appendix a: practica experiment ems-1: am modulation / demodulation

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APPENDIX A: PRACTICA
EXPERIMENT EMS-1: AM MODULATION / DEMODULATION
OBJECTIVES: To theoretically analyse AM modulation and demodulation in the time and frequency domains and
investigate a chopper modulator/demodulator implementation thereof.
I PREREQUISITES
Knowledge of elementary Fourier theory (time-frequency relationships and Fourier-identities), amplitude modulation (AM),
convolution/correlation and filter analysis and synthesis is required.
This work must be done in your lab book that is especially kept for this purpose. Note that it must be your own work, and in
your own handwriting (i.e. your personal observations, conclusions, etc.)! No copies of problems and theoretical work, with
the exceptions of sketches that has been taken from books and sources for references purposes, may be pasted into your lab
books!!!
II REFERENCES
[1] B.P. Lathi and Z. Ding, Modern Digital and Analog Communication Systems, 4 th Edition, Oxford University
Press, 2010. ISBN: 978-0-19-538493-2
[2] B. P. Lathi, Modern Digital and Analog Communication Systems,3 rd Edition, Oxford University Press, 1998.
ISBN: 0-19-511009-9
[3] John G. Proakis and Masoud Salehi, Communication Systems Engineering, 2 nd Edition, Prentice-Hall, 2002,
Chapter 3. ISBN: 0-13-061793-8
[4] S. Haykin, Communication Systems, 3rd edition, Wiley, 1994, Chapter 3 pp 121-151. ISBN: 0-471-57176-8
III THEORETICAL PREPARATION AND PRELIMINARY REPORT
P.1) Consult and study the references above. Identify three distinct methods whereby information can be transmitted by
employing a cosinusoidal ‘carrier’ signal as information bearer.
P.2) Is a modulator a linear time-invariant (LTI) system? Illustrate/motivate your answer.
P.3) Investigate the multiplication operation in the following cases:
P.3.1) multiply two decimal numbers, say 3330 x 1110. Establish whether there is any resemblance with convolution.
Explain/Motivate your answer.
P.3.2) Now multiply the numbers 33 x 11, using their binary representation with 8-bit word lengths. Identify the use of any
specific algorithm, if any, in doing this. Is there any relation to convolution? How does the algorithm cope with negative
numbers?
P.3.3) Propose a linear analogue device (give complete specifications) capable of multiplying any two analogue values
(voltages) in the range ±1 V and specify it’s circuit diagram. What happens if the amplitude range of the multiplier terms
is increased to ±5 V?
P.3.4) Propose a binary device that can perform the multiplication in 3.2, and provide full circuit details.
P.4) Identify at least two commercial applications of AM and specify the version of AM modulation used in each case. Give
as many modulation parameters/specifications possible for each application (e.g, IF frequency used, typical modulation
bandwidths, modulation indices etc).
P.5) The Fig. 3-1 below shows the circuit diagram of an AM chopper modulator. Distinguish between unbalanced and
balanced chopper operation. Analyse the operation of the given circuit electronically and show that it is indeed a
balanced modulator. Give all assumptions and approximations made.
P.6) Also analyse the operation of the circuit in the time and frequency domains and state what effect(s) a balanced
configuration has on the modulator/demodulator output spectrum. Your preparation must theoretically illustrate and
verify as many possible aspects of the experiment that has to be performed in the lab.
P.7 ) Illustrate how you will use the circuit as an AM modulator. Assume an input signal m(t)=Acos(ωmt) and a balanced

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external switching signal Ps(t) with amplitude ±B=±5Volt and frequency fc=1/Tc >> fm Hz] to drive the electronic switch.
What is the effective (internal) carrier signal Peff (t) equal to in this case, so that y(t)= Peff (t).m(t) ?
Fig. 3-1 Chopper modulator block diagram.
HINT: Carry out separate analyses for the amplifier with the switch in the closed and open states, respectively.
P.6.3) Specify the output filter for both unbalanced and balanced operation, so that an undistorted AM output signal will be
obtained in both cases.
P.6.4) What effect does this filter have on the final output time signal v(t), compared to the signal y(t)? Illustrate with sketches
in the frequency and time domains
P.6.5) How would you modify the circuit to yield an unbalanced chopper modulator?
V EQUIPMENT NEEDED
Two signal generators (one for the information signal, the other for the carrier), oscilloscope, spectrum analyser, power supplies,
volt meters, etc. Furthermore, it is expected from each student to build his own experimental modules, if necessary, when
such building blocks are not provided as part of the laboratory equipment.
VI EXPERIMENTAL PREPARATION
E.1) Build and test the chopper modulator beforehand, and verify correct operation by inspecting the output waveform with
elementary modulation signals as input. Also duplicate the chopper modulator in Figure 1, for the purpose of using it as a
synchronous AM demodulator.
E.2) Devise a method whereby the effective switching signal, Peff(t), of the chopper modulator in Figure 1 can be determined and
measured. Use this signal to derive an expression for the chopper modulator output, v(t)=m(t).Peff(t).
E.3) All subsequent experimental measurements should be taken in both the time and frequency domains simultaneously.
VII EXPERIMENT ASSIGNMENTS
AM Modulation
M.1) Investigate AM modulation of the chopper modulator. Observe both time and frequency domain waveforms and
corresponding spectra and compare with theory.
M.2) Investigate what effect a DC-offset of the modulating signal, m(t), has on the AM output signal. What is this version of
AM modulation called and when/where is it used?
M.3 ) Compare y(t) and v(t) (see Figure 1) and explain any differences (time- and frequency-wise).

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AM demodulation
D.1) Implement a simple low pass filter or band pass filter to use at the output of the AM modulator in Figure 1. Specify the
required bandwidth in each case.
D.2) Add the necessary subsystems to the AM modulator in VII 1 above in order to demodulate the AM signal, v(t). How
would you recover the original modulating signal, m(t), from the output of the AM chopper demodulator:
D.2.1) In the case of AM suppressed carrier modulation?;
D.2.2) In the case of AM with carrier modulation?
Identify all unwanted demodulator terms in each case and describe how you would eliminate them.
D.3) Investigate the effect of AM demodulation with an unsynchronised (out-of-phase) carrier at the AM receiver.
VIII LABORATORY RESULTS
1. Even though this experiment requires group work, each student must be able to demonstrate each experimental assignment in
VII, document his/her own observations and results in his/her lab book and compare it with theory.
2. Each student in the group must be able to answer questions individually.
IX EVALUATION TEST
A short optical mark reader evaluation test will be taken in class on the date and time to be determined.
REMARKS:
The given AM chopper modulator will be used again in subsequent experiments, which will investigate various forms of
synchronisation and carrier recovery in AM and digital modulation systems. It is therefore important to take particular care
with the construction of the chopper modulator and to keep it intact for future lab sessions.

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OBJECTIVES
EXPERIMENT EMS-2: FM MODULATION / DEMODULATION
To theoretically analyse FM-modulation in the time and frequency domains and investigate several
modulator/demodulator implementation methods
I PREREQUISITES
Knowledge of elementary Fourier theory (time-frequency relationships and Fourier-identities), frequency modulation (FM), basic
electronic design principles and filter analysis and synthesis is required.
This work must be done in your lab book, which is especially kept for this purpose. Note that it must be your own work, and in
your own handwriting (i.e. your personal observations, conclusions, etc.)! No copies of problems and theoretical work, with
the exceptions of sketches that has been taken from books and sources for references purposes, may be pasted into your lab
books!!!
II REFERENCES
[1] B.P. Lathi and Z. Ding, Modern Digital and Analog Communication Systems, 4 th Edition, Oxford University
Press, 2010. ISBN: 978-0-19-538493-2
[2] B. P. Lathi, Modern Digital and Analog Communication Systems,3 rd Edition, Oxford University Press, 1998.
ISBN: 0-19-511009-9
[3] John G. Proakis and Masoud Salehi, Communication Systems Engineering, 2 nd Edition, Prentice-Hall, 2002,
Chapter 3. ISBN: 0-13-061793-8
III THEORETICAL PREPARATION
NOTE: Your preparation/analysis must theoretically illustrate and verify as many possible aspects of the experiment that has to
be performed in the lab as possible.
P.1) Do problems 5.2-1 and 5.2-6 from [1] (Lathi), and 5.32 and 5.36 from [3] (Proakis).
P.2) Is an FM-modulator a linear time-invariant (LTI) system? Illustrate/motivate your answer.
P.3) Identify at least two applications of FM. Give as many modulation parameters/specifications possible for each
application (e.g, IF frequency used, typical modulation bandwidths, frequency deviation, modulation indices etc).
P.4) Analyse the operation of a narrowband and wideband FM/PM modulator in the time and frequency domains. Draw
phasor diagrams and illustrate the principle of angle modulation, as opposed to amplitude modulation. Quantify the errors
made in the narrowband/wideband angle modulation processes, e.g., residual amplitude modulation, phase error etc.
P.5) The following figure shows the circuit diagram of a parametric FM-generator using a ‘varicap’.
P.5.1) Analyse the operation of the circuit and show that it indeed produces an FM signal. Show that the expression for the
oscillating frequency is given by:
where Cvo denotes the varicap capacitance at nominal reverse bias Vo and Ce=C1.C2/(C1+C2). The instantaneous output frequency,
as a function of the input control signal, m(t), is:
⎡ ∆ C ⎤
v
fi ≈ fo
. ⎢1−
⎥
⎣⎢
2(
Cv + C o e)
⎦⎥
f
o
⎡ 1 ⎤
= ⎢
⎥
⎣⎢
2π L0( Cv + C o e)
⎦⎥

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Where ∆Cv=Cv-CVo= Kc.m(t), and Kc [F/Volt] is the varicap capacitance-to-voltage conversion factor.
P.5.2) Investigate the possibilities of generating narrowband as well as wideband FM-signals with the given circuit. Which are
the limiting factors and can they be overcome? Assume an input signal m(t)=Mcos(ωmt).
CLAPP Voltage Controlled Oscillator (VCO)
P.6) Design an FM-discriminator using a suitable approximation to a differentiator (such as, e.g., the one illustrated in the
accompanying design example).
P.7) Design a simple envelope detector (similar to the one used for AM-detection).
IV EQUIPMENT
Two signal generators (one for the information signal, one for the carrier), oscilloscope, spectrum analyser, power supplies, volt
meters, etc. Furthermore, it is expected from each student to build his own experimental modules, if necessary, when such
building blocks are not provided as part of the laboratory equipment.
V EXPERIMENTAL PREPARATION
E.1) Determine the voltage-to-frequency transfer function of the carrier frequency generator corresponding to your lab work
bench (identify the generator by making a note of its number in your lab book!) Calculate the VCO-constant, kf [Hz/V], from
the measured input-output graph, when the carrier frequency is fc=10 kHz.
E.2) Determine/specify the FM-modulation parameters to experimentally reproduce the spectrum of the FM-signal calculated
in [3] problem 3.32 (but with fc=10 kHz in stead of 1 MHz), using the FM-generator (VCO) measured/quantified in 1) above.
Take as modulating signal, m(t)=Mcos(ωmt), with M=2 Volt and fm=1 kHz.
E.3) All subsequent experimental measurements should be taken in both the time and frequency domains simultaneously.
VI EXPERIMENTAL ASSIGNMENTS
FM-modulation
M.1) Verify the value of the FM-modulation constant in V, E.1) by applying the first-Bessel-zero test.
M.2 Illustrate NBFM.
M.3) Reproduce the spectrum of the WBFM-signal of Problem 3.32 in [3]. Compare your result with theory.
M.4) Illustrate Woodward’s theorem i , by reducing fm to several Hz, and appropriately adjusting the amplitude of m(t).
M.5) Investigate the phenomenon known as β-multiplication at a carrier frequency of f3=3.fc, by generating the appropriate
harmonic of the signal in M.3) above.

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FM-demodulation
D.1) Illustrate FM-demodulation by applying your FM-generator output to the FM-discriminator and envelope detector
designed for this purpose.
VII LABORATORY RESULTS
1. Each student must demonstrate each experimental assignment in VI, document his/her own observations and results in his/her
lab book and compare it with theory.
2. Each student in the group must be able to answer question individually.
VIII EVALUATION TEST
A short optical marksheet test will be taken in class on completion of this experiment.
REMARKS:
Although the best part of the FM-modulator experiment can be conducted by using standard lab equipment, bonus marks will be
in order when students demonstrate their own FM-generator design.
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1 Woodward’s theorem states that the spectrum (or power density spectrum) of an Ultra Wideband FM-signal, v WBFM (t), aproaches and has the same form as the
Amplitude Density Function (a-wdf) of the information signal, m(t).

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OBJECTIVES
Theoretically analyse the PLL
Design and implement a PLL for carrier tracking.
Design and implement a PLL for FM demodulation.
EXPERIMENT EMS-3: PLL-APPLICATIONS
I INTRODUCTION, PREPARATION AND PREREQUISITES
A knowledge of amplitude- (AM) and angle modulation (FM and PM), convolution/correlation, elementary Fourier-theory
(time/frequency-relationships and Fourier-identities) and elementary control theory (feedback) is required. .
It is recommended that sections III to V ( theoretical preparation and system analysis ) be done prior to the scheduled open lab
sessions. Answer all questions in your lab book, which is kept especially for this purpose. Note that it must be your own
work, and in your own handwriting (i.e. your personal observations, conclusions, etc.)! No copies of problems and theoretical
work, with the exceptions of sketches that has been taken from books and sources for references purposes, may be pasted into
your lab books!!!
In advance: Study the specifications and operation of a specific Voltage-Controlled-Oscillator (VCO) signal generator in the
laboratory (make a note of the equipment number for later use). This was also done in practical 2.
II REFERENCES
[1] B.P. Lathi and Z. Ding, Modern Digital and Analog Communication Systems, 4 th Edition, Oxford University
Press, 2010. ISBN: 978-0-19-538493-2
[2] B. P. Lathi, Modern Digital and Analog Communication Systems,3 rd Edition, Oxford University Press, 1998.
ISBN: 0-19-511009-9
[3] John G. Proakis and Masoud Salehi, Communication Systems Engineering, 2 nd Edition, Prentice-Hall, 2002,
Chapter 3. ISBN: 0-13-061793-8
III THEORITICAL PREPARATION
1) Consult and study the references above. Identify at least three applications of a PLL.
2) Study and describe the fundamentals of the operation of a PLL, identify the main elements and ascertain yourself of the
function and purpose of each parameter, subsystem and component. Your preparation must theoretically illustrate and verify
as many possible aspects of the experiment that has to be performed in the lab.
3) Describe, with the aid of the necessary expressions and sketches, the concepts hold-in range and pull-in range (capture
range) of a PLL. How does the noise effective bandwidth, natural frequency and dampening factor of the PLL influence these
ranges?
4) Examine theoretically how the input signal amplitude, Ac [volt], influences the operation of the PLL.
5) Sketch the block diagram of a system which will recover a carrier from a DSB-AM-SC signal. Analyse in detail.
6) The following figure shows the circuit diagram of a balanced chopper modulator. v t)
= Acos(
t )
ω + φ
1(
1 1
6.1) Analyse the operation of the circuit and show that it is indeed a balanced modulator (refer to Exp 1).
6.2) Illustrate how you will use the circuit as a phase detector. Assume an input signal and a balanced switching signal p(t) with
amplitude B=5Volt and frequency fc=1/Tc [Hz].

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HINTS:
Show that the chopper modulator, followed by a LPF, fulfils the function of a correlator, i.e. that it gives a voltage output that is
directly proportional to the relative time shift between the two correlator inputs.
Note that a time shift can be written as a phase difference, which is exactly the function the phase detector fulfills.
Balanced-Chopper Modulator
P.7) Sketch a block diagram of a PLL-based FM demodulator. Analyse it in detail.
P.8) Illustrate how you will use an XOR-gate as a phase detector. Plot the VCO control signal vs. the phase error. Determine the
gain of the XOR phase detector.
IV EQUIPMENT NEEDED
Two signal generators (one with VCO-output), Oscilloscope, spectrum analyser, power supplies, volt meters, etc. Furthermore, it
is expected from each student to build his own experimental modules, if necessary, when such building blocks are not
provided as part of the laboratory equipment. To this purpose students are encouraged to purchase their own set of basic lab
equipment 2 .
V EXPERIMENTAL PREPARATION
E.1) In advance, study the specification and operation of a specific Voltage-Controlled-Oscillator (VCO) signal generator in the
laboratory (make a note of the equipment number for later use).
E.2) Measure and plot the voltage-to-frequency transfer curve of the VCO by plotting the instantaneous frequency, fi(t) [Hz],
against the input (control) voltage vi [Volt], around the nominal PLL-frequency of say fc=10 [kHz], where the latter is
typically the carrier frequency. Calculate, from this, the VCO-constant, kf [Hz/Volt] as well as the VCO-transfer constant, kω
[rad/v-sec]. The latter is an important PLL-design parameter.
E.3) Use the chopper modulator in Figure 1 as phase detector. Determine the effective phase detector constant, KD [V/rad].
E.4) Design and implement a second-order PLL that will be able to lock onto a signal within the VCO frequencyrange of 8 to 11
kHz, without frequency or phase error (refer [3]). Additional PLL-design details is attached. Determine your total PLL-loop
gain? Specify units!
E.5) Design and implement a second-order PLL that will be able to demodulated an FM signal. Choose a carrier frequency in the
order of fc=10 kHz and a sinusoidal message with a frequency of fm=1 kHz.
2 Sharp nosed pliers, cutters, soldering iron, screw driver, basic electronic components, isolated electric wire, set of oscilloscope probes
e tc

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VI EXPERIMENT ASSIGNMENTS
CARRIER RECOVERY
A.1) Choose an arbitrary sinusoidal signal within the PLL frequency range as carrier and demonstrate the PLL’s ability to lock
onto and track this carrier.
A.2) Determine the phase detector transfer function as a function of the phase difference and compare it with theory.
A.3) Measure the pull-in- and hold-in (lock) ranges of the PLL, respectively, and compare it with the theoretical predicted
results.
A.4) Determine what effect the input signal amplitude, Ac [Volt], has on the locking behaviour of the PLL.
A.5) Throughout this experiment plot time and frequency domain measurements at important points in your PLL circuit.
DEMODULATION OF AN FM SIGNAL
F.1) Use a PLL to recover the message m(t) from an FM modulated signal.
F.2) Determine what effect the message bandwidth has on the demodulation performance of the PLL.
F.3) Throughout this experiment plot time and frequency domain measurements at important points in your PLL circuit.
VII LABORATORY RESULTS
1. Each student must document his/her own observations and results in his/her lab book and compare it with theory.
2. Demonstrate each experimental assignment in VI.
3. Each student in the group must be able to answer question individually.