Amplitude modulation, Generation of AM signals

1. 1
TOPIC
• Baseband and Carrier Comm.
• Amplitude modulation
• Quadrature amplitude modulation
• Single side band Modulation
• Vestigial side band Modulation

2. 3/18/2014
Dr. Sarmad Ullah Khan
Baseband versus Carrier Communication
• Modulation is a process that moves signal into a specific
frequency band
• The bandwidth B represents a measure of frequency range.
• It is typically measured in Hz
.
• The bandwidth of a signal indicates the frequency range in which the signal‘s
Fourier transform has a power above a certain threshold (typically
half of the maximum power) 5
2

3. Dr. Sarmad Ullah Khan
Baseband versus Carrier Communication
• Communication systems that do not use modulation are
called baseband communication
• Communication systems that use modulation are called
carrier communication
• Baseband is original message frequency band
• In telephony, baseband is audio band (0 – 3.5 kHz)
• In NTSC television, video baseband is 0 – 4.3 MHz
6

4. • Baseband Communication
Message signals are directly transmitted without any
modulation
Dedicated user channels are assigned to each long distance
communication
Baseband signals have overlapping bands
Severe interference
Waste of channel resources
Modulation and shifting to non overlapping bands save
channel resources

5. • Carrier Communication
Modulation techniques is used to shift the frequency
spectrum of message signal
Modulation changes one of the basic parameter of
carrier signal (Amplitude, Frequency, Phase)
Carrier signal is a sinusoidal signal of high frequency
fc
Parameter variation is proportional to message signal
m(t)
Amplitude modulation is linear
Frequency and Phase modulations are non linears
PAM, PWM, PPM, PCM and DM are baseband signals

6. Amplitude Modulation:

7. Amplitude Modulation (DSB)
•Amplitude modulation (AM) varies the amplitude of a carrier
signal Acos(wct c ) according to a modulating signal m(t).
• The modulated signal is m(t)cos(wct)
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8. Amplitude Modulation (DSB) cont…
Frequency-Shifting Property of Fourier transform:
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9. Amplitude Modulation (DSB) cont…
•This type of modulation shifts the spectrum of m(t) to the carrier
frequency.
If
2
c
c c
m(t)cos w t 
1
M(w  w )  M (w  w )
m(t)  M(w)
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10. Amplitude Modulation (DSB) cont…
•This modulation shifts the frequency spectrum to the right and the
left by wc
• The modulated signal is composed of two parts, above wc and
below wc
– the upper sideband (USB) containing the frequencies |w| > | wc|
– the lower sideband (LSB) containing the frequencies |w| < | wc|
•The modulated signal in this scheme does not have a discrete
component of the carrier frequency wc for this reason this is called
double-sideband suppressed carrier (DSB-SC) modulation
2
1
0
c
c c
m(t)cos w t 
1
M(w  w )  M (w  w )

11. 9
Amplitude Modulation (DSB) cont…
B Vs wc
•If the bandwidth of the original signal m(t) is 2 B, then the
bandwidth of the modulated signal will be 4B, consisting of
– the upper sideband (USB) containing the frequencies |w| > |wc|
– the lower sideband (LSB) containing the frequencies |w| < |wc |
To avoid overlap of the two spectral parts, wc > 2B must be fulfilled
(if c < 2B , the information of m(t) will be partly lost in the process of
modulation)

12. Modulation / Demodulation
Modulation
12
Demodulation

13. Demodulation
•The process of receiving the original signal from the modulated
signal is called demodulation.
•Demodulation is similar to modulation and can be performed by
multiplying the modulated signal again with the carrier signal cos(wc
t)
The resulting signal
2
1
2
c
m(t) m(t)cos(2w t)
c
e(t)  m(t)cos w t 
It has the Fourier transform
E(w)  1
2 M (w)  1
4M (w  2wc
)  M (w  2wc
)
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14. Demodulation
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15. Amplitude Modulation (AM)
For DSB-SC a receiver must generate a carrier in frequency and phase synchronism
with the carrier at the transmitter.
Problem:
Transmitter and receiver may be located thousands of miles away, this call for a
sophisticated receiver and could be costly.
Solution:
Transmit a carrier Acoswct along with the modulated signal m(t)coswct so no
need to generate a carrier at the receiver.
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16. Amplitude Modulation (AM)
AM
This type of modulation is called amplitude modulation and denoted by  (t)
and is given by:
It has the Fourier spectrum
c
The spectrum of AM
(t) is the same as m(t)coswct plus two additional impulses at w
are identical with
•DSB-SC signal m(t)coswct and AM signal
A+m(t) as modulating signal instead of m(t)
•To sketch  (t) ,we sketch A+m(t) & -(A+m(t) ) and fill in between the carrier
AM
frequency.
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17. Amplitude Modulation (AM)
As we sketch A+m(t) & -(A+m(t) ):
Consider two cases:
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A m(t)  0 and A m(t)  0

18. Amplitude Modulation (AM)
For simple envelope detection for AM signal is:
A = 0, also satisfies the condition. In this case there is no need to add carrier,
because the envelope of DSB-SC signal m(t)coswct is m(t)
Such a DSB-SC signal can be detected by envelope detection
Assume for all t
Let mp is the peak amplitude (positive or negative) of m(t)
Then
Hence the condition is equivalent to
Thus the minimum carrier amplitude required for the envelope detection is mp
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19. Amplitude Modulation (AM)
We define the modulation index  as:
A = carrier amplitude
mp = constant ofm(t)
As A is the carrier amplitude and there is no
upper bound on A,
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This is the condition for the viability of demodulation of Am signal by an
envelope detector

20. Amplitude Modulation (AM)
Sideband and carrier power:
There is a disadvantage of envelope detection in terms of power waste, as the
carrier term does not contain any information
The carrier power Pc is given by
The sideband power Ps is given
by
Hence the power efficiency  is given by:
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21. Amplitude Modulation (AM)
For the special case of tone modulation:
m(t)  Acos wmt and
Hence
With condition
Thus under best condition only one third of the transmitted power is used for
carrying message, for practical signals the efficiency is even worst
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22. Generation of AM signals
• Am signals can be generated by any DSB-SC modulators.
• The input should be A + m(t) instead of just m(t).
•The modulating circuit do not have to be balanced because there is no need to
suppress the carrier
Switching action is provided by a single diode
and controlled by c cos wctwith
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23. Generation of AM signals
infect
The diode opens and short periodically with
multiplying the input signal by w(t).
bb/
23
The voltage across is:

24. Demodulation of AM Signals
The AM signal can be demodulated coherently by a locally generated carrier. E.g.
A m(t)coswctcoswct No benefit of sending carrier on the channel
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There are two well known methods of demodulation of AM signals:
1) Rectifier detection 2) Envelope detection
Rectifier detector:
AM signal is applied to a diode and resistor circuit, the negative part of the the
AM wave will be suppressed.
The output across the resistor is the half wave rectified version of the AM signal
means multiplying AM with w(t).

25. 49
Rectifier Detector
c
v  
A m(t)cosw t

w(t)
R
 
 5
3 c
c
c
c  2
 A m(t)cosw t
1

2 cosw t 
1
cos3w t 
1
cos5w t ...

 

1
A  m(t) otherTerms

The rectified output VR

26. 50
Rectifier Detector (cont…)

27. 51
Envelope Detector
In an envelope detector, the output follows the envelope of the modulated signal.
The following circuit act as an envelope detector:
•During the positive cycle of the input signal, the diode conducts and the
capacitor C charges up to the peak voltage of the input signal.
•When input signal falls below this peak value, the diode is cut off. (because the
diode voltage which is nearly the peak voltage is greater than the input signal
voltage causing the diode to open ).
•At this stage the capacitor discharge at the slew rate (with a time constant RC)
• during the next positive cycle the process repeats.

28. 52
Envelope Detector (cont…)
During each positive cycle the capacitor charges up to the peak voltage of the
input signal and then decays slowly until the next positive cycle.
This behavior of the capacitor makes output voltage Vc(t) follow the envelope of
the input signal.
Capacitor discharges during each positive peaks causes a ripple signal of
frequency wc at the output

29. Envelope Detector (cont…)
The ripple can be reduced by increasing the time constant RC so the capacitor
discharges very little between positive peaks of the input signals
Making RC too large, makes capacitor voltage impossible to follow the envelope.
Conditions:
RC should be large compared to 1/wc, but should be small compared to
Where B is the highest frequency in m(t)
1
2B
Also requires a condition which is necessary for well defined envelope.
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30. Envelope Detector (cont…)
The envelope detector output is with a ripple of frequency wc
The DC term A can be blocked by a capacitor or a simple RC high pass filter, and
the ripple may be reduced further by another low-pass RC filter.
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31. Quadrature Amplitude Modulation
The DSB signals of AM require twice the bandwidth required for the baseband
signal!
Idea: Try to send two signals m1(t) and m2(t) simultaneously by modulating them
with two carrier signals of same frequency but shifted in phase by –/2
The combined signal is
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m1(t)  m2 (t)  m1(t)cos wct  m2 (t)sin wct

32. Quadrature Amplitude Modulation (cont…)
Both modulated signals occupy the same band
•At the receiver the two baseband signals can be separated by using a second
carrier that is shifted in phase by –/2
•The first signal m1(t) can be detected by a multiplication with 2cos(ct) followed
by a low-pass filter
The second signal x2(t) can be detected accordingly by a multiplication with
sin(ct) followed by a low-pass filter
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33. Quadrature Amplitude Modulation
(cont…)
•Thus, two baseband signals, each of bandwidth B, can be simultaneously
transmitted over a channel with bandwidth 2B
•This principle is called quadrature amplitude modulation (QAM), because
the carrier frequencies are in phase quadrature.
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34. Bandwidth Efficient Amplitude
Modulation
• DSB spectrum has two sidebands (USB, LSB)
• Both carry complete information
• Bandwidth requirement is 2B
• How to improve spectral efficiency?
–Utilize spectral redundancy
– remove spectral redundancy
• Single Sideband (SSB) removes either LSB or USB
• Quadrature amplitude modulation (QAM) utilize

35. • Single Sideband (SSB) Modulation
–LSB or USB can be suppressed by band pass filter
–A scheme in which only one sideband is transmitted is known as single-
sideband ( SSB) transmission
–In SSB transmission the required bandwidth is half compared to DSB signal
–An SSB signal can be coherently (synchronously) demodulated. E.g
–For example multiplying USB signal by coswct shifts its spectrum to the left
and right by wc

36. Amplitude Modulation (Single Sideband SSB)
• The DSB spectrum has two sidebands: USB and LSB
•Both USB and LSB contain complete information of the baseband
signal.
•A scheme in which only one sideband is transmitted is known as
single-sideband ( SSB) transmission.
•In SSB transmission the required bandwidth is half compared to DSB
signal.
• An SSB signal can be coherently (synchronously) demodulated. E.g.
For example multiplying USB signal by cos wct shifts its spectrum to
the left and right by wc
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37. Single Sideband SSB (cont..)
Low pass filtering will give the required baseband signal at the
receiver.
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38. 38
Generation of SSB Signals
Two methods are generally used to generate SSB signals.
1) Sharp cutoff filters
2) Phase shifting networks
Selective Filtering Method:
•In this method the DSB-SC signal is passed through a sharp cutoff
filter to eliminate the undesired sideband.
•To obtain USB , the filter should pass all components above wc,
attenuated and completely suppress all components below wc
•Such an operation requires an ideal filter that is practically not
possible.

39. Generation of SSB Signals
•This method of generating SSB signal can be used when there is
some separation between the passband and stopband.
• In some application this can be achieved e.g. voice signals
Voice signals spectrum shows little power content at the
origin. Thus filtering the unwanted sideband is relatively
easy.
Tests have shown that frequency components
below 300Hz are not important.
600Hz transition region around the cutoff
frequency wc , makes filtering easy and
minimize the channel interference
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40. Generation of SSB Signals(cont…)
Phase-Shift Method:
The basis of this method is the following equation
SSB (t)  m(t)cos wct  mh (t)sin wct
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41. Generation of SSB Signals(cont…)
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42. Amplitude Modulation: Vestigial
Sideband (VSB)
• VSB is a compromise between DSB and SSB
• It combines the advantages of DSB and SSB
while avoid disadvantages at small cost
• Its generation is relatively easy and bandwidth
requirement is 25% greater than SSB

43. 3/18/2014
Amplitude Modulation: Vestigial
Sideband (VSB)
• If vestigial shaping filter produce VBS from DSB is
Hi(f), Its spectrum will be
• VBS filter allows transmission of one sideband but
suppress other side band gradually, NOT completely
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44. 3/18/2014
Amplitude Modulation: Vestigial
Sideband (VSB)
• For demodulation,
85
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