.
2.2 Theory of Frequency Modulation (FM)
In frequency modulation (FM), the amplitude of the carrier is kept
constant but the frequency fc of the carrier is varied by the
modulating signal. The carrier frequency fc varies at the rate of the
*signal frequency fs ; the frequency deviation being proportional to
the instantaneous amplitude of the modulating signal. Note that
maximum frequency deviation is (fc (max) – fc) and occurs at the
peak voltage of the modulating signal. Suppose we modulate a 100
MHz carrier by 1V, 1 kHz modulating signal and the maximum
frequency deviation is 25 kHz. This means that the carrier
frequency will vary sinusoidally between (100 + 0.025) MHz and
(100 – 0.025) MHz at the rate of 1000 times per second. If the

18. amplitude of the modulating signal is increased to 2V, then the
maximum frequency deviation will be 50 kHz and the carrier
frequency will vary between (100 + 0.05) MHz and (100 – 0.05)
MHz at the rate of 1000 times per second.
Suppose a modulating sine-wave signal es (= Es cos ωs t)
is used to vary the carrier frequency fc. Let the change in
carrier frequency be kes where k is a constant known as the
frequency deviation constant. The instantaneous carrier frequency
fi is given by ;
fi = fc + k es
= fc + k Es cos ωs t
A graph of fi versus time is shown in Fig. 2-3. It is
important to note that it is frequency-time curve and not
amplitude-time curve. The factor k Es represents the maximum
frequency deviation and is denoted by Δf i.e.
Max. frequency deviation, Δf = ** k Es
∴ fi = fc + Δ f cos ωs t
figure 2-3
Equation of FM wave. In frequency modulation, the
carrier frequency is varied sinusoidally at signal frequency. The
instantaneous deviation in frequency from the carrier is
proportional to the instantaneous amplitude of the modulating
signal. Thus the instantaneous angular frequency of FM is given
by;
ωi= ωc + Δωc cos ωst

19. Total phase angle θ = ωt so that if ω is variable, then,
𝜃 = ∫ 𝜔𝑖 𝑑𝑡
𝑡
0
= ∫ (𝜔𝑐 + 𝛥𝜔𝑐 𝑐𝑜𝑠 𝜔𝑠 𝑡 ) 𝑑𝑡
𝑡
0
* Note this point. It means that modulating frequency is the rate of
frequency of deviations in the RF carrier. For example, all signals
having the same amplitude will deviate the carrier frequency by the
same amount, say 50 kHz, no matter what their frequencies. On
similar lines, all signals of the same frequency,say, 3 kHz, will
deviate the carrier at the same rate of 3000 times per second, no
matter what their individual amplitudes.
** Note that k is in kHz or MHz per volt.
Principles of Electronics
∴ 𝜃 = 𝜔𝑐 𝑡 +
∆𝜔 𝑐
𝜔 𝑠
sin 𝜔𝑠 t
The term
∆𝜔 𝑐
𝜔 𝑠
is called modulation index mf .
∴ θ = 𝜔𝑐 𝑡 + 𝑚 𝑓 sin 𝜔𝑠
The instantaneous value of FM voltage wave is given by ;
e = 𝐸𝑐cos θ
or e = 𝐸𝑐 cos (𝜔𝑐 𝑡 + 𝑚 𝑓 sin 𝜔𝑠 𝑡) ...(i)
Exp. (i) is the general voltage equation of a FM wave. The
following points may be noted carefully : (i) The modulation
index mf is the ratio of maximum frequency deviation (Δf) to
the frequency (= fs) of the modulating signal i.e.
Modulation index, mf =
∆𝜔 𝑐
𝜔 𝑠
=
𝑓 𝑐(max)− 𝑓𝑐
𝑓𝑠
=
∆𝑓
𝑓𝑠
(ii) Unlike amplitude modulation, the modulation index (mf) for
frequency modulation can be greater than unity.
Frequency Spectrum. It requires advanced mathematics to
derive the spectrum of FM wave.We will give only the results
without derivation. If fc and fs are the carrier and signal
frequencies respectively, then FM spectrum will have the
following frequencies :
fc ; fc ± fs ; fc ± 2fs ; fc ± 3fs and so on.
Note that fc + fs, fc + 2fs, fc + 3fs ...... are the upper sideband
frequencies while fc – fs, fc – 2fs, fc – 3fs ...... are the lower
sideband frequencies.

20. 2.3 Narrow Band FM
Narrowband FM is in many ways similar to DSB or AM signals.
By way of illustration let us consider the NBFM signal.
Using the approximations cos = 1 and sin , when is very small.
Equation-26 shows that a NBF M signal contains a carrier
component and a quadrature carrier linearly modulated by (a
function of) the baseband signal. Since s(t) is assumed to be
bandlimited to fm therefore (t) is also bandlim-ited to fm,. Hence, the
bandwidth of N BF M is 2fm, and the N BF M signal has the same
bandwidth as an AM signal.
2.4 Narrow Band FM Modulator
According to Equation above, it is possible to generate NBFM signal
using a system such as the one shown in Fig-2.4 . The signal is
integrated prior to modulation and a DSB modulator is used to
generate the quadrature component of the NBFM signal. The carrier
is added to the quadrature component to generate an approximation
to a true NBFM signal.

21. fig 2.4
Because of the difficulty of analyzing general angle-modulated
signals, we shall only consider a sinusoidal modulating signal. Let
the modulating signal of a narrowband FM signal be :
mf(t) = am cos 2fmt
we have,
tfmf  2sin
where βf= kf am /(2fm).
βf is called the frequency modulation index and f is only defined
for a sinusoidal modulating signal.
Differentiating φ(t) and substituting d(t )/dt into Δf , we have :
B
f
f


where B = fm is the bandwidth of the modulating signal. Then, we
have :
tf
f
ak
t m
m
mf


 2sin
2
)( 

22. The bandwidth of the narrowband FM signal is 2fm Hz.
Figure 2.5 shows the vector representation of a narrowband FM
signal and an AM signal.
fig 2.5
2.5 Wide Band FM Modulator
There are two basic methods for generating FM signals known as
direct and indirect methods. The direct method makes use of a

23. device called voltage controlled oscillator (VCO) whose oscillation
frequency depends linearly on the modulation voltage.
A system that can be used for generating an FM signal is shown
in Figure-2.6.
fig 2.6
The combination of message diferentiation that drive a VCO
produces a PM signal. The physical device that generates the FM
signal is the VCO whose output frequency depends directly on the
applied control voltage of the message signal. VCO0s are easily
implemented up to microwave frequencies.
Consider the angle-modulated signal :
sc(t) = A cos (2fct + sin 2fmt)
with sinusoidal modulating signal
m(t) = am cos 2fmt.
It can be shown that sc(t) can also be written as :
tnffJAts mc
n
nc )(2cos)()(  



where :
dxeJ nxxj
n 






 )sin(
2
1
)(
The integral is known as the Bessel function of the first kind of the
n-th order and cannot be evaluated in closed form. Figure 2.7

24. shows some Bessel functions for n = 0, 1, 2, 3, and 8. Clearly, the
value of Jn() becomes small for large values of n.
Figure 2.7 Bessel functions.
It can be shown that :







oddnJ
evennJ
J
n
n
n
..),.....(
..),.......(
)(



Therefore, we can write :
sc(t) = A{J0()cos 2fct -
J1()[cos 2(fc – fm)t - cos 2(fc + fm)t
] +

25. J2()[cos 2(fc - 2fm)t + cos 2(fc +
2fm)t ] -
J3()[cos 2(fc - 3fm)t - cos 2(fc +
3fm)t ] + ...}….(4-3)
Figure 2.8 shows the amplitude spectra of FM signals with a
sinusoidal modulating signal and fixed fm.
Figure 2.7 Amplitude spectra of FM signals with sinusoidal
modulating signal and fixed fm.
2.6 Generation of Wideband FM :
2.6.1 -Indirect Method :
In this method, a narrowband frequency-modulated signal is first
generated using an integrator and a phase modulator. A frequency
multiplier is then used to increase the peak frequency deviation
from f to nf. Use of frequency multiplication normally
increases the carrier frequency from fc to n fc. A mixer or double-
sideband modulator is required to shift the spectrum down to the

26. desired range for further frequency multiplication or transmission.
This is shown in Figure 2.8 .
Figure 2.8 Indirect method of generating WFM.
2.6.2 -Direct Method :
Here the carrier frequency is directly varied in accordance with
the modulating signal. A common method used for generating
direct FM is to vary the inductance L or capacitance C of a
voltage-controlled oscillator (VCO). This is shown in Figure 2.9 .
Figure 2.9 Direct method of generating WFM.
The oscillator uses a high-Q resonant circuit. Variations in the
inductance or capacitance of the oscillator will change its
oscillating frequency. Assuming that the capacitance of the tuned
circuit varies linearly with the modulating signal m(t), we have :
C = k m(t) + C0
C = C + C0
Where ,

27. C = k m(t)
k is a constant and C0 is the capacitance of the VCO when the
input signal to the oscillator is zero. The instantaneous frequency
is given by :
0
0 12
1
2
1
C
C
LC
f
LC
f
i
i






2
1
02
1






 

C
C
ff ci
where the zero-input-signal resonance frequency is :
02
1
LC
fc


For C << C0, we can write :
fff
C
tkm
ff
C
C
ff
ci
ci
ci













 

0
0
2
)(
1
2
1

28. where ,
cc f
C
C
f
C
tkm
f
00 22
)( 

Although the change in capacitance may be small, the frequency
deviation f may be quite large if the resonance frequency fc is large. We
can alternatively vary the inductance to achieve the same effect.
Advantage - Large frequency deviations are possible and thus less
frequency multiplication is needed.
Disadvantage - The carrier frequency tends to drift and additional
circuitry is required for frequency stabilization.
To stabilize the carrier frequency, a phase-locked loop can be used. This
is shown in Figure 2.10 .
Figure 2.10 Direct method of generating WFM
with frequency stabilization.

30. 3.1 Demodulation
The process of recovering the audio signal from the modulated
wave is known as demodulation or detection.
At the broadcasting station, modulation is done to transmit the
audio signal over larger distances to a receiver. When the
modulated wave is picked up by the radio receiver, it is
necessary to recover the audio signal from it. This process is
accomplished in the radio receiver and is called demodula-tion.
Necessity of demodulation. It was noted previously that
amplitude modulated wave consists of carrier and sideband
frequencies. The audio signal is contained in the sideband
frequencies which are radio frequencies. If the modulated wave
after amplification is directly fed to the speaker as shown in
Fig. 3.1, no sound will be heard. It is because diaphragm of the
speaker is not at all able to respond to such high frequencies.
Before the diaphragm is able to move in one direction, the
rapid reversal of current tends to move it in the opposite
direction i.e. diaphragm will not move at all. Consequently, no
sound will be heard.
fig 3.1
From the above discussion, it follows that audio signal must
be separated from the carrier at a suitable stage in the receiver.
The recovered audio signal is then amplified and fed to the
speaker for conversion into sound.

31. 3.2 Essentials in Demodulation
In order that a modulated wave is audible, it is necessary to
change the nature of modulated wave. This is accomplished by a
circuit called detector. A detector circuit performs the following
two functions :
(i) It rectifies the modulated wave i.e. negative half of the
modulated wave is eliminated. As shown in Fig. 3.2 (i), a
modulated wave has positive and negative halves exactly equal.
Therefore, average current is zero and speaker cannot respond. If
the negative half of this modulated wave is eliminated as shown in
Fig. 3.2 (ii), the average value of this wave will not be zero since
the resultant pulses are now all in one direction. The average
value is shown by the dotted line in Fig. 3.2 (ii). There-fore, the
diaphragm will have definite displacement cor-responding to the
average value of the wave. It may be seen that shape of the
average wave is similar to that of the modulation envelope. As the
signal is of the same shape as the envelope, therefore, average
wave shape is of the same form as the signal.
(ii) It separates the audio signal from the carrier.
The rectified modulated wave contains the audio signal
and the carrier. It is desired to recover the audio signal.
This is achieved by a filter circuit which removes the car-
rier frequency and allows the audio signal to reach the load i.e
speaker
fig 3.2

32. 3.3 Demodulation of FM Signals
An FM demodulator is required to produce an output voltage
that is lin-early proportional to the input frequency variation.
One way to realize the requirement, is to use discriminators-
devices which distinguish one frequency from another, by
converting frequency variations into amplitude variations. The
resulting amplitude changes are detected by an envelope
detector, just as done by AM detector.
the discriminator output will be
where kd is the discriminator constant. The
characteristics of an ideal discriminator are shown in
Fig. 3.3. Discriminator can be realized by using afilter in
the stopband region, in a linear range, assuming that the
afliter is diferentiation in frequency domain.
fig 3.3

33. An approximation to the ideal discriminator characteristics can be
ob-tained by the use of a diferentiation followed by an envelope
detector (see Figure 3.3) . If the input to the diferentiator is Sm(t),
then the output of the diferentiator is
With the exception of the phase deviation (t), The output of the
diferen-tiator is both amplitude and frequency modulated. Hence
envelope detection can be used to recover the message signal. The
baseband signal is recovered without any distortion if
which is easily achieved in most
practical systems.
3.4 FM Demodulator Classification
• Coherent & Non-coherent
– A coherent detector has two inputs —one for a reference signal,
such as the synchronized oscillator signal, and one for the
modulated signal that is to be demodulated.
– A noncoherent detector has only one input, namely, the
modulated signal port.
• Demodulator Classification
– Frequency Discrimination
• Noncoherent demodulator
• FMAMEDm(t)
– Phase Shift Discrimination
• Noncoherent demodulator
• FMPMm(t)
– Phase-Locked Loop (PLL) Detector
• Coherent demodulator
• Superior performance; complex and expensive

34. 3.5 frequency discrimination

35. fig 3.5

37. FREQUENCY MODULATION CIRCUIT DIAGRAM:4.1-
fig 4.1

38. 4.2 - FM DEMODULATOR:
fig 4.2.1
fig 4.2.2 fig 4.2.3
4.3 - AIM:

39. To construct & Design Frequency modulator using IC XR2206 &
demodulate the Frequency modulated wave by using IC565.
COMPONENTS REQUIRED:
FREQUANCY MODULATION REQUIRED :
QuantityRangeApparatus
1_IC XR 2206
1100KΩResistors
110KΩResistors
34.7KΩResistors
1220KΩResistors
122µFcapacitors
11µFcapacitors
110µFcapacitors
10.01µFcapacitors
FREQUANCY DEMODULATION REQUIRED :
QuantityRangeApparatus
1_IC LM 565
110KΩResistors
14.7KΩResistors
1620ΩResistors
1220KΩResistors
11µFcapacitors
10.1µFcapacitors
20.01µFcapacitors
4.4 - THEORY :

40. Frequency modulation is also called as angle modulation. Frequency
modulation is defined as changing the frequency of the carrier with
respect to the message signal amplitude. Here the amplitude of the
carrier remains fixed & timing parameter frequency is varied. When
the modulating signal has zero amplitude,then the carrier has frequency of
Fc as amplitude of the modulating signal increases. The frequency of the
carrier increases, similarly, as the amplitude of the modulating signal
decreases, the frequency of the carrier decreases.
PIN DIAGRAM (XR-2206):
fig 4.3

41. fig 4.4
fig 4.5

42. 4.5 - The results
4.5.1 - Frequency Modulation
In the practical part of the FM after performing experiments through Input signal
change we noticed the negative of half cycle of the signal made the carrier dropped
below frequency and the positive half cycle made the carrier rise frequancy and
increase the frequency that will increase the close and wide of modulated signal and
the best case in 410 HZ
fig 4.5.1
fig 4.5.2

43. fig 4.5.3
fig 4.5.4

44. 4.5.2 - Frequency Demodulation
In demodulation the received signal like the original signal after remove the carrier
signal
fig 4.5.5
fig 4.5.6

45. fig 4.5.7

46. References
[1] B.P. Lathi. Modern Digital and Analog Communication
Systems, 3rd Ed. Oxford University Press, 1998
[2] . "Communication Systems" 4th Ed, Simon Haykin, 2001
[3] S. Haykin, Communication Systems, 3rd
Edition, Wiley, 1994
[4] Armstrong, E. H. (May 1936). "A Method of Reducing
Disturbances in Radio Signaling by a System of Frequency
Modulation". Proceedings of the IRE (IRE) 24 (5): 689–740
[5] . Leon W. Couch II, Digital and Analog Communication
Systems, 8th edition, Pearson / Prentice, Chapter 4
[6] B. Boashash, editor, "Time-Frequency Signal Analysis and Processing –
A Comprehensive Reference", Elsevier Science, Oxford, 2003
[7] Contemporary Communication Systems, First Edition by M F
Mesiya– Chapter 5