1. Analog Communications
Dr. M. Venu Gopala Rao
A.M.I.E.T.E, M.Tech, Ph.D(Engg)
Cert. in R.S.T ( City & Guild’s London Institute, London)
F.I.E.T.E, L.M.I.S.T.E, I.S.O.I., S.S.I., M.I.A.E.
Professor, Dept. of ECE, K L University

3. AM: The amplitude of the carrier signal is varied according to the instantaneous value of m(t)

4. FM: The frequency of the carrier signal is varied according to the instantaneous value of m(t)

6. Lectrure - 1
AMPLITUDE MODULATION

7. AM-FC (or simply AM) Introduction
Single Tone, Multi tone and Baseband signals
Time domain description
Frequency domain description
Power and Current Relations
2. Modulators
Switching Modulator
Square Law Modulator
3. Demodulators
Square Law Demodulator
Detector Demodulator
Envelope Detector
4. Virtues and Limitations of AM

8. Types of Amplitude Modulation
Non Linear AM
AM DSB FC
Amplitude Modulation
AM DSB SC
Linear AM
SSB
VSB

9. Amplitude Modulation (AM)
The Amplitude Modulation (A.M.) is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating signal.
DSB C: Double Side Band with Carrier
DSB FC: Double Side Band with Full Carrier
DSB LC: Double Side Band with Large Carrier
DSB TC: Double Side Band with Transmitted Carrier

10. Block Schematic Diagram of AM.

12. Time Domain Description:
The standard form of an A.M wave as a function of time is represented by where is constant called the amplitude sensitivity of the modulator. The term is called envelope of AM wave

13. Case1: Single Tone Modulation
Let a message signal be Time domain Representation of AM
where
Modulation index

15. Freq. domain Representation

16. Single Tone Modulation

17. AM Signal
Modulating Signal
Carrier Signal

18. Transmission Bandwidth
The AM transmission bandwidth is defined as the frequency range occupied between upper sideband and lower sideband. The AM transmission Band Width = USB – LSB = which is the twice of the message signal frequency (or bandwidth).
LSB
USB

19. AM for various Modulation Indices

20. Power Relations in AM where the total power, carrier power,
lower sideband power
higher sideband power
Carrier Signal

21. Power Relations in AM
If R = 1 ohm

22. Transmission Efficiency of AM

23. Carrier Power Vs Sideband Power

24. Ex2: In an AM transmission the carrier signal
is modulated with a message signal
(a) What is the modulation index? (b) What are the frequency components are available in AM signal? (c) Determine the Carrier power, sideband power and total power. (d) What is the efficiency of the AM system?

25. (c) Determine the Carrier power, sideband power and total power
(c) Determine the Carrier power, sideband power and total power. (d) What is the efficiency of the AM system?

26. Ex3: A 400W carrier is modulated to a depth of 75%
Ex3: A 400W carrier is modulated to a depth of 75%. Calculate the total power in the modulated wave. Also determine the total sideband power

27. Ex4: A broadcast radio transmitter radiates 10 KW, when the modulation percentage is 60. Determine the carrier, lower sideband, upper sideband and total sideband power.

28. Current Calculations
Let be un-modulated current and be the total or modulated current of an AM transmitter both being rms values. If R is the resistance,

29. ,
Ex5: The antenna current of an AM transmitter is 8 A, when only the carrier is sent, but increases to 8.93 A, when the carrier is modulated by a single sine wave. Find the percentage of modulation. Determine the antenna current when the modulation index is 0.8.
Ans: (a) Given that the carrier current and total currents are
and
The total current and carrier currents are related by or
(b) For

30. Given an Amplitude Modulated waveform as shown.
(a) Find the modulation index
(b) Find the expression of the modulated wave.
(c) Determine the total average power of the modulated signal, the carrier power, the USB power and the LSB power (assume unit load).
(d) Determine the modulation efficiency.

31. Develop Matlab codes for the following problem

32. Case2: Multitone Modulation
Let a message signal consists of three sinusoidal signals of different frequencies.

33. Case2: Multitone Modulation
The frequency domain representation Multitone modulation is expressed as

34. Case2: Multitone Modulation

35. Modulation Index for Multitone modulating signal
The total magnitude of several sinusoidal modulating signals may be represented as
By dividing by on both sides of the equation, we get
For three sinusoidal signals
and

36. Ex6: A certain transmitter radiates 9 KW power with the carrier un-modulated and KW, when the carrier is simultaneously modulated. Calculate the modulation index. If another sine wave corresponding to 40% modulation is transmitted simultaneously, determine the resultant modulation index and total radiated power.

37. Ex7: The antenna current of an AM broadcast transmitter modulated to a depth of 40% by an audio sine wave is 11 A. It increases to 12 A as a result of simultaneous modulation by another audio sine wave. What is the modulation index due to the second wave?

38. Case3: Baseband Signal Modulation
Let a message signal is band limited to ‘W’ Hz
Then AM signal is represented in time domain by
Frequency domain representation of AM wave is given by

39. AM (Baseband Signal)

40. Transmission Bandwidth = USB-LSB = 2W

41. Conditions for AM
The amplitude of is always less than unity. That is for all t.
If envelope distortion is introduced.
2. The carrier frequency is much higher than that of message frequency W ,
Otherwise envelope cannot be visualized satisfactorily at the receiver.

42. End