DSB-SC Generation

Sri Lanka Institute of Information Technology Faculty of Engineering Department of Electrical and Computer Engineering EC3611: Communication Engineering Lab 01: DSB-SC Generation

Name

:

S.A.D.T.P Kaushalya

Index Number

:

EN14535468

Group

:

3A

Submission Date

:

08/03/2016

Table of Contents Objectives ....................................................................................................................................... 3 Introduction ..................................................................................................................................... 4 Experiment 1 ................................................................................................................................... 5 Procedure .................................................................................................................................... 5 Experiment 2 ................................................................................................................................... 5 Procedure .................................................................................................................................... 5 Observations ................................................................................................................................... 6 Calculations..................................................................................................................................... 7 Discussion ....................................................................................................................................... 8 Conclusion .................................................................................................................................... 10 References ..................................................................................................................................... 10

List of Figures Figure 1: Waveforms of Modulations ............................................................................................. 4 Figure 2: Frequency Spectrum of DSB-SC .................................................................................... 4 Figure 3: TIMS Block Diagram ...................................................................................................... 5 Figure 4: DSB-SC Signal along with the Message signal .............................................................. 6 Figure 5: DSBSC-Signal ................................................................................................................. 6 Figure 6: Message Signal ................................................................................................................ 6 Figure 7: Overloaded DSB-SC signal ............................................................................................. 6 Figure 8: Envelop phase change ..................................................................................................... 6 Figure 9: TIMS Setup ..................................................................................................................... 6 Figure 10: Waveforms and Frequency Spectrum ........................................................................... 8 Figure 11: Generation of DSB-SC .................................................................................................. 8

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Objectives Purpose of this laboratory is to understand the concept of double sideband suppressed carrier signal DSBC. This laboratory will also cover the conceptual understanding of spectrum estimation, multipliers, and modulators using MULTIPLIER, VCO, 60 kHz LPF, and TUNEABLE LPF modules of TIMS.

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Introduction Double-sideband suppressed-carrier transmission (DSB-SC) is transmission in which frequencies produced by amplitude modulation (AM) are symmetrically spaced above and below the carrier frequency and the carrier level is reduced to the lowest practical level, ideally being completely suppressed. In the DSB-SC modulation, unlike in AM, the wave carrier is not transmitted; thus, much of the power is distributed between the sidebands, which implies an increase of the cover in DSB-SC, compared to AM, for the same power used. DSB-SC is basically an amplitude modulation wave without the carrier, therefore reducing power waste, giving it a 100% efficiency. This is an increase compared to normal AM transmission (DSB), which has a maximum efficiency of 33.333%, since 2/3 of the power is in the carrier which carries no intelligence, and each sideband carries the same information. Single Side Band (SSB) Suppressed Carrier is 100% efficient. [1] In DSB-SC AM, there is no dc offset added to the baseband signal. As a result, the carrier component of the AM signal is suppressed. The formula for the modulated signal s(t) and its Fourier transformation S(f) becomes,

Figure 1: Waveforms of Modulations

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Figure 2: Frequency Spectrum of DSB-SC

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Experiment 1 Procedure 

Following block diagram shown below was setup using TIMS.



Here the Audio Oscillator block was used to obtain the message signal (𝐴 cos 𝜇𝑡 ) and the Master signal block was used to generate the carrier wave (𝐵 cos 𝜔𝑡).



Outputs from the Master signal block and Audio oscillator block was fed on to the Multiplier block



Corresponding waveforms of the message signal and the modulated signals were observed from the Oscilloscope.

Figure 3: TIMS Block Diagram

Experiment 2 Procedure 

For the same setup, the carrier wave from the master signals module was directed to buffer amplifier module to amplify it.



Then it was routed to the multiplier module and multiplied with the same message signal.



Corresponding waveforms from the oscilloscope was obtained.

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Observations

Figure 4: DSB-SC Signal along with the Message signal

Figure 6: Message Signal

Figure 8: Envelop phase change

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Figure 5: DSBSC-Signal

Figure 7: Overloaded DSB-SC signal

Figure 9: TIMS Setup

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Calculations

Amplitude of the Message signal (Channel 1) 𝐴=

4.20 = 2.1 𝑉 2

Peak to Peak Amplitude of the DSB-SC signal (Channel 2) 𝐷𝑆𝐵𝑆𝐶 𝑉𝑝 − 𝑝 = 4.025 𝑉 Amplitude of the Carrier Signal Since the Amplitudes are nominally equal in TIMS 𝐵=

4.20 = 2.1 𝑉 2

Scale factor ′𝒌′ of the Multiplier 𝒌=

𝐷𝑆𝐵𝑆𝐶 𝑉𝑝 − 𝑝 2𝐴𝐵

𝒌=

4.025 2 × 2.1 × 2.1

𝒌 = 0.4563

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Discussion Double-sideband Suppressed-carrier (DSB-SC) modulation is an amplitude modulation, which consists only of the two symmetrical sidebands and no carrier band. Here in the below figure shows that The spectrum of DSB-SC modulated wave consists of impulse functions located at ωc ±ω m −ωc ±ω m .

Figure 10: Waveforms and Frequency Spectrum

The generation of a DSB-SC modulated wave consists simply of the product of the message signal m(t) and the carrier wave. A device for achieving this requirement is called a product modulator or a Multiplier block which we used in TIMS. Generation can be described by the following figure.

Figure 11: Generation of DSB-SC

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Here the frequency of the carrier wave should be much greater than the message signal frequency 𝑓𝑚 ≪ 𝑓𝑐 . This is to ensure that there is no overlapping of the positive and negative frequency bands of the frequency spectrum that would cause distortion of the modulated signal. DSB-SC modulation, which is of linear nature, shifts the frequency spectrum of a message signal to the frequency of the carrier that is used to modulate the message signal, increasing the bandwidth of the message signal to twice its original value. The presence of the Upper Sideband and the Lower Sideband makes it known as ‘Double Sideband’ The DSBSC is a member of a class known as linear modulated signals. Here the spectrum of the modulated signal, when the message has two or more components, is the sum of the spectral components which each message component would have produced if present alone. For the case of non-linear modulated signals, on the other hand, this linear addition does not take place. DSB-SC Transmission can be summarized as 

Frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency



The carrier level is reduced to the lowest practical level, ideally completely suppressed.

In the double-sideband suppressed-carrier transmission (DSB - SC) modulation, unlike AM, the wave carrier is not transmitted; thus, a great percentage of power that is dedicated to it is distributed between the sidebands, which imply an increase of the cover in DSB-SC, compared to AM, for the same power used. Using the 3-WAY SWITCH module, a Buffer Amplifier was put into the carrier path to the Multiplier. This involves setting the switch which connects to Multiplier input Y to the middle position. This increased the input amplitude of the carrier signal and overloading occurs at the output of the multiplier. The multipliers are quite sensitive to variations and imbalances in the surrounding resistor values. Due to temperature shifts and aging. As a result, the waveforms we observed appeared somewhat different from what we expect.

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Conclusion Today vast amounts of information are communicated using radio communications systems. Both analogue radio communications systems and digital or data radio communications links are used. One of the fundamental aspects of any radio communications transmission system is modulation, or the way in which the information is superimposed on the radio carrier. Without modulation all signals at same frequencies from different transmitters would be mixed up. Also Since the size of the antenna is directly proportional to the wavelength, increasing the frequency of the signal or modulation will decrease the size of the antenna which is a vital factor in transmission. [2] This experiment is an introduction to the MULTIPLIER and the double sideband suppressed carrier signal, or DSBSC. Through this experiment I understood the spectrum estimation, multipliers, and modulators using MULTIPLIER, VCO, 60 kHz LPF, and TUNEABLE LPF modules of TIMS.

References

[1] W.-.

DSBSC,

"https://en.wikipedia.org/wiki/Double-sideband_suppressed-

carrier_transmission," [Online]. [2] http://www.icbse.com/topics/modulation, "Modulation in Communication System," [Online].

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