1. Analog Communications

2. Overview Communication systems Analog Modulation Digital ModulationAM FM
Digital Modulation
ASK
FSK
Modems

3. Communication systems
Digital
Analog
The block diagram on the top shows the blocks common to all communication systems

4. Remember the components of a communications system:
Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating
Transmitter: The device that sends the transduced signal
Transmission channel: The physical medium on which the signal is carried
Receiver: The device that recovers the transmitted signal from the channel
Output transducer: The device that converts the received signal back into a useful quantity

5. Analog Modulation
The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel
The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer
For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of Hz
The “hello” wave is a baseband signal:

6. Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation
Remember the radio spectrum:
For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band)
The FM radio system must operate with frequencies in the range of MHz (VHF band)
AM radio
FM radio/TV

7. Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorily
This process is accomplished by a device called a modulator
The transmitter block in any communications system contains the modulator device
The receiver block in any communications system contains the demodulator device
The modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrum
For example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum
For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum)
The demodulator extracts the original baseband signal from the received modulated signal
To Summarize:
Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal
Modulation is done to bring information signals up to the Radio Frequency (or higher) signal

8. Basic analog communications system
Baseband signal (electrical signal)
EM waves (modulated signal)
Transmitter
Input transducer
Transmission Channel
Modulator
EM waves (modulated signal)
Carrier
Baseband signal (electrical signal)
Receiver
Output transducer
Demodulator

9. Types of Analog Modulation
Amplitude Modulation (AM)
Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant
Frequency Modulation (FM)
Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant
Phase Modulation (PM)
Another form of analog modulation technique which we will not discuss

10. Amplitude Modulation

11. Amplitude Modulation Uses a higher frequency carrier
Most efficient use of frequency
Time and Frequency Domain
Susceptible to Noise

12. Sine Wave Peak Amplitude (A) Frequency (f) Phase () General Sine wave
maximum strength of signal
volts
Frequency (f)
Rate of change of signal
Hertz (Hz) or cycles per second
Period = time for one repetition (T)
T = 1/f
Phase ()
Relative position in time, from 0-2*pi
General Sine wave

13. S(t) = Ac(1 + Ka m(t))cos(2fct) Where
Ka is a constant.
m(t) is the baseband message. For an audio signal, the spectrum of m(t) would typically be in the range of 300 Hz - 3 kHz.
fc is the carrier frequency
m(t) = Am cos(2fmt)
Where fc >> fm.

14. Varying Sine Waves

15. Higher Frequency Carrier
Signal
time
Carrier
time

16. Higher Frequency Carrier
Power Spectral Density
watts
Carrier
Signal
frequency
Baseband

17. Time Domain
Signal
time
Carrier
time

18. Time Domain Continued
time

19. Amplitude Modulation Carrier wave Baseband signal Modulated wave
Amplitude varying-frequency constant

20. Some AM Waveforms

21. Detection of Signal
time
time

22. Frequency Domain Unmodulated Modulated watts Carrier Signal frequency
Baseband
watts
Modulated
Carrier
Signal
frequency
Baseband
Baseband

23. The AM Waveform
The three components that form the AM waveform are listed below:
1. The lower-side frequency (fc - fi)
2. The carrier frequency (fc)
3. The upper-side frequency (fc + fi)
If a radio signal of frequency with1000KHz is mixed with an audio signal of 1 KHz the output will be
A signal at 1000 KHz (Carrier wave)
A signal at 1001 KHz (upper sideband)
A signal at 999 KHz (lower sideband)

24. Upper and Lower Sidebands

25. Upper and Lower Sidebands
In most systems the intelligence signal is a complex waveform containing components from roughly 200Hz to 3KHz.
If this complex waveform is used to modulate the carrier there would be a whole band of side frequencies.
The band of frequencies above the carrier is term the upper sideband.
The band of frequencies below the carrier is called the lower sideband.

26. Example
A 1.4MHz carrier is modulated by a music signal that has frequency components from 20Hz to 10kHz. Determine the range of frequencies generated for the upper and lower sidebands.

27. Answer

28. Percentage Modulation Determination

30. Example 2
An unmodulated carrier is 300Vp-p. Calculate %m when its maximum p-p value reaches:
a. 400
b. 500
c. 600

31. Answers
33.3%
b. 66.7%
c. 100%

32. Points to be Remembered
Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant
The function of the carrier in AM is simply to provide a signal to heterodyne (mix) with the modulated audio, to convert all the AF components to a higher frequency.
The bandwidth of an AM signal is equal to twice the highest freq.
[ In commercial AM broadcast tx’s the freq of the modulating audio is permitted to be as high as 4.5 KHz, and since a double side system is used , commercial stations therefore have a bandwidth of 9 KHz]
The bandwidth does not depend on the power of the modulating signal.