AM Demommmmmmmmmmmmmmmmmmmmmdulation.pptx

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1. AM Demodulation

2. Generation of AM wave

3. Analog Communication - AM Demodulators The process of extracting an original message signal from the modulated wave is known as  detection  or  demodulation . The circuit, which demodulates the modulated wave is known as the  demodulator . The following demodulators (detectors) are used for demodulating AM wave. Square Law Demodulator Envelope Detector

4. Square Law Demodulator Square law demodulator is used to demodulate low level AM wave. This demodulator contains a square law device and low pass filter. The AM wave  V1(t) is applied as an input to this demodulator.

5. Square Law Demodulator The standard form of AM wave is We know that the mathematical relationship between the input and the output of square law device is

6. Square Law Demodulator

7. Envelope Detector Envelope detector is used to detect (demodulate) high level AM wave. This envelope detector consists of a diode and low pass filter. Here, the diode is the main detecting element. Hence, the envelope detector is also called as the  diode detector . The low pass filter contains a parallel combination of the resistor and the capacitor.

8. Envelope Detector In the positive half cycle of AM wave, the diode conducts and the capacitor charges to the peak value of AM wave. When the value of AM wave is less than this value, the diode will be reverse biased. Thus, the capacitor will discharge through resistor R till the next positive half cycle of AM wave. When the value of AM wave is greater than the capacitor voltage, the diode conducts and the process will be repeated. We should select the component values in such a way that the capacitor charges very quickly and discharges very slowly. As a result, we will get the capacitor voltage waveform same as that of the envelope of AM wave, which is almost similar to the modulating signal.

9. Double Sideband Suppressed Carrier (DSB-SC) Modulation Definition :  DSB-SC  is an amplitude modulated wave transmission scheme in which only  sidebands  are  transmitted  and the  carrier  is not transmitted as it gets  suppressed . DSB-SC is an acronym for  D ouble  S ideband  S uppressed  C arrier. The carrier does not contain any information and its transmission results in loss of power. Thus only sidebands are transmitted that contains information. This results in saving of power used in transmission.

10. Double Sideband Suppressed Carrier (DSB-SC) Modulation This saved power can be inserted into the 2 sidebands. Hence, ensuring a stronger signal that transmits over long distances. As during suppression, the baseband signal does not get affected in any way. As we know that transmission power and bandwidth are the two important parameters in a communication system. Thus, in order to save power and bandwidth, DSB-SC  modulation  technique is adopted. A normal system also termed as  Double Sideband Full Carrier (DSB-FC)  system,  transmits carrier  along with the  two sidebands . Hence, resulting in a considerable power loss. So, DSB-SC system is used in order to overcome the drawback of DSB-FC system.

11. Generation of DSB-SC signal Here, by observing the above figure, we can say that a product modulator generates a DSB-SC signal. The signal is obtained by the multiplication of baseband signal x(t) with carrier signal cos ω c t By frequency shifting property of Fourier transform-

12. Generation of DSB-SC signal From the above equation, it is clear that only 2 components are present in the spectrum. These two are the two sidebands that are placed at  + ω c   and  - ω c . Let’s have a look at the pictorial representation of a waveform for DSB-SC system

13. Mathematical Expression Consider the baseband or modulating signal,

14. Mathematical Expression

15. Carrier suppression in DSB-SC (Balanced Modulator) The carrier without any information content is suppressed by a balanced modulator. Its principle of operation is such that, when two signals of the different frequency are passed through a non-linear resistance then an amplitude modulated signal with the suppressed carrier is achieved at the output. It can be a diode,  JFET  or BJT that possess non-linear resistance characteristic. A non-linear device has the capability to produce 2 sidebands with a carrier. But, a balanced mode connection of 2 non-linear devices produces a DSB-SC signal.

16. Carrier suppression in DSB-SC (Balanced Modulator)

17. Carrier suppression in DSB-SC (Balanced Modulator)

18. Carrier suppression in DSB-SC (Balanced Modulator)

19. Carrier suppression in DSB-SC (Balanced Modulator)

20. Carrier suppression in DSB-SC (Balanced Modulator)

21. Advantages of DSB-SC modulation It provides 100% modulation efficiency. Due to suppression of carrier, it consumes less power. It provides a larger bandwidth.

22. Disadvantages of DSB-SC modulation It involves a complex detection process. Using this technique it is sometimes difficult to recover the signal at the receiver. It is an expensive technique when it comes to demodulation of the signal.

23. Applications of DSB-SC modulation During the transmission of binary data, DSB-SC system is used in phase shift keying methods. In order to transmit 2 channel stereo signals, DSB signals are used in Television and FM broadcasting. DSB-SC technique allows us to have a transmission that reduces overall power consumption rate, thereby ensuring a stronger signal at the output.