Camellia Institute of Polytechnic

Digital & Microwave Communication Lab Manual

EXPERIMENT-1
TITLE: - To study the generation of TDM Signal and its waveform
THEORY: - Time-division multiplexing is defined as a type of multiplexing wherein FDM,
instead of sharing a portion of the bandwidth in the form of channels, in TDM, time is shared. Each
connection occupies a portion of time in the link.
In Time Division Multiplexing, all signals operate with the same frequency (bandwidth) at different
times.
CIRCUIT DIAGRAM: -

TECHNICAL SPECIFICATIONS

• To study the Time Division Multiplexing & Demultiplexing

• Circuit diagram printed on PCB
• Data Generator 1 K b/s/Channel
• Low High, Logic Low sources
• Time Division Multiplexer
• Time Division Demultiplexer
• LED indication for channel outputs
• Interconnection: 2/4mm banana sockets

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 • DC Supply: Built-in IC-regulated power supplies
• 220V ±10%, 50Hz mains operated
• Enclosed in an attractive ABS plastic cabinet with cover
• Standard Accessories: User's Manual with patch cords

Waveform: -

CONCLUSION:- Thus, we have studied and constructed the TDM waveform.

EXPERIMENT-2

TITLE:- To study the generation of FDM signal and its detected waveform.

THEORY:- Frequency division multiplexing is defined as a type of multiplexing where the

bandwidth of a single physical medium is divided into a number of smaller, independent
frequency channels.
Frequency Division Multiplexing is used in radio and television transmission.
In FDM, we can observe a lot of inter-channel cross-talk, due to the fact that in this type of
multiplexing the bandwidth is divided into frequency channels. In order to prevent the
inter-channel cross talk, unused strips of bandwidth must be placed between each channel.
These unused strips between each channel are known as guard bands.

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TECHNICAL SPECIFICATIONS

• To study the Frequency Division Multiplexing & Demultiplexing

• Circuit diagram printed on PCB
• Data Generator 1 K b/s/Channel
• Low High, Logic Low sources
• Frequency Division Multiplexer
• Frequency Division Demultiplexer
• LED indication for channel outputs
• Interconnection: 2/4mm banana sockets
• DC Supply: Built-in IC-regulated power supplies
• 220V ±10%, 50Hz mains operated
• Enclosed in an attractive ABS plastic cabinet with cover
• Standard Accessories: User's Manual with patch cords

CIRCUIT DIAGRAM: -

Waveform:-

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CONCLUSION:- Thus, we have constructed and studied the FDM waveform.

EXPERIMENT-3

TITLE:- To study the generation of ASK signal and its detected waveform.
THEORY:- Amplitude Shift Keying ASK is a type of Amplitude Modulation that represents
the binary data in the form of variations in the amplitude of a signal.
Any modulated signal has a high-frequency carrier. The binary signal, when ASK modulated,
gives a zero value for Low input while it gives the carrier output for High input.
The following figure represents ASK modulated waveform along with its input.

The ASK modulator block diagram comprises of the carrier signal generator, the binary
sequence from the message signal and the band-limited filter. Following is the block diagram
of the ASK Modulator. The carrier generator, sends a continuous high-frequency carrier. The

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binary sequence from the message signal makes the unipolar input to be either High or Low.
The high signal closes the switch, allowing a carrier wave. Hence, the output will be the carrier
signal at high input. When there is low input, the switch opens, allowing no voltage to appear.
Hence, the output will be low.
The band-limiting filter, shapes the pulse depending upon the amplitude and phase
characteristics of the band-limiting filter or the pulse-shaping filter.

CIRCUIT DIAGRAM:-

Waveform:-

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CONCLUSION: - Thus, we have studied and constructed the ASK waveform.

EXPERIMENT-4
TITLE:- To study the generation of FSK signal and the detected waveforms.
THEORY:- Frequency Shift Keying FSK is the digital modulation technique in which the
carrier signal frequency varies according to the digital signal changes. FSK is a scheme of
frequency modulation.
The output of an FSK-modulated wave is high in frequency for a High binary input and is low
in frequency for a Low binary input. The binary 1s and 0s are called Mark and Space
frequencies.
The following image is the diagrammatic representation of FSK modulated waveform along
with its input.

CIRCUIT DIAGRAM:-

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Waveform:-

CONCLUSION:- Thus, we have studied the FSK waveform signal.

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 EXPERIMENT-5

TITLE:- To study the characteristics of klystron.

THEORY:- The essential elements of Klystron are electron beams and cavity resonators.
Electron beams are produced from a source and the cavity klystrons are employed to amplify
the signals. A collector is present at the end to collect the electrons. The electrons emitted by
the cathode are accelerated towards the first resonator. The collector at the end is at the same
potential as the resonator. Hence, usually, the electrons have a constant speed in the gap
between the cavity resonators.Initially, the first cavity resonator is supplied with a weak high-
frequency signal, which has to be amplified. The signal will initiate an electromagnetic field
inside the cavity. Due to this field, the electrons that pass through the cavity resonator are
modulated. On arriving at the second resonator, the electrons are induced with another EMF at
the same frequency. This field is strong enough to extract a large signal from the second cavity.

CIRCUIT DIAGRAM:-

Working of Klystron:- To understand the modulation of the electron beam, entering the first
cavity, let's consider the electric field. The electric field on the resonator keeps on changing its
direction of the induced field. Depending on this, the electrons coming out of the electron gun,
get their pace controlled.

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As the electrons are negatively charged, they are accelerated if moved opposite to the direction
of the electric field. Also, if the electrons move in the same direction of the electric field, they
get decelerated. This electric field keeps on changing, therefore the electrons are accelerated
and decelerated depending upon the change of the field. The following figure indicates the
electron flow when the field is in the opposite direction. While moving, these electrons enter
the field free space called as the drift space between the resonators with varying speeds, which
create electron bunches. These bunches are created due to the variation in the speed of travel.
These bunches enter the second resonator, with a frequency corresponding to the frequency at
which the first resonator oscillates. As all the cavity resonators are identical, the movement of
electrons makes the second resonator to oscillate. The following figure shows the formation of
electron bunches.
The induced magnetic field in the second resonator induces some current in the coaxial cable,
initiating the output signal. The kinetic energy of the electrons in the second cavity is almost
equal to the ones in the first cavity and so no energy is taken from the cavity.
The electrons while passing through the second cavity, few of them are accelerated while
bunches of electrons are decelerated. Hence, all the kinetic energy is converted into
electromagnetic energy to produce the output signal.
Amplification of such two-cavity Klystron is low and hence multi-cavity Klystrons are used.
This microwave generator, is a Klystron that works on reflections and oscillations in a single
cavity, which has a variable frequency.
Reflex Klystron consists of an electron gun, a cathode filament, an anode cavity, and an
electrode at the cathode potential. It provides low power and has low efficiency.

CONCLUSION:- Thus, we have studied the working principle of Gunn Diode.

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 EXPERIMENT-6
TITLE: - To study the characteristics of Gunn Diode.
THEORY: - A Gunn diode is a passive semiconductor device with two terminals, which
composes only an n-doped semiconductor material, unlike other diodes, consisting of a p-n
junction. Gunn diodes can be made from materials that consist of multiple, initially-empty,
closely-spaced energy valleys in their conduction band, like Gallium Arsenide (GaAs),
Indium Phosphide (InP), Gallium Nitride (GaN), Cadmium Telluride (CdTe), Cadmium
Sulphide (CdS), Indium Arsenide (InAs), Indium Antimonide (InSb) and Zinc Selenide
(ZnSe).

General manufacturing involves growing an epitaxial layer on a degenerate n+ substrate to

form three n-type semiconductor layers (Figure 1a), where-in the extreme layers are heavily
doped compared to the middle, active layer.

Further, the metal contacts are provided at either end of the Gunn diode to facilitate biasing.
The circuit symbol for the Gunn diode is as shown in Figure 1b and differs from that normal
diode to indicate the absence of a p-n junction. On applying a DC voltage across the
terminals of the Gunn diode, an electric field is developed across its layers, most of which
appears across the central active region. At the initial stages, the conduction increases due to
the movement of electrons from the valence band into the lower valley of the conduction
band.
The associated V-I plot is shown by the curve in region 1 (colored in pink) of Figure 2.
However, after reaching a certain threshold value (V th), the conduction current through the
Gunn diode decreases, as shown by the curve in Region 2 (colored in blue) of the figure.
This is because, at higher voltages the electrons in the lower valley of the conduction band
move into its higher valley where their mobility decreases due to an increase in their effective
mass. The reduction in mobility decreases the conductivity which leads to a decrease in the
current flowing through the diode.
As a result, the diode is said to exhibit a negative resistance region (region spanning from
Peak point to Valley Point) in the V-I characteristic curve. This effect is called the transferred
electron effect; thus, the Gunn diodes are also called Transferred Electron Devices.

CIRCUIT DIAGRAM

V-I Characteristics:-

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CONCLUSION:- Thus, the V-I characteristics of Gunn Diode is studied and plotted.

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