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Signals and Systems Lecture 14

Signals and Systems Lecture 14. Multiplication Property Communication System. Chapter 4 Fourier Transform. §4.5 The Multiplication property (modulation property) （调制特性）.

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Signals and Systems Lecture 14

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  1. Signals and SystemsLecture 14 Multiplication Property Communication System

  2. Chapter 4 Fourier Transform §4.5 The Multiplication property (modulation property) （调制特性）

  3. Chapter 4 Fourier Transform Example 4.23 Determine the Fourier transform of the signal

  4. Chapter 4 Fourier Transform 例 某连续时间LTI系统的单位冲激响应为 若输入信号 ， 试求整个系统的输出。

  5. Chapter 4 Fourier Transform 例 如图所示系统中， 若输入信号 ，试分别A、B、C各点信号 的频谱。 A B C

  6. Chapter 4 Fourier Transform §4.5.1 Frequency-Selective Filtering with Variable Center Frequency 中心频率可调的频率选择性滤波器

  7. Chapter 8 Communication Systems Communication Channel 信道 Source Signals Modulator 调制器 Modulated Signals Be best suited for transmitting

  8. Chapter 8 Communication Systems The general process of embedding an information -bearing signal into a second signal. Modulation : Extracting the information-bearing signal. Demodulation : Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) Modulation : Amplitude Modulation Modulating Signal Modulated Signal Carrier Signal

  9. Chapter 8 Communication Systems c(t) x(t) y(t)=x(t)c(t) §8.1 Complex Exponential and Sinusoidal Amplitude Modulation Modulating system model: x(t) --- modulating signal c(t) --- Carrier signal

  10. Chapter 8 Communication Systems (1) Modulation Theory 8.1.1 Amplitude Modulation with Complex Exponential Carrier Exponential carrier: For convenience, let c=0, so Output signal(modulated signal):

  11. Chapter 8 Communication Systems

  12. Chapter 8 Communication Systems (1) Implementation

  13. Chapter 8 Communication Systems 8.1.2 Amplitude Modulation with Sinusoidal signal For convenience, choose c=0, so

  14. Chapter 8 Communication Systems

  15. Chapter 8 Communication Systems

  16. Chapter 8 Communication Systems Envelope §8.1 Complex Exponential and Sinusoidal Amplitude Modulation

  17. Chapter 8 Communication Systems 8.2.1 Synchronous demodulation (1) Demodulation process 8.2 Demodulation for Sinusoidal AM

  18. Chapter 8 Communication Systems In time domain: In frequency domain: Expected signal:

  19. Chapter 8 Communication Systems

  20. Chapter 8 Communication Systems (2) Synchronous problem

  21. Chapter 8 Communication Systems When , it is referred to as synchronous demodulation. Time domain: The output of lowpass filter: Ideal output: x(t)

  22. Chapter 8 Communication Systems 8.2.2 Asynchronous demodulation Amplitude-modulated signal:

  23. Chapter 8 Communication Systems

  24. Chapter 8 Communication Systems Communication Channel LPF Receiver Synchronous Demodulation Transmitter Modulation §8.2.1 Synchronous Demodulation ① Complex Exponential Carrier • Suppose the modulator and demodulator are not • synchronized in phase. 2. Suppose the modulator and demodulator are not synchronized in frequency.

  25. Chapter 8 Communication Systems Communication Channel LPF Receiver Synchronous Demodulation Transmitter Modulation ② Sinusoidal Carrier • Suppose the modulator and demodulator are not • synchronized in phase. 2. Suppose the modulator and demodulator are not synchronized in frequency.

  26. Chapter 8 Communication Systems BPF BPF BPF §8.3 Frequency-Division Multiplexing（频分复用）

  27. Chapter 8 Communication Systems LPF Communication Channel LPF Quadrature Multiplexing（正交多路复用）

  28. Chapter 8 Communication Systems Upper sideband Upper sideband Lower sideband §8.4 Single-Sideband Sinusoidal Amplitude Modulation （单边带） Double Sideband Modulation (DSB)

  29. Chapter 8 Communication Systems Upper sideband Upper sideband Lower sideband Retain the upper sideband Retain the lower sideband Single Sideband Modulation (SSB)

  30. Chapter 8 Communication Systems Figure 8.21 SSB system using a 90o phase-shift network. 90ophase-shift

  31. Chapter 8 Communication Systems Lower Sideband

  32. Chapter 4 Fourier Transform Summary Multiplication Property and Communication Systems AM and DSB、SSB System Modulation and Demodulation

  33. Readlist • Signals and Systems: • 5.1, 5.2 • Question: • DTFT

  34. Chapter 8 Communication Systems Homework: 8.1 8.3 8.22 8.28

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