1. Lecture 5: DSB-SC AM Modulation 1st semester 1439 - 2017
By: Elham Sunbu

2. Modulation Concept
- The transmitter modifies the message signal into a form suitable for transmission over the channel.

3. Modulation Concept
- Modulation: is the process of facilitating the transfer of information over a medium.
- Modulation is done by putting message signal (baseband signal) onto a high frequency carrier signal for transmission.

4. Modulation - The modulation process involve two waves:
- The baseband signal (called a Modulating Signal) and the Carrier Signal which is a sinusoid signal.
- The output of the modulation process is called as the Modulated Signal.

5. Types of Modulation

6. Continuous-Wave Modulation (CW)
- The Continuous-Wave Modulation (CW): is used to transmit an analog baseband signal using the high frequency carrier signal.
- In CW modulation, the analog baseband signal (modulating signal) is used to change a parameter of the carrier signal
( amplitude, frequency, or phase).
- The CW modulation is used in the analog communication system.

7. Types of CW Modulation Amplitude Modulation (AM).
Frequency Modulation (FM).
Phase Modulation (PM).

8. 1. Amplitude Modulation
- Amplitude Modulation: is defined as the process in which is the amplitude of the carrier signal is varied according to the baseband signal.
- The carrier’s frequency and its phase remain fixed, just the amplitude is varies.

9. Types of Amplitude Modulation
- There are several different ways of amplitude modulation:
Double-sideband suppressed carrier (DSB-SC) modulation.
Amplitude modulation (AM).
Single-sideband modulation (SSB).
Vestigial-sideband modulation (VSB).
- Each of these schemes has its own distinct advantages, disadvantages, and practical applications.
There are several different ways of amplitude modulating the carrier signal by the baseband signal ( modulating signal), each of which results in different spectral characteristics for the transmitted signal (modulated signal):

10. Double-Sideband Suppressed Carrier (DSB-SC) Modulation
- Spectrum
- Upper and Lower Sidebands
- Bandwidth
- Demodulation

11. Modulation - The Modulating Signal (message or baseband signal)
- The Carrier Signal is
- The Modulated Signal

13. The DSB-SC modulated signal undergoes a phase reversal whenever the message signal m(t) cross zero.
Consequently, the envelop of a DSB-SC modulated signal is different from the message signal.

14. Spectrum
The process of DSB-SC modulation shifts the spectrum of the modulating signal m(t) to the left and the right by ωc (carrier frequency).

15. Upper and Lower Sidebands
The modulated signal spectrum centered at ωc is composed of two parts: a portion that lies above ωc , known as the upper sideband (USB), and a portion that lies below ωc , known as the lower sideband (LSB).
Similarly, the spectrum centered at - ωc has upper and lower sidebands.
Hence, this is a modulation scheme with double sidebands

16. Bandwidth
- If the bandwidth of the modulating signal is B Hz, then the bandwidth of the modulated signal is 2B Hz.

17. Bandwidth
- In order to avoid the overlap of the spectra centered at ωc and –ωc , the ωc should be ≥ ωM.
- If ωc < ωM, the LSB of ωc will overlap with LSB of –ωc.
- Which make it impossible to get back the m(t) from the modulated signal m(t) cos (ωct).

18. Demodulation
- The DSP-SC demodulation consists of multiplication of the incoming modulated signal m(t) cos (ωct ) by a carrier cos (ωct) followed by a low pass filter.
- The receiver need to generate a local carrier that has the same frequency and the same phase of the carrier used for modulation.

20. Demodulation
- This method of recovering the baseband signal is called synchronous detection or coherent detection, because the receiver uses a carrier of exactly the same frequency and phase as the carrier used for modulation

21. Thank You