1. EKT343 –Principle of Communication Engineering
PULSE MODULATION
CHAPTER 4
Part 1
EKT343 –Principle of Communication Engineering

2. EKT343 –Principle of Communication Engineering
Chapter Outline
PART 1:
Basic sampling technique
Generation and recovery
Pulse Amplitude Modulation (PAM)
Pulse Duration Modulation (PDM)
Pulse Position Modulation (PPM)
Advantages & Disadvantages
EKT343 –Principle of Communication Engineering

3. EKT343 –Principle of Communication Engineering
Sampling
To convert a signal from continuous time to discrete time, a process called sampling is used.  The value of the signal is measured at certain intervals in time. Each measurement is referred to as a sample. 
When the continuous analog signal is sampled at a frequency F, the resulting discrete signal has more frequency components than did the analog signal.  To be precise, the frequency components of the analog signal are repeated at the sample rate. 
EKT343 –Principle of Communication Engineering

4. EKT343 –Principle of Communication Engineering
Sampling
Sampling a signal: Analog → Digital conversion by
reading the value at discrete points
A process of taking samples of information signal at a rate of Nyquist’s sampling frequency.
EKT343 –Principle of Communication Engineering

5. Nyquist’s Sampling Theorem :
The original information signal can be reconstructed at the receiver
with minimal distortion if the sampling rate in the pulse modulation
system equal to or greater than twice the maximum information
signal frequency.
fs >= 2fm (max)
EKT343 –Principle of Communication Engineering

6. EKT343 –Principle of Communication Engineering
infinite bandwidth cannot be sampled.
the sampling rate must be at least 2 times the highest frequency, not the bandwidth.
EKT343 –Principle of Communication Engineering

7. EKT343 –Principle of Communication Engineering
Example 1
A complex low-pass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal?
Solution:
The bandwidth of a low-pass signal is between 0 and f, where f is the maximum frequency in the signal. Therefore, we can sample this signal at 2 times the highest frequency (200 kHz). The sampling rate is therefore 400,000 samples per second.
EKT343 –Principle of Communication Engineering

8. EKT343 –Principle of Communication Engineering
Example 2
A complex bandpass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal?
Solution :
We cannot find the minimum sampling rate in this case because we do not know where the bandwidth starts or ends. We do not know the maximum frequency in the signal.
EKT343 –Principle of Communication Engineering

9. Undersampling & Oversampling
Undersampling is essentially sampling too slowly, or sampling at a rate below the Nyquist frequency for a particular signal of interest. Undersampling leads to aliasing and the original signal cannot be properly reconstructed
Oversampling is sampling at a rate beyond twice the highest frequency component of interest in the signal and is usually desired.
Usually DESIRED??????
EKT343 –Principle of Communication Engineering

10. EKT343 –Principle of Communication Engineering
Aliasing effect
If the required condition of the sampling theorem that fs >= 2fmmax is not met, then errors will occur in the reconstruction.
When such errors arise due to undersampling, aliasing is said to occur
Undersampling: Sampling rate is too low to capture high-frequency variation
EKT343 –Principle of Communication Engineering

11. EKT343 –Principle of Communication Engineering
Aliasing effect
EKT343 –Principle of Communication Engineering

12. Example 3
For an intuitive example of the Nyquist theorem, let us sample a simple sine wave at three sampling rates:
a) fs = 2f (Nyquist rate)
b) fs = 4f (2 times the Nyquist rate),
c) fs = f (one-half the Nyquist rate).
Figure shows the sampling and the subsequent recovery of the signal.
SOLUTION:
It can be seen that sampling at the Nyquist rate can create a good approximation of the original sine wave (part a). Oversampling in part b can also create the same approximation, but it is redundant and unnecessary. Sampling below the Nyquist rate (part c) does not produce a signal that looks like the original sine wave.

13. Recovery of a sampled sine wave for different sampling rates
EKT343 –Principle of Communication Engineering

14. EKT343 –Principle of Communication Engineering
Natural Sampling
Tops of the sample pulses retain their natural shape during the sample interval.
Frequency spectrum of the sampled output is different from an ideal sample.
Amplitude of frequency components produced from narrow, finite-width sample pulses decreases for the higher harmonics
Requiring the use of frequency equalizers
EKT343 –Principle of Communication Engineering

15. EKT343 –Principle of Communication Engineering
Natural Sampling
EKT343 –Principle of Communication Engineering

16. EKT343 –Principle of Communication Engineering
Flat-top Sampling
Common used in PCM systems.
Accomplish in a sample-and-hold circuit
To periodically sample the continually changing analog input voltage & convert to a series of constant-amplitude PAM voltage levels.
The input voltage is sampled with a narrow pulse and then held relatively constant until the next sample is taken.
EKT343 –Principle of Communication Engineering

17. EKT343 –Principle of Communication Engineering
Cont’d…
Sampling process alters the frequency spectrum & introduces aperture error.
The amplitude of the sampled signal changes during the sample pulse time.
Advantages:
Introduces less aperture distortion
Can operate with a slower ADC
EKT343 –Principle of Communication Engineering

18. EKT343 –Principle of Communication Engineering
Flat-top Sampling
EKT343 –Principle of Communication Engineering

19. ANALOG PULSE MODULATION (PM)
Sampling analog information signal
Converting samples into discrete pulses
used to represent an analog signal with digital data
among the first of the pulse techniques to be utilized
Carrier signal is pulse waveform and the modulated signal is where one of the carrier signal’s characteristic (either amplitude, width or position) is changed according to information signal.
EKT343 –Principle of Communication Engineering

20. EKT343 –Principle of Communication Engineering
Pulse Amplitude Modulation (PAM)
The amplitude of pulses is varied in accordance with the information signal.
Width & position constant.
2 types –
double polarity
single polarity
2 types-single n double polarity
EKT343 –Principle of Communication Engineering

21. Natural Sampling (PAM)
A PAM signal is generated by using a pulse train, called the sampling signal (or clock signal) to operate an electronic switch or "chopper". This produces samples of the analog message signal, as shown in Figure
EKT343 –Principle of Communication Engineering

22. Flat Top Sampling (PAM)
a sample-and-hold circuit is used in conjunction with the chopper to hold the amplitude of each pulse at a constant level during the sampling time
Flat-top sampling – generation of PAM signals.
EKT343 –Principle of Communication Engineering

23. EKT343 –Principle of Communication Engineering
Cont’d
Pulse duration (τ) supposed to be very small compare to the period, Ts between 2 samples
Lets max frequency of the signal, W
If ON/OFF time of the pulse is same, frequency of the PAM pulse is
Fs >= 2 W
Ts =< 1/2W
T « Ts =< 1/2W
EKT343 –Principle of Communication Engineering

24. Transmission BW of PAM Signal
Bandwidth required for transmitter of PAM signal will be equal to maximum frequency
EKT343 –Principle of Communication Engineering

25. Advantages & Disadvantages PAM
it allows multiplexing, i.e., the sharing of the same transmission media by different sources (or users). This is because a PAM signal only occurs in slots of time, leaving the idle time for the transmission of other PAM signals.
Disadvantage:
require a larger transmission bandwidth (very large compare to its maximum frequency)
Interference of noise is maximum
Needed for varies transmission power
EKT343 –Principle of Communication Engineering

26. Pulse Density Modulation (PDM)
Sometimes called Pulse Duration Modulation/ Pulse Width Duration (PWM).
The width of pulses is varied in accordance to information signal
Amplitude & position constant.
PDM is used in a great number of applications Communications
The width of the transmitted pulse corresponds to the encoded data value
EKT343 –Principle of Communication Engineering

27. EKT343 –Principle of Communication Engineering
PDM
Immune to noise
Power Delivery
Reduce the total amount of power delivered to a load
Applications: DC Motors, Light Dimmers, Anti-Lock Breaking System
PWM is essentially a means of transmitting information in a series of pulses, where the data being transmitted is encoded on the width of the transmitted pulse
􀁺 PWM is used in reducing the total power provided to a load - average power delivered is proportional to the modulation duty cycle
EKT343 –Principle of Communication Engineering

28. EKT343 –Principle of Communication Engineering
PWM signal output is generated by comparing summation result with reference level
EKT343 –Principle of Communication Engineering

29. EKT343 –Principle of Communication Engineering
Cont’d...
EKT343 –Principle of Communication Engineering

30. Advantages & Disadvantages PDM
Noise performance is better compare to PAM.
Disadvantages:
require a larger power transmission compare to PPM
Require very large bandwidth compare to PAM
EKT343 –Principle of Communication Engineering

31. Pulse Position Modulation (PPM)
Modulation in which the temporal positions of the pulses are varied in accordance with some characteristic of the information signal.
Amplitude & width constant.
The higher the amplitude of the sample, the farther to the right the pulse is position within the prescribed time slot.
EKT343 –Principle of Communication Engineering

32. Advantages & Disadvantages PPM
The amplitude is held constant thus less noise interference.
Signal and noise separation is very easy
Due to constant pulse widths and amplitudes, transmission power for each pulse is same.
Require less power compare to PAM and PDM because of short duration pulses.
Disadvantages:
Require very large bandwidth compare to PAM.
EKT343 –Principle of Communication Engineering

33. Transmission BW of PDM/PPM Signal
PPM and PDM need a sharp rise time and fall time for pulses in order to preserve the message information.
Lets rise time, tr
From formula above, we know that transmission BW of PPM and PDM is higher than PAM
tr« Ts
EKT343 –Principle of Communication Engineering

34. Transmission BW of PAM Signal
Pulse duration (τ) supposed to be very small compare to the period, Ts between 2 samples
Lets max frequency of the signal, W
If ON/OFF time of the pulse is same, frequency of the PAM pulse is
Fs >= 2 W
Ts =< 1/2W
T « Ts =< 1/2W
EKT343 –Principle of Communication Engineering

35. EKT343 –Principle of Communication Engineering
Example 4
For PAM transmission of voice signal with W = 3kHz. Calculate BT if fs = 8 kHz and τ = 0.1 Ts
SOLUTION
EKT343 –Principle of Communication Engineering

36. EKT343 –Principle of Communication Engineering
Example 5
For the same information as in example 1, find minimum transmission BW needed for PPM and PDM. Given tr= 1% of the width of the pulse.
SOLUTION
EKT343 –Principle of Communication Engineering

37. EKT343 –Principle of Communication Engineering
Pulse Modulation
EKT343 –Principle of Communication Engineering

38. EKT343 –Principle of Communication Engineering
PAM
PDM
PPM
Relation with modulating signal
Amplitude of the pulse is proportional to amplitude of modulating signal
Width of the pulse is proportional to amplitude of modulating signal
Relative position of the pulse is proportional to amplitude of modulating signal
BW of the transmission channel
depends on width of the pulse
Depends of rise time of the pulse
Depends on rising time of the pulse
Instantaneous power
varies
Remains constant
Noise interference
High
Minimum
Complexity of the system
Complex
Simple
simple
EKT343 –Principle of Communication Engineering

39. EKT343 –Principle of Communication Engineering
PAM, PWM, PPM
EKT343 –Principle of Communication Engineering

40. Advantages & Drawbacks of Pulse Modulation
Noise immunity.
Relatively low cost digital circuitry.
Able to be time division multiplexed with other pulse modulated signal.
Storage of digital streams.
Error detection & correction
Requires greater BW to transmit & receive as compared to its analog counterpart.
Special encoding & decoding methods must be used to increased transmission rates & more difficult to be recovered.
Requires precise synchronization of clocks between Tx & Rx.
EKT343 –Principle of Communication Engineering