analogue communication trainer

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).

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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 CONTROL Adjust RF Frequency to

be at 100KHz

FUNCTION SELECTOR

Select the switch to Sine


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:


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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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 CONTROL Adjust to 500Hz frequency

output


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


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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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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.


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


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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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1. Study the circuit configuration given on the front panel of the trainer.

AF OSCILLATOR:


FREQ CONTROL Adjust To 1KHz frequency

Output


RF OSCILLATOR:


FUNCTION

SELECTOR

Set the Switch to Square

FREQ CONTROL 5 KHz - 100KHz


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.


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

analogue communication trainer

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