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LAB ELECTRONICS
NEW#5, II FLOOR, 10TH AVENUE, ASHOK NAGAR, CHENNAI-83
ANALOGUE COMMUNICATION TRAINER
MODEL - X15A
FEATURES:
Lab Analogue Communication Trainer is a versatile instrument, which
includes all principles of modulation & demodulation techniques. It comes with the
following features given below.
LIST OF THE EXPERIMENTS:
1. Amplitude modulation / demodulation.
2. FM modulation / demodulation.
3. Balanced modulation.
4. Pulse Amplitude modulation and demodulation.
This unit consists of the signal sources as mentioned below:
AF Oscillator:
Function : Sine.
Frequency X1: 20Hz to 200Hz.
X 10: 200Hz to 2 KHz.
Amplitude : 0-10V (P-P). .
RF Oscillator:
Function : Sine/Square.
Frequency X1: 2 KHz to 20 KHz.
X 10: 20 KHz to 200 KHz.
Amplitude : 0 -10V (P-P).
LAB ELECTRONICS
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EXPERIMENT - 1
AMPLITUDE MODULATION AND
DEMODULATION
OBJECTIVE:
1. To construct an Amplitude Modulator using transistor and to demonstrate how
much intelligence can be added to a carrier and observe the amplitude
modulated waveforms and check the percentage of modulation.
2. To demonstrate how intelligence can be recovered from amplitude modulated
carrier by using diode demodulator. It has got two parts namely AM modulator
and AM demodulator.
INTRODUCTION:
MODULATION TECHNIQUES:
Communication is defined as a process by which information is exchanged.
In Electronics, it is the transmission and reception of information. Likewise,
information is defined as "the communication of knowledge or intelligence”. For the
purpose of this course, it is defined as any electrical signal representing data. Thus,
the purpose of any communication system is to convey or transfer information from
one point to another.
INFORMATION TRANSFER:
Communication of the written word developed from hand-carried letters and
newspapers to the mail system, telegraph, and now electronic mail. Spoken
communications evolved from face-to-face contact into telephone and radio
communication. All of these steps were taken in an effort to increase the
communications distance and speed.
The most significant advance in increasing communications range was radio.
Basically, the audio or sound waves are converted to an electrical signal then into
audio waves and transmitted to a distant receiving station. However, if the audio
signal is transmitted at its original frequency, a number of problems are met. First to
be efficient, the transmitting antenna must be at least 1/4 to 1/2 wavelength long.
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This means that for a 3000 Hz signal, the antenna problem was solved, only
one station could transmit at a time. This is because all stations would be operating
on the same "audio” frequencies. Second transmission system at these frequencies
is very inefficient.
All these problems can be solved by using a higher frequency signal as a
"carrier” for the audio information. In essence, the speech signal is transferred to a
much higher frequency for transmission, and then converted back to audio
frequencies by the receivers. The former is called MODULATION, while the latter is
DEMODULATION.
TYPES OF MODULATION:
Since, three characteristics of the sine wave carrier can be varied, it follows
that there are three types of modulation. These are amplitude modulation (AM)
frequency modulation (FM), and phase modulation (PM). However, in practice, it is
very difficult to distinguish between phase and frequency modulation. Therefore,
these two types of modulations are grouped together under the title of Angle
modulation. Thus, there are two basic types of modulation: Amplitude and Angle.
The next section we will discuss both of these in detail.
MODULATION:
In the process of modulation, some characteristics of a high frequency sine
wave is varied in accordance with the information or modulation signal. This signal
may be an audio waveform, a digital pulse train, a television picture or any other
form of information. The important consideration is that it is transferred to a higher
frequency for efficient transmission.
As mentioned before, the modulated high frequency sine wave is called the
carrier. The mathematical expression for an unmodulated sine wave or carrier is
e = A sin (t +)
Where e = instantaneous value of the wave (voltage or current)
A = maximum amplitude
= angular velocity (2f)
t = time
= phase angle
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This equation shows that there are characteristics of the wave that can be
varied or modulated. These are: amplitude (A), angular velocity or frequency (), and
phase angle ().
AMPLITUDE MODULATION
With amplitude modulation, the amplitude of the carrier is varied in
accordance with the modulating signal. There are several circuits that accomplish
this and they will be examined. Right now, we wish to limit our discussion to the
characteristics of the modulated waveform itself.
THE AM WAVEFORM:
Figure 1. shows a very simple AM circuit. Here a radio frequency carrier is
applied at "A" and the modulating audio tone at "B". The circuit consists of a
nonlinear device such as a diode or transistor. The two signals "mix" in this circuit
and produce the AM waveform shown at figure 1-C. Notice that both the negative
and positive peaks of the output waveform correspond exactly to the modulating
tone's waveform.
FIGURE: 1 - THE BASIC METHOD OF OBTAINING AMPLITUDE MODULATION
The amplitude and frequency of the modulating tone determines the shape of
the output waveform or the modulation envelope. For example, figure 2-A shows a
high amplitude audio signal. The resultant modulated waveform is shown in figure 2-
B. On the other hand, figure 2-C shows low amplitude, higher frequency audio
signal. The modulated waveform is in figure 2-D.
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FIGURE: 2 - EXAMPLES OF HOW THE MODULATED WAVEFORM VARIES WITH THE
MODULATING SIGNAL
PERCENT OF MODULATION:
The waveforms of figure 2-B and figure 2-D are said to have different degrees
of modulation. The degree of modulation is normally expressed as a percentage
from 0 to 100. However, it is also known as the modulation factor, which varies from
0 to 1. An unmodulated carrier as shown in figure 3-A has 0% modulation. For
comparison purpose, let's assume that the carrier has peak-to-peak amplitude of 40
volts as shown in figure 3A.
FIGURE: 3 - MEASURING THE PERCENT OF MODULATION
Figure 3-B shows the same carrier modulated to 100%. Here, the amplitude of
the modulated waveform falls to zero volts for an instant during each cycle of the
modulating wave.
80V
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Also, the amplitude increases to 80V peak-to-peak once during each cycle of each
modulating wave. The average peak-to-peak amplitude is still 40 volts.
In figure 3-C the carrier is shown modulated to 50%. The peak-to-peak amplitude
varies from 60 volts to 20V. However, the average peak-to-peak amplitude is still
40V.
The equation for determining the percent of modulation is
Percent of modulation =
For example,
Generally it is desirable to keep the percent of modulation high. For a given
transmitter power, a high percent of modulation will produce a stronger audio tone in
the receiver. The reason for this can be visualized from figure - 4.
FIGURE: 4 - THE RELATIVE AMPLITUDE OF THE RECOVERED AUDIO DEPENDS ON
THE MODULATION PERCENTAGE
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Since the AM receiver recovers just the modulation envelope of the transmitted
wave, it is easy to see that the higher modulated waveform in "B" will produce a
louder signal than at "A". While it is a good idea to keep the percent of modulation
high, over-modulation must be avoided.
Over-modulation is shown in figure 5-C. It occurs when the amplitude of the
modulating signal is too high compared to the unmodulated carrier.
Obviously, the minimum amplitude of the carrier is zero volts. It cannot drop
below this level regardless how high the modulating signal is. If the modulating signal
is too high, it will cause the carrier to cut off for a portion of each cycle. As a result,
part of the envelope will be distorted. That is, the envelope will not be an accurate
representation of the modulating wave.
Figure 5-A shows the high modulating waveform. The unmodulated carrier is
shown in figure 5-B. The modulated waveform, shown in figure 5-C, cuts off for a
portion of each cycle. At the receiver, the envelope is detected and since it is
distorted, the detected waveform is also distorted. The detected envelope is shown
in figure 5-D.
FIGURE: 5 - OVER-MODULATION CAUSES SEVERE DISTORTION IN THE RECEIVED
SIGNAL
DETECTED
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SIDEBANDS:
In the first section of this unit, you learned that any complex waveform could
be broken down into its component sine waves. The same is true for amplitude-
modulated waveform such as that shown in figure 6-A. This wave is a 1 MHz carrier
modulated by a 10 KHz sine wave. At first glance, you might say that the wave is
composed of a 1 MHz sine wave and a 10 KHz sine wave. However, if we apply the
waveform to both a 10 KHz band pass filter and a 1 MHz band pass filter as shown
in figure 6-B, we would see an output only at the 1 MHz filter. This shows that there
is no 10 KHz signal present in the modulated wave; the modulation envelope only
represents the audio signal. But we do know that some other sine wave components
must exist in this complex wave.
FIGURE: 6 - FREQUENCY DOMAIN ANALYSIS OF AN AM WAVE
If we had a tunable band pass filter or a spectrum analyzer, we could search
the spectrum and determine what other frequencies are contained within the
modulated signal. Doing this, we would find a signal at 1.01 MHz and another at
0.99MHz. These two signals are called sidebands.
They can be extracted from the modulated waveform by using sharply tuned
filters as shown in figure 6-C.
The higher frequency (1.01 MHz) is called the upper sideband. Its frequency
is always equal to the carrier frequency plus the modulating frequency. That is
Upper sideband = fC + fm
Where fc = carrier frequency
fm = modulating frequency
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In our Example: Upper sideband = 1 MHz + 10 KHz
= 1.01 MHz
The lower frequency (0.99 MHz) is called the lower sideband. Its frequency is
equal to the carrier frequency minus the modulating frequency. In this case, it is:
Lower sideband = fc - fm
= 1 MHz - 10 KHz
= 0.99 MHz.
If we carefully observe the filter outputs of figure 6-C, we would find that the
sideband and carrier amplitudes do not vary. In fact, we would find that the carrier
amplitude never varies whether it is modulated or not. You might ask, if the individual
frequency components do not change, how can the modulated waveform change to
follow the modulating signal? Figure 7 gives a detailed look at what exactly happens.
It shows that the constant amplitude sidebands are at different frequencies and
therefore, are in phase and out of phase with one another at various times. For
example, at point A, they are exactly in phase with each other. At this time, the
constant amplitude carrier is also in phase. The result is a high amplitude peak in the
modulated waveform. Now observe the wave relationships at point B. Once, again
the sidebands are in phase with each other. However, they are 180 out of phase
with the carrier wave. The result is a low point, or trough, in the modulated
waveform.
FIGURE: 7 - THE PHASE RELATIONSHIPS OF AN AM WAVE
From this analysis, you can see that the shape of the modulation envelope is
dependent on the sidebands. And, the sidebands are in turn dependent on the
modulating signal.
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That is, the frequency of the sidebands determines their phase relationship
and therefore, the peaks and troughs or frequency of the modulation envelope. The
sideband amplitude will also determine the envelope's amplitude, or percent of
modulation. This is because they will be either adding to or subtracting from the
constant amplitude carrier. This illustrates an important fact about amplitude
modulation. The modulating intelligence or information is contained only in the
sidebands.
The frequency spectrum charts of figure-8 will further illustrate this point.
Since these are voltage diagrams, the sideband amplitudes shown will add or
subtract directly from the carrier to produce the modulated envelope.
FIGURE: 8 - THE SIDEBAND SPECTRUM OF AM WAVES
For example, figure 8-A shows the sideband amplitudes as being exactly one
half that of the carrier. This is the condition for 100% modulation, because when all
signals are in phase, the waveform amplitude will be twice the carrier and when the
sidebands is out of phase with the carrier, the waveform amplitude will be zero.
Figure 8-B shows a 50% modulated signal. Note that the carrier amplitude
remains the same while the sideband amplitudes have decreased. The frequency of
the sidebands has also changed.
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Since the sidebands are further from the carrier, the modulating frequency has
increased. This is shown in the modulated waveform to the right. The result of a
square wave-modulating signal is shown in figure 8-C. In this case, since the
modulating signal is actually the fundamental and all odd-order harmonics, there is a
sideband for each sine wave in the modulating signal.
BANDWIDTH:
It is readily apparent from the frequency spectrum charts shown in figure - 8
that, with amplitude modulation, the transmitted signal is actually a band of
frequencies rather than just the carrier. The carrier contains no information. If we
transmitted or received just the carrier, no information would be conveyed. In AM
systems, both the carrier and the sidebands must be transmitted and received.
The bandwidth of an AM signal extends from the lowest sideband frequency
to the highest sideband frequency. Therefore, the bandwidth is always twice the
highest modulating frequency. Thus, if the highest modulating frequency is 15 KHz,
then the bandwidth will be 30 KHz. In the case of a complex modulating wave, such
as a square wave, the bandwidth is twice the highest harmonic contained in the
wave. However, each AM transmitter has bandwidth limitations above, which it
cannot go. In this case, the transmitter itself would limit the maximum bandwidth.
AM DETECTORS:
As shown earlier, an audio signal is impressed onto a carrier wave in the form
of amplitude variations. It is then amplified and applied to a transmitting antenna.
This modulated signal is then radiated and propagated, and a small fraction of it is
collected by the receiving antenna. The receiver must amplify this extremely weak
signal and, since the signal is one of many collected by the antenna, the receiver
must select the desired signal while rejecting all others. Finally, since modulation
took place in the transmitter, demodulation must be performed in the receiver to
recover the original modulating signal. The circuit that performs this function is called
a demodulator or a detector.
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THE DIODE DETECTOR:
The most popular AM demodulator is the diode detector. This circuit is very
simple and is used in virtually all AM receivers. Its purpose is to recover the
envelope from the AM waveform.
The diode detector is shown in figure - 9. The switch, S1, is included merely
for explanation. The input to the circuit is the AM waveform that has been selected
and amplified by previous stages in the receiver. It is applied to diode D1, which acts
as a half-wave rectifier. The positive half cycles cause D1 to conduct, developing
positive pulses across R1. D1 cuts off the negative half cycles of the RF input. The
center waveform shows the voltage developed across R1 if S1 is open.
When S1 is closed, C1 is placed in parallel with R1. C1 quickly charges through
D1 to the peak of each positive pulse. Between pulses, C1 attempts to discharge
through R1. However, the RC time constant is chosen so that C1 discharges only
slightly. The result is that the voltage across C1 follows the envelope of the AM
waveform. Thus, the output looks like the upper envelope with a small amount of
ripple. Normally, the carrier frequency is many times higher than the envelope
frequency, and therefore, the ripple is not noticeable.
FIGURE: 9 - THE DIODE DETECTOR
Figure-9 illustrates the diode detector's operation in the time domain. Let's
analyze its operation in the frequency domain.
The AM input consists of three frequency components: the carrier, the upper
sideband, and the lower sideband. These signals are applied to D1 and are mixed
across its nonlinear resistance. The difference signal is the modulating information.
The next step is to separate this low frequency signal from the high frequency RF.
This is accomplished by C1, which acts as a short circuit to ground for the RF
signals and a high reactance for the audio signals.
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The final step in AM detection is to separate the audio from the DC
component. This is done quite simply using a coupling capacitor such as C2 in figure-
10.
FIGURE: 10 - COMPLETE DIODE DETECTOR CIRCUIT
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AM MODULATION:
STEP-BY-STEP PROCEDURE:
1. Connect oscilloscope probe across AF sine output and GND. Adjust the AF
oscillator setting as mentioned below
AF OSCILLATOR:
FREQ SELECTOR X10 POSITION
FREQ CONTROL Adjust to 1KHz
AMP CONTROL 10V(P-P)
2. Connect CRO across RF Oscillator output and ground. Adjust the RF Oscillator
setting as mentioned below.
RF OSCILLATOR:
FREQ SELECTOR X10 POSITION
FREQ CONTROL Adjust RF Frequency to
be at 100KHz
FUNCTION SELECTOR
Select the switch to Sine
AMP CONTROL 10V(P-P)
3. Patch the circuit as shown in the wiring diagram for AM modulation. Switch ON
the TRAINER.
4. Connect your oscilloscope to the AM output. Set the vertical input to 2V/cm and
the sweep to 1 ms/cm (APPROX).
5. Set 100K trimmer to mid-range in the AM MOD/DEMOD section and 10K trimmer
to max position
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FIGURE: 11 - AM TRANSMITTER CIRCUIT
6. The circuit in the figure-11 is an AM transmitter. What type of modulation is being
used? According to the figure - 11, Transistor Q1 is the transmitter's _________.
7. Adjust the oscilloscope trigger controls for a stable display.
8. Adjust the (100K trimmer) of sine wave in the trainer at mid-range, measure the
percent of modulation.
% Modulation = __________.
9. Adjust 100K in clockwise direction to 100% modulation. ______________.
10. At this point you may wish to vary the modulating frequency and amplitude and
observe the effects on the modulated waveform.
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DISCUSSION:
In the AM transmitter of figure -11 series modulation is being used. Transistor
Q2 is the RF amplifier and Q1 is the modulator. 100K Trimmer controls the percent
of modulation. The percent of modulation is approximately.
AM DEMODULATION:
STEP-BY-STEP PROCEDURE:
1. Turn off your trainer and Patch the modulator output to demodulator input (refer
wiring diagram for demodulation). Connect your oscilloscope to the output of the
diode detector circuit (i.e demodulated output). If you have a dual-trace
oscilloscope, you can monitor the AM signal on one channel and the detected
output on the other.
2. Switch ON your trainer. You should see the demodulated signal on your
oscilloscope. Now use 100K trimmers to vary the percent of modulation. What
happens to the detected signal?
3. Turn 100K trimmers clockwise until the AM waveform is over modulated. What
happens to the detected output?
FIGURE-12 DIODE DETECTOR
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DISCUSSION:
You have constructed an AM generator. You have seen that amplitude
control varied the percent of modulation. You constructed a diode detector circuit.
Then you verified its operation and the fact that as the percent of modulation
changes. You proved that over modulation causes severe distortion of the output
wave.
OBSERVATION CHART: (AM MODULATION & DEMODULATION)
ADJUSTMENT AM O/P
AF FREQ-1KHZ
AMP-10V (P-P)
RF FREQ- 5KHZ -100KHZ
AMP-10V (P-P)
TRIMMER 100K – MID RANGE
10K – MAX POSITION
CRO
ADJUSTMENTS
AM OUTPUT DEMOD OUTPUT
TIME /DIV-
0.5ms/DIV VOLT
/DIV-2V/DIV
TIME /DIV-
0.5ms/DIV
VOLT /DIV-0.2V/DIV
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WIRING DIAGRAM
INDICATES THE PATCHING CONNECTIONS
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EXPERIMENT - 2
FREQUENCY MODULATION AND
DEMODULATION
OBJECTIVE:
1. To construct an FM generator and observe its waveform.
2. To construct a phase-locked loop FM demodulator and observe its operation.
INTRODUCTION:
Communication is defined as a process by which information is exchanged.
In electronics, it is the transmission and reception of information. Likewise,
information is defined as "the communication of knowledge or intelligence". For the
purpose of this course, it is defined as any electrical signal representing data. Thus,
the purpose of any communication system is to convey or transfer information from
one point to another.
THEORY:
Figure-1 shows a frequency modulated or FM waveform. The information or
modulating waveform is shown in figure 1-A, while the unmodulated carrier is shown
in figure 1-B. With FM, the modulating signal changes the frequency of the carrier
rather than its amplitude. The resulting frequency modulated waveform is shown in
figure 1-C.
At time T0 the modulated waveform is at its center frequency. As the
modulating signal swings positive, the frequency of the carrier is increased. The
carrier reaches its maximum frequency when the modulating signal reaches its
maximum amplitude at time T1.
At time T2, the modulating signal returns to 0 and the carrier returns to its
center frequency. After T2, the modulating signal swings negative. This forces the
carrier below its center frequency. The carrier again returns to its center frequency
when the modulating signal returns to 0 volts at time T4. Between times T4 and T8,
the modulating signal repeats its cycle. As a result, the carrier again shifted in
frequency. It swings first above then below its center frequency. Notice that it returns
to its center frequency each time the modulating signal passes through 0 volts.
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FIGURE: 1(A) THE MODULATING SIGNAL, B) THE UNMODULATED CARRIER, (C) THE
FREQUENCY MODULATED WAVEFORM
The carrier changes equally above and below its center frequency. The
amount of frequency change is called the frequency deviation. For example, let's
assume that a carrier continuously swings from 100 MHz, up to 100.1 MHz and back
to 100 MHz. The frequency deviation is +0.1MHz or 100 MHz.
The rate of frequency deviation is determined by the frequency of the
modulating signal. For example, if the modulating signal is a 1 KHz audio one, the
carrier will swing above and below its center frequency 1000 times each second. A
10 KHz audio tone will still cause the carrier to deviate +10 KHz; but this time at the
rate of 10,000 times each second. Thus, the frequency of the modulating signal
determines the rate of frequency deviation but not the amount of deviation.
C
T1 T2
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The amount that the carrier deviates from its center frequency is determined
by the amplitude of the modulating signal. A high amplitude audio tone may cause a
deviation of 100 KHz. A lower amplitude tone of the same frequency may cause a
deviation of only 50 KHz.
Thus the frequency-modulated waveform has the following characteristics:
1. It is constant in amplitude but varies in frequency.
2. The rate of carrier deviation is the same as the frequency of the modulating
signal.
3. The amount of carrier deviation is directly proportional to the amplitude of the
modulating signal.
MODULATION INDEX:
In AM, the degree of modulation is measured as a percentage from 0% to
100% or as a modulation factor from 0 to 1. In angle modulation, the degree of
modulation is measured by the modulation index. The equation for modulation index
is:
m = fd/fm
Where, fd = The frequency deviation
fm = The modulating frequency
While the modulation factor in AM is limited to a decimal between 0 and 1, it
must be emphasized that the modulation index in angle modulation can reach quite
high numerical values. For example, the maximum deviation in FM broadcasting is
75 KHz. If a 1 KHz audio signal causes full deviation, the modulation index is:
m = 75 KHz/1 KHz = 75
Another measure of angle modulation is the deviation ratio. This is the ratio of
the maximum deviation to the maximum audio frequency, thus, it is a total system
measurement rather than the instantaneous measurement of modulation index.
Using the FM broadcast system as an example, maximum deviation is 75 KHz and
the maximum audio frequency is 15 KHz.
The deviation ratio is:
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SIDEBANDS:
One of the distinct differences between amplitude and angle modulation is the
number of sidebands. Of course, AM has only two, the upper and lower sidebands.
In angle modulation, the number of sidebands is theoretically infinite. This is because
the numerous frequency shifts produced by the modulating signal cause the
generation of many additional frequencies. Fortunately, many higher order
sidebands contain an insignificant amount of energy and can, therefore, be
disregarded.
In AM, the sidebands add to or subtract from the constant amplitude carrier,
which results in the modulation envelope. However, in angle modulation, the
waveform remains at constant amplitude regardless. This means that, as sideband
number, amplitude, or distribution change, the carrier must also change to keep the
resultant waveform's amplitude constant. This interrelationship between carrier and
sidebands is orchestrated by the modulation index. That is, the modulation Index
determines the number of significant sidebands, their amplitude, and the carrier's
amplitude.
FIGURE: 2 - GRAPH OF SIDEBAND AND CARRIER AMPLITUDE IN AN ANGLE
MODULATED SIGNAL
You can see that, with a modulation index of zero, the carrier amplitude is one
and there are no sidebands. As the modulation index increases, more sidebands are
added, their amplitude increases, and the carrier decreases.
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This happens until at an index of 2.4, the carrier disappears entirely. This is known
as the first carrier "null", since it also occurs at still higher modulation indices. At an
index of 3.1 the carrier's amplitude is -0.3. This indicates that it is 180 out of phase
with the components above the zero axis.
Figure: 3 - Table of sideband and carrier distribution for several modulation
indices.
Figure-3 is a table of sideband and carrier amplitudes at various modulation
indices. Although the sidebands theoretically stretch out to infinity, any sidebands
with an amplitude less than 1% of the original carrier are insignificant and, therefore,
left out. As an example, suppose the modulation index is 0.5, from the table in figure-
3, a modulation index of 0.5 means that carrier amplitude is 0.94, the first sidebands
have amplitude of 0.24, and the second and last significant sidebands, have
amplitude of 0.03. This sideband distribution is shown in figure-4 along with several
others.
FIGURE: 4 - SIDEBAND DISTRIBUTION FOR ANGLE MODULATION
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BANDWIDTH:
In angle modulation, the number of sidebands and their amplitude is
determined by the modulation index. However, the frequency of each sideband
depends on the modulating frequency. The first- order sidebands are fc + fm and fc -
fm. The second-order sidebands are fc + 2fm and fc - 2fm. This progression continues
for each higher order sideband. The bandwidth, therefore, depends on the number of
sidebands in the wave. This is determined by the modulating frequency and
modulation index. Since modulation index is fd/fm, frequency deviation is also a
determining factor in bandwidth.
Thus, if we know the modulating frequency and the frequency deviation, we
can easily determine the required bandwidth. As an example, what is the bandwidth
of an angle modulated signal in which the modulating frequency is 3 KHz and the
maximum deviation is 18 KHz? We must first find the modulation index.
m = fd/fm
= 18 KHz/3 KHz = 6.
From the table in figure - 3, we find that a modulation index of 6 has 9 significant
sideband pairs. Therefore, the bandwidth is
BW = fm x highest order sideband x 2
= 3 kHz x 9 x 2 = 54 KHz
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STEP-BY-STEP PROCEDURE:
1. Switch ON the trainer.
2. Connect oscilloscope probe across Sine output and GND. Adjust the AF
oscillator setting as mentioned below
AF ADJUSTMENT:
FREQ SELECTOR X10 POSITION
FREQ CONTROL Adjust to 500Hz frequency
output
AMP CONTROL 10V(P-P)
3. Connect sine wave input of 10Vp-p and adjust the frequency control in clockwise
direction and select frequency selector switch to X 10. The frequency will be
500Hz.
4. Connect your oscilloscope to pin 2 of the XR-2206 IC (output) and set the
oscilloscope Time/div control to 2s/div and the vertical input to 2V/div. Your
oscilloscope should display a sine wave output.
5. Adjust carrier frequency (47K) to maximum position and set it to 100 KHz.
FIGURE: 5 - FM GENERATOR
+12V
220
4
7
+12v
V
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6. Patch the circuit as shown in the wiring diagram and connect the oscilloscope the
FM output and ground. Adjust oscilloscope time/div to 2s and volts/div to 2V/div.
7. Turn the 100K trimmers to mid range in the FM MOD/DEMOD section in the
trainer. Your oscilloscope should show a slightly blurred sine wave such as that
shown in figure-6. This graphically illustrates the frequency deviation of the FM
output. It occurs because the oscilloscope triggers each move at the same point
on the display. However, since each cycle has a slightly different frequency, the
blurred display results. By varying the setting of 100K trimmer you can observe
the change in deviation. What quantity is 100K trimmer changing in order to vary
the frequency deviation +03?
FIGURE: 6 - FM OUTPUT
8. Turn the 100K trimmer to its midrange setting. Now slowly increase the carrier
generator frequency from minimum (fully counter clockwise) to maximum (full
clockwise). What happens to the output frequency deviation? _____________ .
Is the output of the XR-2206 IC frequency or phase modulated?
9. Switch off your trainer and read the following discussion.
DISCUSSION:
In step 7, you saw a visual display of the generator's frequency deviation.
100k was used to change the amount of deviation. This was possible because 100K
controls the amplitude of the audio input signal. As the audio amplitude increases so
does the frequency deviation. Then, you increased the audio modulating frequency.
The frequency deviation should have remained constant. Any slight deviation
changes were due to brief generator output amplitude changes. Since the deviation
remained constant regardless of the modulating frequency, the XR-2206 IC
generates a true FM output.
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STEP-BY-STEP PROCEDURE (CONTINUED):
10. Patch the FM output to the input of demodulator (as shown in the patching
diagram).
AF ADJUSTMENT:
FREQ SELECTOR
X10POSITION
FREQ CONTROL
Adjust from s500Hz to
1KHz
AMP CONTROL
10V(P-P)
CARRIER FREQ. ADJUSTMENT: 100KHZ,
TRIMMER: 100K-MID RANGE, 47K-MAXIMUM RANGE
FIGURE: 7 - PHASE LOCKED LOOP DEMODULATOR
11. Set deviation control 100K fully clockwise, for maximum deviation. Also set the
generator frequency control to be 500Hz
12. Connect your oscilloscope to pin 7 (audio output) of the 565 phase-locked loop.
Set the Time/div control to 5 ms/div and the vertical input to 0.2 V/div. At this
point, you may or may not have an audio output signal displayed on the
oscilloscope. You must adjust the 565 PLL to the correct operating frequency. To
do this, adjust 10K TRIMMER until you obtain a sine wave output on the
oscilloscope. At this point, the VCO operating frequency is the same as the input
frequency. The sine wave output is the error Voltage required to keep the VCO
locked on to the input FM signal.
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13. Using the CARRIER GENERATOR FREQUENCY ADJUST and 10K the
deviation control, verify that the output of the phase-locked loop is directly
proportional to the modulating signal.
14. Turn OFF your trainer.
DISCUSSION:
In this part of the experiment, you constructed a phase locked loop FM
demodulator. You adjusted the VCO to the input frequency and from there, the PLL
locked onto the incoming signal. You verified that it was indeed demodulating the FM
wave.
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WIRING DIAGRAM
FREQUENCY MODULATION & DEMODULATION
INDICATES THE PATCHING CONNECTIONS
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EXPERIMENT - 3
BALANCED MODULATOR
OBJECTIVES:
1. To construct and properly adjust a balanced modulator and study its operation.
2. To observe the double side banded output with a suppressed-carrier signal.
3. To adjust it for optimum carrier suppression.
4. To verify the input audio-level that directly affects the double side band output
amplitude.
5. To observe that the output is minimum with zero audio input.
6. To measure the carrier only output and the peak side-band output and to
calculate the carrier suppression.
INTRODUCTION:
The purpose of communication system is to transmit information bearing
signals or baseband signals through a communication channel separating the
transmitter from the receiver. The term base band is used to designate the band of
frequencies representing the original signal as delivered by a source of information.
The efficient utilization of the communication channel requires 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.
A shift of the range of frequencies in a signal is accomplished by some
characteristics of a carrier which are varied in accordance with a modulating wave.
The base band signal is referred to as the modulating wave and the result of the
modulation process is referred to as the modulated wave. At the receiving end of the
communication system, we usually require the original baseband signal or
modulating wave to be restored. This is 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.
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THEORY:
One circuit that lends itself extremely well to balanced modulator applications
is the differential amplifier. A simplified diagram of a differential amplifier is shown in
figure 1.
FIGURE: 1 – A DIFFERENTIAL AMPLIFIER USED AS A BALANCED MODULATOR
Q3 acts as the current source for Q1 and Q2. If the RF input is applied to the
bases of Q1 and Q2 in phase, current through both transistors will be identical and
the voltage difference across the output will be zero. This is the common-mode
rejection of the differential amplifier and it has balanced out the carrier.
The audio input is applied to the base of Q3. This upsets the circuit balance.
As a result, the audio and RF signals are mixed across Q1 and Q2. This is non-linear
mixing and, therefore, side bands appear at the output. However, the carrier or RF
input does not. Since it is a common-mode signal, it is rejected.
A differential amplifier must be constructed with transistors whose
characteristics are very closely matched. Forming the transistors on a single chip of
silicon as is done with ICs which ensures this necessary match. Therefore, the
differential amplifier ideally suits for the construction of integrated circuit construction.
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FIGURE: 2 – CIRCUIT DIAGRAM OF BALANCED MODULATOR USING IC 1496
IC BALANCED MODULATORS:
Figure-2 shows IC that has been specifically designed for use as balanced
modulator. Figure-2 is the 1496 balanced modulator, which is manufactured by
Motorola, National and Signetics. This device uses a differential amplifier
configuration similar to what was previously described. Its carrier suppression is
rated at a minimum of -5 dB with a typical value of -65dB at 500 KHz.
FIGURE: 3 - SHOWS THE INPUT AND OUTPUT WAVEFORMS OF BALANCED
MODULATOR
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STEP-BY-STEP PROCEDURE:
This section contains a balanced modulator using a 1496 integrated circuit.
You will verify that it does suppress the carrier and also adjust it for optimum carrier
suppression.
1. Study the 1496 balanced modulator circuit on the front panel of the trainer.
2. Switch ON the trainer.
3. Connect 200Hz sine wave from the AF Oscillator Section and (5 KHz -100 KHz
range) SQUARE WAVE from RF Oscillator section as shown in wiring diagram.
In the Balanced Modulator Section adjust the 1K trimmer and 50K linear trim pot
to mid-range. Connect your oscilloscope to the output and set the vertical input
control to 2V/DIV and the sweep to 1ms/DIV.
4. Adjust the oscilloscope's trigger control for a stable display. You may also use the
trainer square wave output as an external trigger control for the oscilloscope.
What is the output waveform?
_________________________________________________________.
5. Vary the amplitude control of AF OUTPUT both clockwise & counter clockwise.
What effect does it have on the output? _______________________________.
6. Disconnect the AF input. The output should now be close to zero. Set your
oscilloscope's vertical input to 0.2 V/Div Now adjust 50K pot for minimum output.
7. If possible, increase the oscilloscope's vertical input sensitivity to measure the
output voltage.
E out carrier only = _______________________________.
8. Set the vertical input control to 1V/div. Connect the sine input and adjust
amplitude control for maximum output without producing clipping. Measure the
peak side band output voltage.
Epeak sidebands = _______________________________.
9. Calculate the carrier suppression in dB.
Epeak side band
dB = 20log Eout carrier only dB = ______________________.
10. SWITCH OFF your trainer and disconnect your circuit.
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DISCUSSION:
You checked the balanced modulator's operation and verified that the output
was a double sideband, suppressed carrier signal. You verified that the input audio
level directly affects the double side band output amplitude.
You adjusted 50K to balance the circuit. This was indicated by a minimum
output with zero audio input. In the next steps, you measured the carrier-only output
and the peak side band output. From these figures, you calculated the carrier
suppression. While the 1496 is rated at 65 dB suppression, with this circuit you
should obtain about 35 dB. This is due to stray coupling capacitance, inductance,
etc.
OBSERVATION CHART: BALANCED MODULATION
ADJUSTMENTS BALANCED MODULATOR
OUTPUT
AF FREQ-200HZ
AMP-10V(P-P)
RF (SQUARE) FREQ-5KHz -100KHZ
AMP-10V (P-P)
TRIMMER 1K – MID RANGE
50K – MID RANGE
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WIRING DIAGRAM
BALANCED MODULATOR
INDICATES THE PATCHING CONNECTIONS
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EXPERIMENT - 4
PULSE AMPLITUDE MODULATION (PAM) &
PULSE AMPLITUDE DEMODULATION
OBJECTIVES:
1. To construct a pulse amplitude modulation generator and to observe the
characteristics of both single and dual-polarity pulse amplitude modulation.
2. To observe how its output can control a C-MOS sampling switch.
3. To identify that this output wave is a dual-polarity PAM.
4. To adjust the depth and frequency of modulation.
5. To observe a single-polarity PAM from this circuit, by adding a DC reference level
to the input sine wave.
6. To observe that the output waveform is a single polarity PAM.
7. To observe the demodulated waveform using detector.
INTRODUCTION:
In amplitude and angle modulation, some characteristic of the carrier
amplitude, frequency, or phase is continuously varied in accordance with the
modulating information. However, in pulse modulation, a small sample is made of
the modulating signal and then a pulse is transmitted. In this case, some
characteristic of the pulse is varied in accordance with the sample-of the modulating
signal. The sample is actually a measure of the modulating signal at a specific time.
There are several types of pulse modulating systems. Three of the more
common types are; pulse amplitude modulation (PAM), pulse duration modulation
(PDM) and pulse position modulation. In each of these systems, a characteristic of,
the pulse such as amplitude duration or position is continuously varied in accordance
with the modulating signal. This type of pulse modulation, where a pulse
characteristic is continuously varied, is called analog pulse modulation.
Another type of pulse modulation is pulse code modulation (PCM), which is
digital pulse modulation. With PCM, the modulating signal is sampled and then
quantized. In quantization, each sample is assigned a specific numerical value
according to its amplitude.
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This numerical value is then represented by a group of pulses of equal
amplitude and duration. The absence or presence of pulses represents the
modulating signal's value in the binary number system. This system has many
advantages and, therefore, has many applications in modern communications.
PULSE AMPLITUDE MODULATION:
The simplest form of pulse modulation is pulse amplitude modulation (PAM).
In PAM, the amplitude of the pulse varies in proportion to the amplitude of the signal.
This is illustrated in figure -1. The modulating signal is shown in figure 1-A and the
sampling signal in figure 1-B. Figure 1-C shows a dual-polarity PAM signal. This
results if the waveform in "A" is centered on- a zero-volt axis or, in other words, is a
true AC wave. If a DC level is added to the modulating signal, single-polarity PAM
results, as shown in figure 1-D. In this case, sufficient DC level is added to ensure
that the pulses are always positive. Likewise, a negative DC voltage could be used
to obtain negative pulses.
FIGURE: 1 - EXAMPLE OF SINGLE/DUAL POLARITY PAM
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One practical method of generating a PAM signal is shown in figure -2. This
circuit uses a 4016 integrated circuit CMOS switch. Basically, it is a FET logic switch.
When the sampling pulse goes positive, the switch closes and the modulating input
appears across R3 and the output. When the sampling pulse drops to zero, the
switch opens and the output is zero. As shown, the circuit provides dual-polarity
PAM. However, single-polarity PAM can be achieved by adding R1 and R2. These
resistors form a voltage divider that adds a DC level to the input signal. The result is
that the input AC wave now varies around a positive DC reference rather than a
zero-volt reference.
The demodulator for a PAM signal is merely a low-pass filter. It removes the
sampling signal and its harmonics, and passes the original modulating signal.
However, the roll-off of the filter must be steep enough to pass the highest
modulating frequency and to fully attenuate the lowest sampling frequency
component. That is, the filter's cutoff must fall well within the guard band of the
particular PAM system.
FIGURE: 2 - ONE METHOD OF OBTAINING PAM
When pulse amplitude modulation is used, it is sent over cable or wire, or it
can be used to modulate a radio frequency carrier. When this is done, the PAM
signals normally frequency modulates the carrier rather than amplitude modulating
the carrier. However, PAM is not used very often to transmit information, since it is
more susceptible to noise interference than other forms of pulse modulation.
C
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THE TRANSISTOR DETECTOR
Many low cost transistor receivers use a germanium diode as the detector.
However, due to the limited RF gain usually preceding the detector, a transistor is
sometimes used as the demodulator to provide additional gain. A transistor can
perform as a detector, if it is biased for class B operation. In this way, the PAM signal
is both rectified and amplified at the same time.
FIGURE: 3 - DEMODULATOR CIRCUIT DIAGRAM
Figure-3 shows a typical transistor detector. Resistors R4 and R5 form a
biasing network, which sets the circuit for exactly class B operation. R6 is the
collector load resistor, while C1 filters out the RF components. This leaves the audio
signal to be coupled to the output by C2.
The transistor detector offers a method of achieving additional receiver gain.
However, in anything but very low cost receivers, the diode detector is chosen for its
simplicity and excellent performance.
R4
R5
R6 C1
C2
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STEP-BY-STEP PROCEDURE:
1. Study the circuit configuration given on the front panel of the trainer.
AF OSCILLATOR:
FREQ SELECTOR X10 POSITION
FREQ CONTROL Adjust To 1KHz frequency
Output
AMP CONTROL 10V(P-P)
RF OSCILLATOR:
FREQ SELECTOR X10 POSITION
FUNCTION
SELECTOR
Set the Switch to Square
FREQ CONTROL 5 KHz - 100KHz
AMP CONTROL 10V(P-P)
2. The 4016 integrated circuit is a CMOS bilateral switch which is used as a
sampling switch. A positive voltage on pin 13 closes the CMOS transistor switch
between pins 1 & 2. When pin 13 is at zero volts, the switch is open. The
complete pin diagram of 4016 is shown in figure- 5.
3. Switch ON the trainer.
4. Connect a 1 KHz sine wave of 10V P-P from an AF OSCILATOR at the “AF
INPUT”.
5. Connect the oscilloscope to pin 2 of 4016 IC, adjust the amplitude control in sine
wave to vary the amplitude of the modulating signal. Also adjust the frequency of
the modulating signal to obtain stable display on the oscilloscope. You will
observe the dual polarity PAM on CRO.
6. Modify the circuit by connecting the 10K biasing resistors. By doing so DC level
is added to the input and the signal moves above the DC line. So the output is
single polarity pulse amplitude modulated waveform.
7. Vary the amplitude and frequency of the sine wave signal and observe the
change in the output waveform.
8. Connect the modulated output to the input of the demodulator as shown in wiring
diagram.
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9. Connect channel 1 of the dual trace oscilloscope to the demodulator output and
channel 2 to the input sine wave. Compare the two waveforms you will find that
they are 180 out of phase since the transistor detector operates in a CE
configuration.
FIGURE: 4 - DUAL POLARITY PAM
Figure: 5 PIN DIAGRAM OF 4016
.
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WIRING DIAGRAM
PULSE AMPLITUDE MODULATION &
DEMODULATION (DUAL POLARITY)
INDICATES PATCHING CONNECTIONS
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PULSE AMPLITUDE MODULATION &
DEMODULATION
(SINGLE POLARITY)
INDICATES PATCHING CONNECTIONS