Lecture on Double Sideband Suppressed Carrier (DSB-SC)

Introduction

• Topic: Basics of DSB-SC (Double Sideband Suppressed Carrier)
• Goals:
  • What is DSB-SC?
  • Why is DSB-SC needed?
  • How to generate a DSB-SC signal?
  • Importance of DSB-SC in problem-solving

Amplitude Modulated (AM) Signal

• AM Signal Equation:

  y_d = A_C * cos(ω_C t) + (μ * A_C / 2) * cos((ω_C + ω_M) t) + (μ * A_C / 2) * cos((ω_C - ω_M) t)
  

  • Carrier Signal: A_C * cos(ω_C t)
  • Upper Sideband Signal: (μ * A_C / 2) * cos((ω_C + ω_M) t)
  • Lower Sideband Signal: (μ * A_C / 2) * cos((ω_C - ω_M) t)
• Carrier vs. Sidebands:
  • Carrier signal contains no information.
  • Information is in the sidebands.

Double Sideband Suppressed Carrier (DSB-SC)

• Concept: Eliminate the carrier signal, only transmit sidebands.
• Definition: DSB-SC is an AM signal without the carrier.
  • Only upper and lower sidebands are transmitted.

Power Transmission in AM and DSB-SC

• Power Components in AM:
  • Carrier Power: P_C
  • Sideband Power: (μ^2 / 2) * P_C
• Power Calculation Example: Modulation index (μ) = 1
  • Transmitted Power: P_C + 0.5 * P_C = 1.5 * P_C
  • Power in Sidebands: 0.5 * P_C (33% of total power)
  • Power in Carrier: P_C (66% of total power)
• Advantage of DSB-SC: Saves carrier power, transmitting only sidebands.

Block Diagram for Generating DSB-SC

• Components:
  • Modulating Signal: Low Frequency
  • Carrier Signal: High Frequency
  • Product Modulator: Multiplies the two signals
  • Output Signal: y_d = x_m(t) * cos(ω_C t)

Generating DSB-SC Signal

• Step-by-Step:
  1. Consider modulating signal (e.g., red waveform).
  2. Create similar negative envelope.
  3. Multiply modulating signal with carrier signal.
  4. Observe 180-degree phase reversal at zero crossings.

Frequency Response of DSB-SC Signal

• Frequency Components:
  • Lower Sideband: ω_C - ω_M
  • Upper Sideband: ω_C + ω_M
  • Carrier: ω_C (not transmitted)
• Bandwidth: Same as AM, 2 * ω_M
• Transmission: Only sidebands are sent, saving power.

Summary

• DSB-SC: Efficient in power transmission compared to standard AM.
• Generation: Simple using product modulators and understanding signal phase reversals.
• Next Steps: Explore different methods to generate DSB-SC signals.

Conclusion

• Importance: Understanding DSB-SC enhances communication system efficiency.
• Further Study: Different modulation techniques for DSB-SC generation.

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• Suggestions and Queries: Encouraged to provide feedback and ask questions for better comprehension.