DEPARTMENT OF ELECTRONICS AND

COMMUNICATION ENGINEERING

19ECE283 – Linear Integrated Circuits

DONE BY:
Akshy Ram B-CH.EN.U4ECE22007
Arun Siva Prasath A-CH.EN.U4ECE22009
Aryan S Nair-CH.EN.U4ECE22010
Mugilvarshan M-CH.EN.U4ECE22029
Sriramkumaran S-CH.EN.U4ECE22048

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 TABLE OF
CONTENTS
S.n Topic Page
o no.
1 Abstract 3
2 Keywords 4-5
3 Introduction 6-8
4 Literature Survey 9-13
5 Proposed 14-
Method/Changes 15
6 Result 16-
17
7 References 18

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 ABSTRACT
This project presents the design and implementation of a Double-Sideband
Suppressed Carrier (DSB-SC) modulator and demodulator utilizing operational
amplifiers (op-amps) and passive elements. DSB-SC modulation is a key
technique in communication systems for transmitting baseband signals over
radio frequency carriers efficiently. The modulator architecture is based on op-
amp circuits combined with passive elements such as resistors, capacitors. The
project aims to achieve a compact and cost-effective modulator design suitable
for various communication applications. Design considerations, circuit analysis,
and simulation results are discussed to validate the proposed approach's
effectiveness in generating DSB-SC signals. Additionally, practical
implementation challenges and performance evaluations are addressed to
ensure the modulator meets specified requirements. The project contributes to
the advancement of modulation techniques by offering a practical
implementation solution leveraging op-amp technology and passive
components, with potential applications in wireless communication systems,
radar systems, and instrumentation.

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 KEYWORDS
1.Amplitude Modulation:
Amplitude Modulation (AM) is a modulation technique where the amplitude of
a carrier signal is varied in accordance with the instantaneous amplitude of the
modulating signal. This allows information to be encoded in the variations of
the carrier signal's amplitude, facilitating transmission of audio or other data
over radio waves.
2. DSB-SC Signal:
A Double-Sideband Suppressed Carrier (DSB-SC) signal is a type of amplitude
modulation where both the upper and lower sidebands are retained, while the
carrier signal is completely suppressed. This results in efficient transmission of
the modulating signal without redundant carrier information, suitable for
applications like radio communication and signal processing.

3. Baseband Signals:
Baseband signals refer to signals that occupy the lowest frequencies within a
communication system, typically ranging from DC to a certain maximum
frequency. These signals represent the original information content, such as
audio, video, or data, before modulation for transmission over a carrier
frequency in a communication channel.
4.Schottky Diode:

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A Schottky diode is a semiconductor device featuring a metal-semiconductor
junction, typically composed of a metal (e.g., gold, platinum) and a
semiconductor material (e.g., silicon). It exhibits low forward voltage drop and
fast switching characteristics due to its unique rectifying properties, making it
suitable for high-frequency and high-speed applications.

5.BPF:
BPF stands for Bandpass Filter, a type of electronic filter that allows signals
within a specific frequency range to pass through while attenuating or rejecting
frequencies outside of this range. It is commonly used in communication
systems to select and isolate desired frequencies for further processing or
transmission.
6.LPF:
LPF stands for Low Pass Filter. It's a type of electronic filter that allows signals
with frequencies lower than a certain cutoff frequency to pass through
unaffected, while attenuating (reducing) signals with frequencies higher than
the cutoff frequency. LPFs are commonly used in audio and communications
systems to remove unwanted high-frequency noise.

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 INTRODUCTION
In the process of Amplitude Modulation, the modulated wave consists of the
carrier wave and two sidebands. The modulated wave has the information only
in the sidebands. Sideband is nothing but a band of frequencies, containing
power, which are the lower and higher frequencies of the carrier frequency.
The transmission of a signal, which contains a carrier along with two sidebands
can be termed as Double Sideband Full Carrier system or simply DSBFC.

However, such a transmission is inefficient. Because, two-thirds of the power is

If this carrier is suppressed and the saved power is distributed to the two
sidebands, then such a process is called as Double Sideband Suppressed
Carrier system or simply DSBSC.

Figure 2.a – Frequency domain plot of DSB-FC

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 Figure 2.b – Frequency Domain Plot of DSB-SC
In the above images fc and fm are carrier and message signal frequencies
respectively.

Design:
Modulator –
To design a Double-Sideband Suppressed Carrier (DSB-SC) modulator, use a
balanced modulator or a mixer to multiply the message signal by a high-
frequency carrier signal. This process suppresses the carrier while retaining the
upper and lower sidebands, effectively achieving DSB-SC modulation.
Additional circuitry may include filters to remove unwanted harmonics and
amplifiers to adjust signal levels. The modulator's design aims for efficient
multiplication and suppression while maintaining signal integrity.

Figure 3 – Block Diagram of a DSB-SC Modulator

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Demodulator –
A DSB-SC demodulator extracts the original baseband signal from the
modulated DSB-SC waveform. One common demodulator design involves
multiplying the modulated signal by a local oscillator signal at the carrier
frequency, followed by a low-pass filter to remove the high-frequency
components. This process cancels out the carrier, leaving only the original
baseband signal. Finally, the filtered signal is amplified and processed for
further use in the receiver circuitry.

Figure 4- Block Diagram of a DSB-SC Demodulator

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 Literature Survey
Author Circuit from paper/blog Findings
B.Kanman Use of two
i transform
er to step
up the
signal,
BJT’s used
as
multipliers
and tank
circuit
used to
select
certain
frequency.

Figure 5.a – Model Circuit 1

UNKNOW Usage of
N IC 8038 as
carrier
generator
and BJT’s
used as
multipliers
. Again LC
tank is
used to
select
certain
frequency

Figure 5.b – Model Circuit 2

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 UNKNOW Usage of
N ring
modulator
to multiply
signals.
Square
carrier
Figure 5.c- Model Circuit -3 wave used
instead of
Sine
UNKNOW Finalised
N circuit –
Workings
explained
in detail
below

Figure 5.d – Model Circuit - 4

Chosen circuit and its working: -

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 Figure 6 – Envelope creator
Figure 6 shows the part of the circuit that creates the envelope for the DSB-SC
signal.

Figure 7 – Output of Envelope generator

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 Figure 8

Figure 8 shows the BJT part of the circuit which fills the envelope with carrier
signal supplied through the base

Figure 9
Figure 9 shows the BP filter required to generate the final DSB-SC signal

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 Figure 10
Figure 10 shows the op-amp circuit used to amplify the final DSB-SC signal and
remove ripples if present in the final output

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 Proposed Method/Changes
It was found out that the feedback capacitance in figure 10 can be removed
and the value of feedback resistance can be increased to smoothen out the
ripples as increasing the feedback resistance increases the amplification and
thus ripples are smoothened out.
It was also found out through experimentation that changing certain values of
resistors in the circuit (Figure 5.d) makes the output even more efficient.
Moreover, a final Demodulation Circuit was constructed as per the theory of
demodulation using Schottky diode (Very Fast Switching Diode).
The final modulation circuit is displayed in Figure 11 while the demodulation
circuit is shown in Figure 12.

Figure 11 – Final Modulation Circuit after installing changes

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 Figure 12 – Final Demodulation circuit

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 RESULT

Figure 13 – Output before method changes

Figure 14 – Output after method changes

Comparing the output to figure 15 we find out that the output gives an under
modulated DSB-SC signal with an modulation index of 71%

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 Figure 15- Types of Modulation

Figure 16 – Output from Demodulator(in green) compared to modulated wave

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 REFERENCES
1. https://www.multisim.com/content/VpZnDC39RKe6tLZvJMa98k/
amplitude-modulation-using-op-amps-18bec0626/open/
2. Modified switching modulator for AM-DSBSC Modulation – by
B.Kanmani
3. https://www.aurora.ac.in/images/pdf/departments/ece-downloads/
academic-manuals/lab-manual-2014-15-1sem/ac-lab-manual-3ece/
3ece-ac-lab-manual-2-11.pdf
4. https://wiki.analog.com/university/courses/alm1k/alm-signals-labs/alm-
am-modulatior-lab

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