1. Chapter 2: Modulation

2. Communication System Chart Communication System Continuous Wave Digital Wave Amplitude Modulation (AM) Pulse Modulation (PM) Angle Modulation Frequency Modulation (FM) Analogue Pulse Modulation Digital Pulse Modulation

3. Modulation is defined as the process of modifying a carrier signal (radio wave) systematically by the modulating signal (audio)” This process makes the signal suitable for the transmission and compatible with the channel. The resultant signal is called the modulated signal MODULATION

4. MODULATION Modulated signal Carrier signal ( a transmitted electromagnetic pulse or wave high frequency of alternation on which information can be imposed by increasing signal strength, varying the base frequency, varying the wave phase, or other means) Baseband signal

5. Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave

8. Modulation 1 Analogue Modulation Amplitude Modulation

11. Figure AM band allocation

13. AMPLITUDE MODULATION modulating Signal v m (t) Modulated Signal Carrier wave V c cos w c t

14. Envelope – the imaginary line on the carrier waveform Vc max V modulated signal v am

15. Carrier signal Modulating signal

16. The amplitude-modulated wave can then be expressed as

17. where notation m is termed the modulation index . It is simply a measurement for the degree of modulation and bears the relationship of V m to V c Therefore the full AM signal may be written as

18. Modulation Index m (Coefficient of Modulation/Modulation Factor/Degree of Modulation) What is the degree of modulation required to establish a desirable AM communication link? Answer is to maintain m<1.0 (m<100%). This is important for successful retrieval of the original transmitted information at the receiver end.

19. Modulation Index m The modulation index can be determined by measuring the actual values of the modulation voltage and the carrier voltage and computing the ratio.

20. Modulation Index m Vm Vc

21. If the amplitude of the modulating signal is higher than the carrier amplitude, which in turn implies the modulation index . This will cause severe distortion to the modulated signal. Modulation Index m

22. The ideal condition for amplitude modulation (AM) is when m=1 , which also means V m =V c . This will give rise to the generation of the maximum message signal output at the receiver without distortion. Modulation Index m

23. Carrier component Upper sideband component Lower sideband component Using

26. DSBFC Frequency Spectrum With single frequency f m B = Maximum freq. - minimum freq. = (fc+fm)-(fc-fm) = fc+fm-fc+fm = 2fm f C f c +f m f c -f m 2f m freq

34. A video signal consists of range of frequencies and maximum frequency is as high as 4.5Mhz. If it is transmitted using the conventional AM system, the required bandwidth is 9.0 Mhz (B=2fm). But according to the standardization, TV signal is limited to 6MHz only. So, to reduce to 6Mhz bandwidth, a part of the LSB is not transmitted. In this case SSB transmission is not applied as it is very difficult to suppress a sideband accurately at high frequency. Amplitude Modulation ~ Vestigial Sideband

35. Frequency spectrum of a Vestigial Sideband Amplitude Modulation ~ Vestigial Sideband Carrier for video Audio Signal (FM) Upper sideband Lower Side band f c -1.25 f c f c +4.5 4.5 MHz Carrier for audio

38. The total transmitted power in AM is the sum of the carrier power and the power in the sidebands. Carrier power : Sideband power: AM Power Distribution

39. Thus, at optimum operation (m = 100%), only 33% of power is used to carry information From previous equation, total current flow in AM is