Download Analog Communication Lab(PEC-453)

COLLEGE OF ENGINEERING ROORKEE (COER)
Department of Electronics and Telecommunication Engineering
Lab Manual
on
Analog Communication Lab
PEC 453
1
INSTRUCTIONS TO STUDENTS
1.
2.
3.
4.
5.
DO’S
Be regular to lab.
Maintain silence.
Know the theory behind the experiment before coming to the lab.
Follow the instructions of your lab demonstrator while conducting the
experiments.
Power must be switched off after disassembling the experimental setup.
DONT’S
1. Do not transfer equipment to other labs without permission.
2. Do not touch the kit with wet or damped hands.
2
TABLE OF CONTENTS
Exp No
Topic
1.
To generate Amplitude Modulated wave & to calculate
the modulation index.
2.
4.
To study the following of the Balanced Modulator
as a 1. Frequency Doublers
2. DSB-SC Generator.
a) To generate the frequency modulated signal and to
find the modulation index.
b) To demodulate the frequency modulated
signal using FM detector.
To study sample and hold circuit.
5.
Measuring the characteristics of line.
6.
Antenna Trainer
7.
To Perform Pulse amplitude modulation and
Demodulation and to draw the observed waveforms.
3.
3
PEC - 453 : ANALOG COMMUNICATION LAB
EXPERIMENT No. : 01
Aim: To generate Amplitude Modulated wave & to calculate the modulation index.
Apparatus Used:
Equipment
Transistor
Diode
Resistors
Capacitors
CRO
Function Generator
Regulated Power supply
Connecting probes
Specification/Range
SL100
OA79
56kΩ,5.6kΩ,560kΩ,
10kΩ - 2, 16kΩ(DRB)
100µF,10µF,0.01µF
(0-20) MHz
1MHz
(0-30V),1A
Quantity
1
1
1 each
1 each
1
2
1
As required
THEORY: Amplitude modulation is defined as a process in which the amplitude of the carrier
wave c(t) is varied linearly with the instantaneous amplitude of the message signal
m(t).The standard form the amplitude modulated wave is defined as
s (t)=Ac[1+Kam(t)] cos 2πfct
where Ka is called amplitude sensitivity of the modulator
The Demodulation circuit is used to recover the message signal from the incoming AM wave at
the receiver. An Envelop detector is simple and yet highly effective device that is well suited for
the demodulation of AM wave for which the % of modulation is less than 100%.An Envelop
detector produces an output signal that follows the envelop of the input wave exactly.
Modulation index is defined as m= (Vmax –Vmin)/ (Vmax+Vmin)
PROCEDURE:
1. The circuit is connected as per the circuit diagram.
2. Switch on +12 V Vcc supply.
3. Apply sinusoidal signal of 1kHz frequency and amplitude 2 Vp-p as modulating signal,and
carrier signal of frequency 11 kHz and amplitude 15 Vp-p.
4. Now slowly increase the amplitude of the modulating signal up to 7V and note down values
Vmax and Vmin.
5. Calculate the modulation index using the equation.
6. Find the value of R from fm=1/2πRC taking C=0.01µF.
7. Connect the circuit diagram for Demodulation.
8. Feed the AM wave to the demodulator circuit and observe the output.
9. Note down the frequency and amplitude of the demodulated o/p waveform.
4
EXPECTED WAVEFORMS:
OBSERVATION TABLE:
S.No
Am(volts)
Vmax
Vmin
m
%m
PRECAUTIONS:
1. Check the connections before giving the power supply
2. Observations should be done carefully.
5
RESULT:
VIVA Questions:
1. AM is Defined as ____________
2. Draw its spectrum___________
3. Draw the phase representation of an amplitude modulated wave___
4. Modulation index is defined as_____
5. The different degrees of modulation _______
6. What are the limitations of square law modulator __________
7. Compare linear and nonlinear modulators
8. AM Demodulator is ___________
6
9. Detection process _________
10. The different types of distortions that occur in an envelope detector are__________
7
8
PEC - 453 : ANALOG COMMUNICATION LAB
EXPERIMENT No. : 02
AIM: To study the following of the Balanced Modulator as a 1.Frequency doublers, 2. DSB- SC
Generator.
APPARATUS USED: Balanced modulator, CRO, function generator and connecting probes.
THEORY: Balanced modulator is used for generating DSB-SC signal.A balanced modulator
consists of two standard amplitude modulators arranged in a balanced configuration so as
to suppress the carrier wave.The two modulators are identical except the reversal of sign of
the modulating signal applied to them.
RFgenerator:Colpitts oscillator using FET is used here to generate RF signal of approximately 100
khz Frequency to use as carrier signal in this experiment.Adjustments for Amplitude and
Frequency are provided in panel for ease of operation.
1. AF Generator:Low Frequency signal of approximately 5khz is generated using OPAMP based wein bridge Oscillator. IC TL 084 is used as an active component,TL 084 is
FET input general purpose quad OP-AMP integrated circuit. Oneof the OP-AMP has been
used as amplifier to improve signal level. Facilityis provided to change output voltage.
2. RegulatedPowerSupply:This consists of bridge rectifier, capacitor filters and three terminal
regulators to provide required dc Voltage in the circuit i.e. +12v, -8v @150 ma each.
3. Modulator:The IC MC 1496 is used as Modulator in this experiment. MC 1496 is
amonolithic integrated circuit Balanced modulator/Demodulator, is versatile and can be
used up to 200 mhz.
4. Multiplier:A balanced modulator is essentially a multiplier. The output of the MC1496
balanced modulator is proportional to the two input signals. If you apply the same
sinusoidal signal to both inputs of a balanced modulator.
The purpose of a communication system is to transmit information-bearing signals baseband
signals through a communication channel separating the transmitter from the receiver. The term
baseband is used to designate the band of frequencies representing the original signal as delivered
by a source-information. The efficient utilization of the communication channel required, a shift of
the range -of baseband-frequencies into other frequency ranges suitable for transmission a
corresponding shift (back to the original frequency range after reception. Shift of the range of
frequencies in a accomplished by using modulation, which is defined as the process by which
some characteristic of a carrier is varied in accordance with a modulating wave. The baseband
signal is referred to as the modulating wave and the result of the modulation process is referred to
9
as the modulated wave. At the receiving end of the communication system, we usually require
original baseband 'Signal' or modulating wave to be restored. This accomplished by using a
process known as demodulation, which is the reverse of the modulation process: In amplitude
modulation, the amplitude of a sinusoidal carrier wave is varied in accordance with the baseband
signal. In standard AM system, the carrier and both sidebands are transmitted just as they appear at
the Output of the modulator. This system is used by standard AM broadcast stations.
10
11
Figure 4: DIODE-RING BALANCED MODULATOR
EXPECTED WAVWFORM:
12
OBSERVATION TABLE:
Signal
Amplitude(Volts)
Frequency(Hz)
OUTPUT:
13
PRECAUTIONS:
1. Check the connections before giving the power supply
2. Observations should be done carefully.
RESULT:
VIVA Questions:
1. The two ways of generating DSB_SC are ________
2. The applications of balanced modulator are ________
3. The advantages of suppressing the carrier ________
4. The advantages of balanced modulator __________
5. The advantages of Ring modulator __________
6. The expression for the output voltage of a balanced modulator is _________
PEC - 453 : ANALOG COMMUNICATION LAB
EXPERIMENT No. : 03
AIM: a) To generate the frequency modulated signal and to find the modulation index.
b) To demodulate the frequency modulated signal using FM detector.
14
APPARATUS USED: FM Modulation & Demodulation trainer kit, Dual trace CRO.
THEORY: The process in which the frequency of the carrier is varied in accordance with the
instantaneous amplitude of the modulating signal is called “Frequency modulation”.An FM
wave can be represented mathematically as
S(t)=Ac cos [2πfct +β sin2πfmt]
Where Ac is the amplitude & fc is the frequency of the carrier wave.
Β is the modulation index of the FM Wave.
EXPECTED WAVEFORMS:
OBSERVATION TABLE:
S.No
Am(Volts)
Tmax(sec)
fmin(kHz)
Δf(kHz)
β
BW
PROCEDURE:
1. Connect the circuit as per the given circuit diagram.
15
2. Switch on the power supply.
3. Measure the frequency of the carrier signal at the FM o/p terminal with input terminals open and
plot the same on graph.
4. Apply the modulating signal of 500 Hz with 1Vp-p.
5. Trace the modulated wave on the CRO and plot the same on graph.
6. Find the modulation index by measuring minimum & maximum frequency deviations from the
carrier frequency using the CRO.
7. Repeat steps 5&6 by changing the amplitude and/or frequency of the modulating signal.
Demodulation:
1. Connections are made as per circuit diagram.
2. Check the functioning by giving square wave to input and observing the output.
3. Frequency of input signal is varied till input and output are locked.
4. Now modulated signal is fed as input and observe the demodulated signal on CRO.
5. Draw the demodulated waveform.
PRECAUTIONS:
1. Check the connections before giving the power supply
2. Observations should be done carefully.
RESULT:
VIVA Questions: 1.Define frequency modulation?
2.Mention the advantages of indirect method of FM generation?
3.Define modulation index and frequency deviation of FM?
4.What are the advantages of FM?
5.What is narrow band FM?
6.Compare narrow band FM and wide band FM?
7.Differrntiate FM and AM?
8.How FM wave can be converted into PM wave?
9,State the principle of reactance tube modulator?
10.Draw the circuit of varactor diode modulator?
11.What is the bandwidth of FM system?
16
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Trinity
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FREQUENCY MODULATION & DEMODULATION
47KQ
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TL084
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AF
OUTPUT INPUT
Min
11
0.01 µ F
-12 V
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FM
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3
12.Want is the function of FM discriminator?
13.How does ratio detector differ from fosterseely discriminator?
14.What is meant by linear detector?
15.What are the drawbacks of slope detector?
17
To Ch2 of CRO
To Chi of CRO
FM
Modulator
AF Signal
generator
Fig-4a FM Modulation Testing arrangement
To Chl of CRO
AF Signal
generator
To Ch2 of CRO
FM
Modulator
FM
Demodulator
Fig-4b FM demodulation Testing arrangement
Trinity Microsystems Pvt Ltd
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TS-1204 Schematics
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PEC - 453 : ANALOG COMMUNICATION LAB
EXPERIMENT NO. : 04
AIM: To study sample and hold circuit.
APPARATUS USED: Sample and Hold circuit trainer, Digital voltmeters etc.
THEORY: A sample and hold amplifier is utilized for freeze the last instantaneous input, output
voltage and present the voltage unaltered to the A/D converter. The A/D then converts the voltage
to a corresponding 411k digital word.
A sample-hold amplifier usually consists of a storage capacitor, It has input and output buffer
amplifier, and a switch and its driver circuitry. During sample the circuit is connected to promote
rapid charging of the capacitor and during hold, the capacitor is disconnected from its charging
source and ideally retains its charge. There are a large number of sample-hold devices available
today, both of IC as well as hybrid and modular designs. However, except for the very highest
limits of performance, most of them tend to fall into only a few categories of design topologies.
The three types are follows:-1. Open loop, Cascaded followers. 2. Closed loop follower output. 3.
Closed loop integrator output.
Circuit Theory: For the following experiment we have used the national Semiconductor LF398
for the closed loop follower output. The LF198/LF298/DF398 are monolithic sample and hold
circuits which utilize BI-FET Technology to obtain ultra-high accuracy with fast acquisition of
signal and low droop rate. Operating as a unity gain follower, to gain accuracy is 0.002%. A
bipolar input stage is used to achieve low offset voltage and wide band width. The wide band
width allows the LF 198 to be included inside the feedback loop of 1MHz op amps without having
stability problems. Input impedance of 10Ω allows high source impedance to be used without
degrading accuracy. In sample and hold circuit: there are two measure parameters involved.
1.
Aperture time 2. Acquisition time
1.
APERTURE TIME: It is s the delay (reaction time) between the time, the control logic
tells the switch SW 1 to open and the time that is actually happen.
2.
ACQUISITION TIME: In a time varying system the input signal to a sample hold,
changes while the sample hold is holding a value, and so the time required for the sample hold to
acquire the new value of input voltage when the sample hold is switched from HOLD to SAMPLE
is important. This is called acquisition time.
OPERATING INSTRUCTIONS: 1. Switch on the Unit and see that supply L.E.D. glows on.
2. Keep the sample/hold switch in the sample mode.
3. Adjust input with input adj potentiometer to 1 volt. Connect input etv jumper link, and
observe the output voltage.
4. Vary the input voltage in steps and observe the corresponding tOc, output voltage on the
output voltmeter. ittk, You should have observed that the output voltage varies in
19
accordance with the input voltage as for as the sample/hold switch is in the sample
position.
5. Now set the input voltage to 2 volt set sample/hold switch to hold mode. The output on the
output meter would be 2 volt.
6. Vary the input voltage to another value say 2.5 volts and observe the output. You should
have observed that output holds the previous value and input voltage variation has no effect
on the output. However output drop slowly due to the capacitor discharging. Set the
sample/hold switch to sample mode and output show 2.5 volt.
7. Repeat the above steps for different input voltage
RESULT: Sample and hold circuit has been studied.
PRECAUTIONS: 1. Connections should be tight.
2. Reading should be taken carefully.
3. Switch off the power supply when not in use.
VIVA QUESTIONS: 1. Define Nyquist criteria.
2.What is aliasing effect?
3. How can aliasing effect be avoided?
4. What is sampling theorem?
5. Why is sample and hold circuit used?
20
21
22
PEC - 453 : ANALOG COMMUNICATION LAB
EXPERIMENT No. : 05
AIM: Measuring the characteristics of line.
APPARATUS USED: Trainer kit, wires, CRO etc.
THEORY:
Transmission line: The trainer uses 100m co-axial (RG 174) cable divided in 4 equal sections of
ST2266 25m each. Both the ends (input/output) of each 25m section are brought-up on 4: the
panel to facilitate interconnections between the different sections and also to observe the line
characteristics at different length.
Operating Instructions and Panel Controls: The trainer is equipped with built in DC-Power
supply, simply attach the three pin mains cord, supplied within the trainer to the 3 pin socket and
connect the other end to a stable 230V AC supply. When ON / OFF switch of the trainer is turned
ON. The power LED indication will lit, indicating that the trainer is ON.
Impedance Matching Resistors: Two 100R potentiometers are connected on source & load side
respectively for impedance matching. A 10 resistance is given at the bottom end of the panel to
facilitate input current.
Measurement:
Transmission Line Trainer Introduction: Transmission lines are a means of conveying signals
or power from one point to another. From such a broad definition, any system of wires can be
considered as forming one or more transmission lines. However, if the properties of these lines
must be taken into account, the lines might as well be arranged in some simple, constant pattern.
This will make the properties much easier to calculate, and it will also make them constant for any
type of transmission line. Thus all practical transmission lines are arranged in some uniform
pattern; this simplifies calculations, reduces costs and increases convenience.
Types of transmission line: One of the simplest forms of a transmission line is the open-wire line
or the twisted pair. Since the two conductors of this type of line have same relationship with
respect to ground, it is a balanced line. But this type of line has very poor shielding properties and
has a tendency to radiate. See Fig 1 Coaxial lines are the more popular of the two in RF
communication. Co axial cable line consists of a central conductor and an outer conductor with the
23
outer conductor referred to as shield normally grounded. Due to the outer conductor normally
grounded the two conductors do not have similar relationship with respect to ground and that is
why a coaxial line is an unbalanced line. However, due to shielding, coaxial lines have extremely
low radiation loss.
Equivalent circuit representation of a transmission line: Since each conductor has a certain length
and diameter, it must have resistance and inductance, since there are two wires close to each other,
there must be capacitance between them. Finally, the wires are separated by a medium called the
dielectric, which cannot be perfect in its insulation; the current leakage through it can be
represented by a shunt conductance. The resulting equivalent circuit is as shown in Fig. 1 . Note
that all the quantities shown are proportional to the length of the line, and unless measured and
quoted per unit length, they are meaning less
In a loss less transmission line R =0, G =0 Therefore, characteristic impedance Zo = L / C The
input impedance of a finite line terminated in its characteristic impedance is equal to its
characteristic impedance only. See Fig 2
Figure 2
Note that all the quantities shown are proportional to the length of the line, and unless measured
and quoted per unit length, they are meaning less
In a loss less transmission line R =0, G =0 Therefore, characteristic impedance Zo = L / C The
input impedance of a finite line terminated in its characteristic impedance is equal to its
characteristic impedance only.
Measuring the characteristics of a line Characteristics of a shielded line: The coaxial lines
used for the transmission of electromagnetic waves consist of an external conductor of cylindrical
shape, with an inner conductor arranged along the axis of the former. The two conductors are
separated by dielectric material of suitable features. One of the advantages of this kind of lines is
that these lines are intrinsically self-shielding, due to the geometry of the arrangement of the two
conductors. Moreover the shielding features of the coaxial lines improve when the frequency
increases. From the electric point of view, a coaxial line can be considered as a cascade of line
trunks. Each one of them can be represented as being composed of resistive, inductive and
capacitive circuit elements of concentrated kind, as R = ohmic resistance for unit length (100 in
24
this trainer) L = inductance for unit length G = conductance for unit length C = capacitance for
unit length.
The transmission characteristics of a line are described in terms of propagation constant y and of
characteristic impedance Z0. These parameters are typical values for each single line. The same is
true for the capacitance, the inductance, the resistance and the conductance for length unit. In the
telecommunications field, these values are generally expressed per meter or kilometer, for
practical reasons. In this case, the symbol used to indicate these magnitudes are the common
symbols. This experiment measures the characteristic parameters such as R, L, C, G, Zo and y for
the transmission line included in this trainer.
Procedure 1) Fig.4 shows the modalities for the measurement to be performed. 2) Make
connections as in Dia.1 3) Both the inductance and the ohmic resistance of the line are measured in
series by short-circuiting end of the line and connecting the measuring instruments to the start of
the line. The capacitance and the conductance are measured in parallel by operating on the open
line. 4) The resistance R and the conductance G can be measured with an ohmmeter or DMM. For
the conductance to be measured an ohmmeter is required which is abls, to perform resistance
measurements with a range greater than 100 MQ
5) For the measurement of series inductance L and the parallel capacitance C, a LCR meter or
measuring bridge is required. The results of these measurements give values of R, L, C and G
referred to the cable length that, in our case, is of 100 meters. Zo can be measured by using the
following formula Zo = JLIC Compare the readings obtained by you with those specified
previously.
25
26
.
2.Measuring the Attenuation of line The ohmic resistance R & the conductance G are
responsible for energy disputation in the form of heat.
27
These losses, which determine the attenuation characteristics, are expressed in terms of
"attenuation " "a" and can be calculated by a = 20 log(V2 V1) Where VI = amplitude of signal
at I/P V2 = amplitude of signal at 0/P a = attenuation for given length, In this experiment we
will measure the attenuation for the different trunks of transmission line available on the
trainer. See fig 5
28
PRECAUTIONS:
1. Check the connections before giving the power supply
2. Observations should be done carefully.
RESULT:
Viva Questions: 1. What is Ohm’s law?
2.What is Faraday’s laws?
3. What is Kirchoff’s law?
4. Which material is not used for transmission and distribution of electrical power?
29
PEC - 453 : ANALOG COMMUNICATION LAB
EXPERIMENT No. : 06
AIM: Antenna Trainer
APARATUS REQUIRED: VCO based microwave signal source, detector, VSWR meter
Theory: As shown, a small portion of the electromagnetic energy escapes from the system
and it is thus radiated. This occurs because the lines of force, travelling toward the open circuit, are
required to undergo a complete phase reversal when they reach it. Not all of them are able to do this,
because they possess the equivalent of mechanical inertia, and thus some do escape, It must be added
that the proportion, of waves escaping the system to those remaining is very small, for two reasons. First,
if we consider the surrounding space as the load for the transmission line, we see that a mismatch exists,
and thus very little power is dissipated in this "load". Second, since the two wires are close together, it is
30
apparent that the radiation from one tip will just about cancel that from the other. This is because they are
of opposite polarities and at a distance apart that is tiny compared to a wavelength. Conversely, this is also
the reason why low-frequency parallel-wire transmission lines do not radiate..
The cure for this problem seems to be an "enlargement" of the open circuit, i.e. spreading of the two
wires, as in Fig. 3. There is now less likelihood of cancellation of radiation from the two wire tips.
By the same token, the radiating transmission line is now better coupled to the surrounding space.
This is another way of saying that more power will be "dissipated" in the surrounding space, i.e. radiated.
Moreover, because of the spreading out, waves travelling along the line find it more difficult to undergo the
phase reversal at the end.
Thus everything points to an increase in radiation.
(c
)
Opened out transmission line
(b)
Conductors Line
31
Half wave dipole
Fig 1: Evolution of the dipole
The radiation efficiency of this system is improved even more when the two wires are bent so
as to be in the same line, as in Fig. 1. The electric (and also the magnetic) field is now fully
coupled to the surrounding space, instead of being confined between the two wires, and the
maximum possible amount of radiation results. This type of radiator is called a dipole. When
the total length of the two wires is a half-wavelength, the antenna is called a half-wave
dipole. It has the form indicated in Fig.1, and now even greater radiation occurs. The reason
for this increase is that the half-wave dipole may be regarded as having the same basic
properties (for the point of view of impedance particularly) as a similar length of transmission
line. Accordingly, we have the antenna behaving as a piece of quarter-wave transmission line
bent out and open-circuited at the far end.This results in the high impedance at the far ends of
the antenna reflected as a low impedance at the end connected to the main transmission line.
This in turn, means that a large current will flow at the input to the half-wave dipole, and
efficient radiation will take place.
Standing Wave Ratio: The standing Wave Ratio (SWR) is defined as the ratio between
maximum and minimum values of voltage (and current) along the line. The SWR is an index
of the mismatch existing between the load and the line feeding it. The SWR equals 1 in the
perfectly-matched case, impossible to reach in practice, and tends to reach very high values
(infinity) for lines shorted or open. In practice SWR values in the range 1.4 to 2 are to be
considered a good matching condition in an antenna system, while rather larger values are
acceptable with our trainer. This is because unlike large power systems where the design
aim is maximum power transfer, in a trainer system the aim is in handiest operability and simple
construction.
The Directional Coupler: To sense the direction of power travel, as well as the amount of power,
is sensing device must have diodes as circuit elements. The directional coupler consists of two line
trunks placed alongside a main transmission line carrying energy from generator to antenna. The
power travelling from input to output of the device will cause induced voltages in the upper and
lower loops. In the lower one the voltage will build across the sensing devices thanks to the
forward conducting diode, while this will not happen in the upper loop. As for the power
travelling from load to generator, the situation is reverted the upper loop will sense, the lower one
will not. Therefore the device allows separate metering of direct and reverse power.
The practical procedure to use the directional coupler to measure the SWR is the following.
1.
Turn on the transmitter
2.
Place the switch of the SWR meter on FORWARD and note the reading. You
can also adjust the Level for full-scale deflection (50 in the case of our trainer. Adjust
RF Level if needed)
3.
Switch the meter to REVERSE. Note the reading. Calculate the SWR by the
formula.
SWR- For + REV
50 + REV
OR
For – REV
50 - REV
Antenna Matching: Let's consider a short circuited transmission line having length 1/4 of the
wavelength of the signal impressed by the generator. At the shorted end there will be a null
voltage and a maximum current while at the other end (generator side), there will be opposite
situation of maximum voltage and zero current. The line therefore appears to the generator
as infinite impedance, since no current is drawn. Let's now consider another line, half
wavelength long, shorted at the end opposed to that of the generator.
The junction point of the generator to the line will be a zero-voltage, maximum current point.
The impedance of the line, as "seen" from the generator, shall be a short circuit (zero
impedance). In all the intermediate cases of a line having length between 1/4 and 1/2 wavelength,
the generator shall see impedance between 0 and infinite. Going on further with the same
reasoning
we
find
out
that
for
shorted
lines
1/4
wave length long to zero length, the impedance goes again from infinity zero.
Since our line is loss less, the impedance must be purely reactive and if we consider the
pattern of the current together to that of the voltage, we soon find out that in the 1/2 to 1/4
wave length interval the impedance goes from 0 to infinite and is capacitive, while in the 1/4
wavelength to zero length interval the impedance goes from infinite to zero and is inductive.
All this leads us to think of a very handy way to match the impedance seen from the generator
by placing in parallel to the mismatched load a trunk of shorted line of a proper length. These
devices are generally called MATCHING STUBS. An adjustable length matching stub can be
adjusted to have a reactive impedance equal in modulus and opposed sign of a mismatched load, in
order to cancel its reactive component and make it appear to the line as purely resistive.
Types of Antennas: Antennas can be broadly classified by the directions in which they
radiate or receive electromagnetic radiation. They can be isotropic, omnidirectional or
directional.
An isotropic antenna is a hypothetical antenna that radiates uniformly in all directions so
that the electric field at any point on a sphere (with the antenna at its center) has the same
magnitude. Such radiation cannot be realized in practice since in order to radiate uniformly in
all directions an isotropic antenna would have to be a point source. The nearest equivalent to an
isotropic antenna is a Hertzian dipole.
The Hertzian dipole is the name given to a dipole which is very small compared to its
wavelength, that is about one-hundredths of the wavelength at its operating frequency;
even in this case its pattern is not truly isotropic.
An omni directional antenna radiates uniformly in one plane. Examples of omni directional
antennas are Monopoles, Dipoles etc. The radiation of a vertical dipole is uniform in the
horizontal plane and a figure of 8 in the vertical plane.
A directional antenna radiates most of is power in one particular direction. Examples
of directional antennas are Yagi UDA, Log-Periodic, and Helical.
Important characteristics of Antenna: An antenna is chosen for a particular application
according to its main physical and electrical characteristics. Further, an antenna must perform
in a desired manner for the particular application. An antenna can be characterized by the
following factors, not all are applicable to all types of antenna. Most of the characteristics
mentioned below can be studied using this trainer.
1.
Radiation resistance.
2.
Radiation pattern.
3.
Beam width
4.
Bandwidth.
5.
6.
7.
8.
9.
Gain of main lobe.
Position and magnitude of side lobes
Front to back ratio
Aperture.
The polarization of the electric field
There are two principal planes in which the antenna characteristics are measured. These are
the horizontal and vertical planes for land based antennas. Some characteristics such as
beam width and side lobes are the same in both planes for symmetrical antennas such as
helical and reflectors. Other characteristics such as gain on bore sight (i.e. where the azimuth
and the elevation planes intersect) can only have a single value. In general, for unsymmetrical
antennas the characteristics are different in the two principal planes.
Radiation Resistance: We can consider an antenna as a load that terminates the transmission
line that feeds it. . In the ideal case this load will have impedance which is purely resistive,
that is, the load will not have any reactive component, such as an inductance or
capacitance. In practice, the impedance of an antenna is made up of a self-impedance and
mutual impedance. The self-impedance is the impedance that would be measured at the
terminals of the antenna when it is in free space, given no other antennas or reflecting objects in
the vicinity. The mutual impedance accounts for the coupling between the antenna and any other
source. When the antenna is sufficiently isolated from other objects, this mutual impedance tends
to zero. On the other hand, in some antennas such as the Yagi array the operation depends on
the mutual coupling between the driven element and the other parasitic passive elements.
When the antenna has the same impedance as the transmission line that feeds it, the antenna is said
to be matched on the line. When this occurs, maximum power is transferred from the
transmission line to the antenna. In general the impedance of the antenna is not the same as that
of the transmission line. When the transmission line has a purely resistive impedance and the
antenna has an impedance that contains a different resistive value as well as a reactive part,
the optimum transfer of power can be achieved via the use of tuning circuits between the
transmission line and the antenna: In general, these circuits consist of an LC circuit in which the
capacitance of the capacitor is altered in order to provide the maximum transfer of power.
In this trainer Antenna match tuning capacitor does this.
Radiation Pattern: The antenna is a reciprocal device,
means it radiates or
receives electromagnetic energy in the same way. Thus, although the radiation pattern is
identified with an antenna that is transmitting power, the same properties would apply to the
antenna even if it was receiving power. Any difference between the received and radiated
powers can be attributed to the difference between the feed networks and the equipment
associated with the receiver and transmitter. The antenna radiates the greatest amount of
power along its bore sight and also receives power most efficiently in this direction.
The radiation pattern of an antenna is peculiar to the type of antenna and its electrical
characteristics as well as its physical dimensions. It is measured at a
constant distance in the far field. The radiation pattern of an antenna is usually plotted in
terms of relative power. The power at bore sight, that is at the position of maximum radiated
power, is usually plotted at 0 degrees; thus, the power in all other positions appears as a
negative value. In other words, the radiated power is normalized to the power at bore sight.
The main reason for using dB instead of linear power is that the power at the nulls is often
of the order of 10,000 times less than the power on the bore sight, which means that the scales
would have to be very large in order to cover the whole range of power values.
For the convenience of the students to plot the polar graph the readings are plotted after
converting them in to dB. A conversion chart is provided in this manual. Also the bore sight
reading is taken as maximum in dB and other readings are plotted in lower values in dB.
The radiation pattern is usually measured in the two principal planes, namely, the azimuth and
the elevation planes. The radiated / received dB is plotted against the angle that is made with
the bore sight direction. If the antenna is not physically symmetrical about each of its
principal planes, then one can also expect its radiation pattern in these planes to be
unsymmetrical. The radiation pattern can be plotted using the Polar or the Rectangular /
Cartesian Co-ordinates.
Polar Plots: In a Polar Plot the angles are plotted radially from the bore sight and the
levels (dBuV/dBuA) are plotted along the radius. The angles may be selected at any
convenient interval. However 5 degrees or 10 degrees may be chosen. Choosing of 1 deg. is
also possible in the trainer but this does not serve any special purpose because the readings
will not change much and will consume more time. The polar plot gives a pictorial
representation of the radiation pattern of the antenna and is easier to visualise than the
rectangular plots. The student will easily understand the polar plot drawn by them.
The beamwidth and gain of main lobe: The beamwidth of an antenna is commonly
defined in two ways. The most well known definition is the -3dB or half-power beamwidth,
but the 10dB beam width is also used, especially for antennas with very narrow beams. The 3dB or half-power beam width of an antenna is taken as the width in degrees at the points on
either side of the main beam where the radiated level is 3dB lower than the maximum lobe
value. The -10dB value is taken as the width in degrees on either side of the main beam
here the radiated level is 10dB lower than the maximum lobe value.
The position and magnitude of sidelobes: The side level is usually quoted as the level
below the bore sight gain. Strictly all peaks on either side of the main lobe are sidelobes.
However, in practice only the "near-in" lobes, those which are adjacent on either side of the
bore sight maxima are referred to as sidelobes. Their amplitude and angle are easily
measured using the Polar Plot . For an antenna that is symmetrical about its main axis, the
radiation pattern is in general also symmetrical. Thus, the level of the sidelobes on opposite
sides of the main beam would be the same. The average value is taken where the two
sidelobes are different. The absolute level of the sidelobes can only be calculated if the
absolute bore sight gain is known.
Bandwidth: The bandwidth of an antenna is a measure of its ability to radiate or receive
different frequencies. It refers to the frequency range over which operation is satisfactory and
is generally taken between the half power points in the direction of maximum radiation.. The
bandwidth is the range of frequencies that the antenna can receive (or radiate) with a power
efficiency of 50% (0.5) or more or a voltage efficiency of 70.7% (that is -3dB points). The
operating frequency range is specified by quoting the upper and lower frequencies, but the
bandwidth is often quoted as a relative value. Bandwidth is commonly expressed in one of
the two ways; 1) As percentage or, 2) As a fraction or multiple of an octave (An octave is a
band of frequencies between one frequency and the frequency that is double or half the first
frequency; for instance, we have an octave between 400 MHz and 800 MHz). When it is
expressed as a percentage bandwidth, its centre frequency should be quoted and the
percentage expressed in octaves, its lower and upper frequency should be also quoted.
The front to back ratio: The front-to-back ratio is a measure of the ability of a
directional antenna to concentrate the beam in the required forward direction. In linear
terms, it is defined as the ratio of the maximum power in the main bean (bore sight) to that
in the back lobe. It is usually expressed in decibels, as the difference between the level on
bore sight and at 180 degrees off bore sight. If this difference is say 35dB then the front-toback ratio of the antenna is 35dB; in linear terms it would mean that the level of the back
lobe is 3,162 times less than the level of the bore sight.
Aperture / Capture area: In simple .words aperture or capture area of antenna is the effective
receiving area of the antenna and may be calculated from the power received and it's comparison
with the power density of the signal being received.
If ,
S = power density of the wave in Watts / sq meter
A = capture area of the antenna
P = Total power absorbed by the antenna
then P= S.A Watts or A = P / S
The aperture size can be defined in two ways; either in terms of actual physical size in meters, or
in terms of wavelength. For instance, if we say that an antenna has an aperture of two
wavelengths, then its actual size depends on its operating frequency. At a frequency of
1GHz, the physical aperture would be 60cms, whereas at 10 GHz it would be only 6cms. It is
more meaningful to refer to an antenna size in terms of its operating wavelength when the
antenna is narrow band or single frequency because its beamwidth and gain are directly related
to the aperture in terms of its operating wavelength. In this case we have to calculate its
wavelength to find its physical dimensions. However, in the case of broadband antennas, its
physical size is more appropriate because there is a range of operating frequencies.
The aperture of an antenna governs the size of its beamwidth. In general, the larger the aperture,
the narrower the beamwidth, and the higher is the gain at a given frequency.
The polarization of electric field: Polarization is used almost exclusively to describe the shape
and orientation of the locus of the extremity of the electric field vector as it varies with time at
a fixed point in space. This locus could be a straight line, an ellipse or a circle.
In the case of linear polarization, the electric field varies sinusoidal in one plane. When this plane
is vertical it is called vertical polarization. When this plane is horizontal, it is called
horizontal polarization. The electric field can also be polarized in any other angle between 0
and 90 degree to the horizontal. In general the only other commonly used angle is 45 degrees,
which is known as the slant polarization.
The polarization of a receiving antenna must match that of the incident radiation in order to
detect the maximum field. If the angles are not the same, only that components that is
parallel to the plane of incident polarization will be detected. If we have a vertically
polarized antenna and the incident radiation is slant polarized, the magnitude of its
component in the vertical plane will be reduced by a factor of cosine 45 degrees.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Experiment 1
Arranging the trainer and performing functional checks
Keep the main unit on the table and connect power cord .Check the mains voltage and switch on
the unit. The indicator lamp should glow. Switch off the main unit.
Assemble the coaxial antenna mast and fix it on the goniometer scale of the main unit. For
details of assembly please read, "Trainer description" in operating manual.
Assemble detector assembly and mount detector unit on the mast as per details given in Trainer
Description of operating manual.
Keep main unit and detector assembly at a distance of 1.5m.
Install folded dipole antenna on the transmitting mast and align the direction and the height of
both transmitting and receiving antennas.
Switch ON the main unit & check for deflection in the meter of directional coupler. Adjust RF
level and FS Adjust (if required). The toggle switch can be in either FWD or REV position.
Check for deflection in detector meter. Adjust Level of detector meter for 3/4 deflection in the
meter.
Rotate transmitting antenna between 0-360° and observe the deflection on the detector assembly.
The variation indicates that the transmitter & the receiver are working and radiation pattern is
formed.
The unit is ready for further experiments.
Experiment 2
Simple Dipole
A simple Dipole is the simplest form of antenna having 2 poles each of length (X,/2) . The
nominal impedance of this antenna is 73.52. The actual value departs from this due to
construction constraints, such on non-zero diameter rods, presence of BNC connector body
and the antenna mast. The effect of all this are partially corrected by "Y match"
arrangement connection.
The radiation pattern of simple Dipole (X/2) is uniform in forward & reverse direction. The
polarization is horizontal. The typical radiation pattern of this antenna is given. For
experimenting proceed as follows.
1. Arrange the Set up as given in Experiment.
2. Mount simple dipole (A,/2) on the transmitting mast.
3. Bring the detector assembly near to main unit and adjust height of both transmitting and
receiving antenna same.
4. Keep detector assembly away from main unit approximately 1.5 m. and align both of
them. Ensure that there are no reflector sort things in the vicinity of the experiment
such as steel structure, pipes, cables etc.
5. Keep the RF Level and FS Adjust to minimum and directional coupler switch to FWD.
6. Keep detector level control in the centre approximately.
7. Increase RF level gradually and see that there is deflection in the detector meter.
8. Adjust RF level and detector level so that the deflection in detector meter is approximately
30-35 11A.
9. Allign arrow mark on the disk with zero of the goniometer scale.
10. Start taking the reading at the interval of 5 or 10 deg. ( if you prefer even 1 deg° ) and
note the deflection on the detector assembly.
11. Convert the uA readings of detector assembly into dBs , with the help of the conversion
chart given at the end of this workbook .
12. Plot the polar graph in degrees of rotation of antenna against level in the detector in dBs.
13. Calculate the following with the help of this graph
(a) Beam wi dt h (b) Front / Back ratio
(c) Gain of antenna.
14. To calculate the above from the graph , please refer to Fig and proceed as follows :
Beam width: Look for math lobe. Draw bore sight maxima line AA'. Mark -3 dB from
maximum on the bore sight line point B. Draw an arc of radius AB. This arc will intersect
main lobe at C & D. Measure angle CAD. This angle is - 3 dB beamwidth. Similarly
calculate 10 dB beamwidth.
Front to back ratio: Look for the main lobe. Draw bore sight maxima line AA'. Look for
back lobe if any ( At 180 deg ). If no back lobe, then, Front to back ratio = AA' dB
If there is back lobe then measure AE. where E is the maximum of back lobe. Then, front to
back ratio = AA' dB
Gain of antenna: Gain of antenna is Maximum radiation intensity/ Maximum radiation
intensity from a ref. antenna (isotropic antenna) with same power input
Since, we cannot have an ideal isotropic antenna we presume here that its maximum
radiation intensity is 1 dB and is 100% efficient. Under this assumption Gain of antenna
(or Directional Gain of antenna) is
G = AA' dB
Result:
Viva Questions:1. What is VSWR?
2,What are Maxwell’s Equations?
3.What is the difference between Directivity and Antenna Gain?
4. What is the impedance of a dipole antenna?
5. What is Friis Transmission formula?
6. What is the relationship between the speed of light, frequency and wavelength?
7. What are the main types of antenna polarization?
PEC - 453 : ANALOG COMMUNICATION LAB
EXPERIMENT No. : 06
AIM: To Perform Pulse amplitude modulation and Demodulation and to draw the observed
waveforms.
APPARATUS REQUIRED: Bread board, power supply, transistor BC 107, CRO, function
generator, connecting wires
THEORY: PAM is the simplest form of data modulation. The amplitude of uniformly
spaced pulses is varied in proportion to the corresponding values of the continuous message
m(t). A PAM consists of a sequence of flat topped pulses. The amplitude of each pulse
corresponds to the value of the message signal x(t) at leading edge of each pulse. The Pulse
amplitude modulation is the process in which the amplitude of regularly spaced rectangular
pulses vary with the instantaneous sample values of a continuous signal in one-one fashion. A
PAM wave is represented mathematically as
Where
x (nTs) represents the nth sample of the message signal x(t)
K is the sampling period
Ka is the constant of amplitude sensitivity
P(t) denotes the pulse.
PAM is of two types
1) Double polarity PAM-This is the PAM wave which consists of both positive and negative
pulses.
2) Single polarity PAM-This PAM consists of either negative or positive pulses.In this fixed
dc level is added to ensure single polarity signal.
EXPECTED WAVEFORMS:
Dual polarity & single polarity PAM:
PROCEDURE:
1. A circuit is constructed as shown in fig
2. Set the modulating frequency to 1KHz and sampling frequency to25KHz.
3. Apply the 12V supply from the RPS.
4. Observe the output on the CRO.
5. Feed the modulated wave to the low pass filter (Demodulation circuit).
6. Note down the amplitude and time period of the demodulated wave.
7. Plot the waveforms on the graph sheet.
PRECAUTIONS:
1. Check the connections before giving the power supply
2. Observations should be done carefully.
RESULT:
VIVA Questions: 1. What is PAM?
2 What are the drawbacks of PAM?
3 What are the advantages of PAM?