1. MODULATION

2. 1. What is modulation?

3. 1. What is modulation?
Modulation is the process of putting information onto a high frequency carrier for transmission (frequency translation).

4. Once this information is received, the low frequency information must be removed from the high frequency carrier. This process is known as “ Demodulation”.

6. 2. What are the reasons for modulation?

7. 2. What are the reasons for modulation?
1. Frequency division multiplexing (To support multiple transmissions via a single channel)
To avoid interference

8. f
M1(f)
M2(f)
M(f) f1 f2
Multiplexed signal +

9. 2. Practicality of Antennas
Transmitting very low frequencies require antennas with miles in wavelength

10. 3.What are the Different of Modulation Methods?

11. 3. What are the Different of Modulation Methods?
1. Analogue modulation- The modulating signal and carrier both are analogue signals
Examples: Amplitude Modulation (AM) , Frequency Modulation (FM) , Phase Modulation (PM)
2. Pulse modulation- The modulating signal is an analogue signal but Carrier is a train of pulses
Examples : Pulse amplitude modulation (PAM), Pulse width modulation (PWM), Pulse position modulation (PPM)

12. 3.What are the Different of Modulation Methods?
3. Digital to Analogue modulation- The modulating signal is a digital signal , but the carrier is an analogue signal.
Examples: Amplitude Shift Keying (ASK), FSK, Phase Shift Keying (PSK)
4. Digital modulation -
Examples: Pulse Code Modulation, Delta Modulation,Adaptive Delta Modulation

13. Topics discussed in this section:
ANALOG AND DIGITAL
Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us.
Topics discussed in this section:
Amplitude Modulation Frequency Modulation Phase Modulation

14. Figure Types of analog-to-analog modulation

15. Figure Amplitude modulation

16. The total bandwidth required for AM can be determined
Note
The total bandwidth required for AM can be determined
from the bandwidth of the audio signal: BAM = 2B.

17. Figure AM band allocation

18. Note
The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.

19. Figure Frequency modulation

20. Figure FM band allocation

21. Figure Phase modulation

22. Note
The total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal: BPM = 2(1 + β)B.

23. 4. What are the Basic Types of Analogue Modulation Methods ?

24. 4. What are the Basic Types of Analogue Modulation Methods ?
Consider the carrier signal below:
sc(t ) = Ac(t) cos( 2fc t +  )
1. Changing of the carrier amplitude Ac(t) produces
Amplitude Modulation signal (AM)
2. Changing of the carrier frequency fc produces
Frequency Modulation signal (FM)
3. Changing of the carrier phase  produces
Phase Modulation signal (PM)

25. Analogue Modulation Methods

27. 5. What are the different Forms of Amplitude Modulation ?

28. 5. What are the different Forms of Amplitude Modulation ?
1. Conventional Amplitude Modulation (DSB-LC)
(Alternatively known as Full AM or Double Sideband with Large carrier (DSB-LC) modulation
2. Double Side Band Suppressed Carrier (DSB-SC) modulation
3. Single Sideband (SSB) modulation
4. Vestigial Sideband (VSB) modulation

29. Conventional Amplitude Modulation (Full AM)

30. 6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)

31. 6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)
1 The carrier signal is
2 In the same way, a modulating signal (information signal) can also be expressed as

32. 3 The amplitude-modulated wave can be expressed as
4 By substitution
5 The modulation index.

33. 6 Therefore The full AM signal may be written as

34. 7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth

35. 7. Draw the Frequency Spectrum of the above AM signal and calculate the BandwidthfC
fc+fm
fc-fm
2fm

36. 8. Draw Frequency Spectrum for a complex input signal with AM

37. 8. Draw Frequency Spectrum for a complex input signal with AMfc
fc-fm
fc+fm

38. Frequency Spectrum of an AM signal
The frequency spectrum of AM waveform contains three parts:
1. A component at the carrier frequency fc
2. An upper side band (USB), whose highest frequency component is at fc+fm
3. A lower side band (LSB), whose highest frequency component is at fc-fm
The bandwidth of the modulated waveform is twice the information signal bandwidth.

39. Because of the two side bands in the frequency spectrum its often called Double Sideband with Large Carrier.(DSB-LC)
The information in the base band (information) signal is duplicated in the LSB and USB and the carrier conveys no information.

40. Example
We have an audio signal with a bandwidth of 5 KHz. What is the bandwidth needed if we modulate the signal using AM?

41. Example
We have an audio signal with a bandwidth of 5 KHz. What is the bandwidth needed if we modulate the signal using AM?
Solution
An AM signal requires twice the bandwidth of the original signal:
BW = 2 x 5 KHz = 10 KHz

42. AM Radio Band

43. 9. What is the significance of modulation index ?
Modulation Index (m)
m is merely defined as a parameter, which determines the amount 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.

44. 9. What is the significance of modulation index ?
Modulation Index (m)

46. 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.

47. 10. Calculate the power efficiency of AM signals
Power distribution in full AM

48. 10. Calculate the power efficiency of AM signals
The ratio of useful power, power efficiency :
In terms of power efficiency, for m=1 modulation, only 33% power efficiency is achieved which tells us that only one-third of the transmitted power carries the useful information.

49. Double Side Band Suppressed Carrier (DSB-SC) Modulation
The carrier component in full AM or DSB-LC does not convey any information. Hence it may be removed or suppressed during the modulation process to attain higher power efficiency.
The trade off of achieving a higher power efficiency using DSB-SC is at the expense of requiring a complex and expensive receiver due to the absence of carrier in order to maintain transmitter/receiver synchronization.

50. 11. Derive the Frequency Spectrum for Double Sideband Suppressed Carrier Modulation (DSB-SC)
1 Consider the carrier
2 modulated by a single sinusoidal signal
3 The modulated signal is simply the product of these two

51. Frequency Spectrum of a DSB-SC AM SignalX
Frequency Spectrum of a DSB-SC AM Signal fc
fc-fm
fc+fm

52. All the transmitted power is contained in the two sidebands (no carrier present).
The bandwidth is twice the modulating signal bandwidth.
USB displays the positive components of sm(t) and LSB displays the negative components of sm(t).

53. Generation and Detection of DSB-SC
The simplest method of generating a DSB-SC signal is merely to filter out the carrier portion of a full AM (or DSB-LC) waveform.
Given carrier reference, modulation and demodulation (detection) can be implemented using product devices or balanced modulators.

54. BALANCED MODULATOR S1(t) AM Modulator 1 Sm(t) S(t) Carrier DSB-SC
Accos(wct)
S2(t)
S1(t)
S(t)
DSB-SC

55. The two modulators are identical except for the sign reversal of the input to one of them. Thus,

56. COHERENT (SYNCHRONOUS) DETECTOR OR DSB-SC (PRODUCT DETECTOR)
DSB-SC Signal s(t)
Local Oscillator
LPF X
Coswct
v(t)
vo(t)
Since the carrier is suppressed the envelope no longer represents the modulating signal and hence envelope detector which is of the non-coherent type cannot be used.

58. It is necessary to have synchronization in both frequency and phase between the transmitter (modulator) & receiver (demodulator), when DSB-SC modulation ,which is of the coherent type, is used.
Both phase and frequency must be known to demodulate DSB-SC waveforms.

59. LACK OF PHASE SYNCHRONISATION
Let the received DSB-SC signal be
if  is unknown,
Output of LPF

60. But we want just
Due to lack of phase synchronization, we will see that the wanted signal at the output of LPF will be attenuated by an amount of cosq.
In other words, phase error causes an attenuation of the output signal proportional to the cosine of the phase error.
The worst scenario is when q=p/2, which will give rise to zero or no output at the output of the LPF.

61. LACK OF FREQUENCY SYNCHRONISATION
Suppose that the local oscillator is not stable at fc but at fc+D f, then
Output of LPF
Thus, the recovered baseband information signal will vary sinusoidal according to cos D wt

62. This problem can be overcome by adding an extra synchronization circuitry which is required to detect q and D wt and by providing the carrier signal to the receiver.
A synchronizer is introduced to curb the synchronization problem exhibited in a coherent system.
Let the baseband signal be
Received DSB-SC signal

63. Mathematical analysis of the synchronizer is shown below:
SYNCHRONISER
( )2
PLL
BPF
 2
Mathematical analysis of the synchronizer is shown below:
Output of BPF

64. DISADVANTAGE OF USING COHERENT SYSTEMS
Output of frequency divider
where k is a constant of proportionality.
DISADVANTAGE OF USING COHERENT SYSTEMS
The frequency and phase of the local oscillator signal must be very precise which is very difficult to achieve.
It requires additional circuitry such as synchronizer circuit and hence the cost is higher.

65. Single Side Band Modulation (SSB)
How to generate SSB signal?
Generate DSB-SC signal
Band-pass filter to pass only one of the sideband
and suppress the other.
For the generation of an SSB modulated signal
to be possible, the message spectrum must have
an energy gap centered at the origin.
Single Side Band Modulation (SSB)

66. Example of signal with -300 Hz ~ 300 Hz energy gap
Voice : A band of 300 to 3100 Hz gives good articulation
Also required for SSB modulation is a highly selective filter

67. Vestigial Side Band Modulation (VSB)
Instead of transmitting only one sideband as SSB, VSB modulation transmits a partially suppressed sideband and a vestige of the other sideband.
Vestigial Side Band Modulation (VSB)

68. Comparison of Amplitude Modulation methods

69. Comparison of Amplitude Modulation methods
Full AM (or DSB-LC)
Sidebands are transmitted in full with the carrier.
Simple to demodulate / detect
Poor power efficiency
Wide bandwidth ( twice the bandwidth of the information signal)
Used in commercial AM radio broadcasting, one transmitter and many receivers.

70. Comparison of Amplitude Modulation methods
DSB-SC
Less transmitted power than full AM and all the transmitted power is useful.
Requires a coherent carrier at the receiver; This results in increased complexity in the detector(i.e. synchroniser)
Suited for point to point communication involving one transmitter and one receiver which would justify the use of increased receiver complexity.
Comparison of Amplitude Modulation methods

71. Comparison of Amplitude Modulation methods
SSB
Good bandwidth utilization (message signal bandwidth = modulated signal bandwidth)
Good power efficiency
Demodulation is harder as compares to full AM; Exact filter design and coherent demodulation are required
Preferred in long distance transmission of voice signals
Comparison of Amplitude Modulation methods

72. Comparison of Amplitude Modulation methods
VSB
Offers a compromise between SSB and DSB-SC
VSB is standard for transmission of TV and similar signals
Bandwidth saving can be significant if modulating signals are of large bandwidth as in TV and wide band data signals.
For example with TV the bandwidth of the modulating signal can extend up to 5.5MHz; with full AM the bandwidth required is 11MHz
Comparison of Amplitude Modulation methods