Amplitude modulation, Generation of AM signals

Amplitude modulation, Generation of AM signals

1. 1
TOPIC
•Basebandand Carrier Comm.
•Amplitudemodulation
•Quadrature amplitudemodulation
•Single side bandModulation
•Vestigial side bandModulation

2. 3/18/2014Dr. Sarmad UllahKhan
Baseband versus CarrierCommunication
•Modulation is a process that moves signal into a specific
frequencyband
•The bandwidth B represents a measure of frequency range.
•It is typically measured in Hz
.
•Thebandwidthofasignalindicatesthefrequencyrangeinwhichthesignal‘s
Fouriertransformhasapoweraboveacertainthreshold(typically
halfofthemaximumpower)
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2

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

4. •BasebandCommunication
Message signals are directly transmitted without any
modulation
Dedicated user channels are assigned to each long distance
communication
Baseband signals have overlappingbands
Severeinterference
Waste of channelresources
Modulation and shifting to non overlapping bands save
channelresources

5. •CarrierCommunication
Modulation techniques is used to shift the frequency
spectrum of messagesignal
Modulation changes one of the basic parameter of
carrier signal (Amplitude, Frequency,Phase)
Carrier signal is a sinusoidal signal of high frequency
f
c
Parameter variation is proportional to message signal
m(t)
Amplitude modulation islinear
Frequency and Phase modulations are nonlinears
PAM, PWM, PPM, PCM and DM are basebandsignals

6. Amplitude Modulation:

7. Amplitude Modulation(DSB)
•Amplitude modulation (AM) varies the amplitude of a carrier
signalAcos(w
ct
c)according to a modulating signalm(t).
•The modulatedsignalism(t)cos(w
ct)
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8. Amplitude Modulation (DSB)cont…
Frequency-Shifting Property of Fouriertransform:
8

9. Amplitude Modulation (DSB)cont…
•This type of modulation shifts the spectrum of m(t) to the carrier
frequency.
If
2
cc c
m(t)coswt
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
leftbyw
c
•The modulated signal is composed of two parts, above w
c and
beloww
c
–the upper sideband (USB) containing the frequencies |w| > | w
c|
–the lower sideband (LSB) containingthefrequencies|w| < | w
c|
•The modulated signal in this scheme does not have a discrete
component of the carrier frequency w
c for this reason this is called
double-sideband suppressed carrier (DSB-SC)modulation
2
1
0
cc c
m(t)coswt
1
M(ww)M(ww)

11. 9
Amplitude Modulation (DSB)cont…
BVsw
c
•If the bandwidth of the original signal m(t) is 2B, then the
bandwidth of the modulated signal will be 4B, consistingof
–the upper sideband (USB) containing the frequencies |w| > |w
c|
–the lower sideband (LSB) containingthefrequencies|w| < |w
c|
To avoid overlap of the two spectralparts,wc> 2B must befulfilled
(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 calleddemodulation.
•Demodulation is similar to modulation and can be performed by
multiplyingthe modulated signal againwith the carriersignalcos(w
ct)
The resulting signal
2
1
2
cm(t)m(t)cos(2wt)
c
e(t) m(t)cosw t
It has the Fouriertransform
E(w)1
2
M(w)1
4
M(w2w
c)M(w2w
c)
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14. Demodulation
14

15. Amplitude Modulation(AM)
For DSB-SC a receiver must generate a carrier in frequency and phase synchronism
with the carrier at thetransmitter.
Problem:
Transmitter and receiver may be located thousands of miles away, this call for a
sophisticated receiver and could becostly.
Solution:
Transmit a carrier Acoswct along with the modulated signal m(t)coswct so no
need to generate a carrier at thereceiver.
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16. Amplitude Modulation(AM)
AMThistype of modulationiscalledamplitude modulationanddenotedby (t)
and is givenby:
It has the Fourierspectrum
c
The spectrumof
AM (t) isthe same asm(t)coswctplustwoadditionalimpulsesatw
are identicalwith•DSB-SC signal m(t)coswct and AMsignal
A+m(t) as modulating signal instead ofm(t)
•Tosketch(t) ,we sketch A+m(t) & -(A+m(t) ) and fill in between thecarrier
AM
frequency.
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17. Amplitude Modulation(AM)
As we sketch A+m(t) & -(A+m(t)):
Consider twocases:
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Am(t)0and
Am(t)0

18. Amplitude Modulation(AM)
For simple envelope detection for AM signalis:
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 ism(t)
Such a DSB-SC signal can be detected by envelopedetection
Assume for all t
Let mp is the peak amplitude (positive or negative) of m(t)
Then
Hence the condition is equivalentto
Thus the minimum carrier amplitude required for the envelope detection ismp
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19. Amplitude Modulation(AM)
We define the modulationindexas:
A = carrieramplitude
mp = constant ofm(t)
As A is the carrier amplitude and there is no
upper bound onA,
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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 carrierpower:
There is a disadvantage of envelope detection in terms of power waste, as the
carrier term does not contain anyinformation
The carrier power Pc is givenby
The sideband power Ps is given
by
Hence thepowerefficiencyis givenby:
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21. Amplitude Modulation(AM)
For the special case of tonemodulation:
m(t)Acosw
mt 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 evenworst
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22. Generation of AMsignals
•Am signals can be generated by any DSB-SCmodulators.
•The input should be A + m(t) instead of justm(t).
•The modulating circuit do not have to be balanced because there is no need to
suppress thecarrier
Switching action is provided by a singlediode
and controlled byc cos w
ctwith
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23. Generation of AMsignals
infectThe diode opens and short periodically with
multiplying the input signal byw(t).
bb
/
23
The voltageacross is:

24. Demodulation of AMSignals
The AM signal can be demodulated coherently by a locally generated carrier.E.g.
Am(t)cosw
ctcosw
ct No benefit of sending carrier on thechannel
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There are two well known methods of demodulation of AMsignals:
1)Rectifierdetection2)Envelopedetection
Rectifierdetector:
AM signal is applied to a diode and resistor circuit, the negative part of the the
AM wave will besuppressed.
The output across the resistor is the half wave rectified version of the AM signal
means multiplying AM withw(t).

25. 49
RectifierDetector
c
vAm(t)coswtw(t)
R
  53 cccc 
2
Am(t)coswt
1

2
coswt
1
cos3wt
1
cos5wt...


 

1
A m(t)otherTerms

The rectified outputVR

26. 50
RectifierDetector(cont…)

27. 51
EnvelopeDetector
In an envelope detector, the output follows the envelope of the modulated signal.
The following circuit act as an envelopedetector:
•During the positive cycle of the input signal, the diode conducts and the
capacitor C charges up to the peak voltage of the inputsignal.
•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 constantRC)
•during the next positive cycle the processrepeats.

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 positivecycle.
This behavior of the capacitor makes output voltage Vc(t) follow the envelope of
the inputsignal.
Capacitor discharges during each positive peaks causes a ripple signal of
frequency wc at theoutput

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 inputsignals
Making RC too large, makes capacitor voltage impossible to follow theenvelope.
Conditions:
RC should be large compared to 1/wc, but should be small compared to
Where B is the highest frequency inm(t)
1
2B
Alsorequires a condition which is necessary for well definedenvelope.
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30. Envelope Detector(cont…)
The envelope detectoroutputis with a ripple of frequencywc
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 RCfilter.
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31. Quadrature AmplitudeModulation
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 signalis
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m
1(t)m
2(t)m
1(t)cosw
ctm
2(t)sinw
ct

32. Quadrature Amplitude Modulation(cont…)
Both modulated signals occupy the sameband
•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-passfilter
The second signal x2(t) can be detected accordingly by a multiplication with
sin(ct) followed by a low-passfilter
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33. Quadrature AmplitudeModulation
(cont…)
•Thus, two baseband signals, each of bandwidth B, can be simultaneously
transmitted over a channel with bandwidth2B
•This principle is called quadrature amplitude modulation (QAM), because
the carrier frequencies are in phasequadrature.
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34. Bandwidth EfficientAmplitude
Modulation
•DSB spectrum has two sidebands (USB,LSB)
•Both carry completeinformation
•Bandwidth requirement is2B
•How to improve spectral efficiency?
–Utilize spectralredundancy
–remove spectralredundancy
•Single Sideband (SSB) removes either LSB orUSB
•Quadratureamplitude modulation (QAM)utilize

35. •SingleSideband(SSB)Modulation
–LSBorUSBcanbesuppressedbybandpassfilter
–Aschemeinwhichonlyonesidebandistransmittedisknownassingle-
sideband(SSB)transmission
–InSSBtransmissiontherequiredbandwidthishalfcomparedtoDSBsignal
–AnSSBsignalcanbecoherently(synchronously)demodulated.E.g
–ForexamplemultiplyingUSBsignalbycosw
ctshiftsitsspectrumtotheleft
andrightbyw
c

36. Amplitude Modulation (Single SidebandSSB)
•The DSB spectrum has two sidebands: USB andLSB
•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 USBsignalbycos w
ct shifts its spectrum to
the left and right bywc
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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 SSBSignals
Two methods are generally used to generate SSB signals.
1)Sharp cutofffilters
2)Phase shiftingnetworks
Selective FilteringMethod:
•In this method the DSB-SC signal is passed through a sharp cutoff
filter to eliminate the undesiredsideband.
•To obtain USB , the filter should pass all components above wc,
attenuated and completely suppress all components belowwc
•Such an operation requires an ideal filter that is practically not
possible.

39. Generation of SSBSignals
•This method of generating SSB signal can be used when there is
some separation between the passband andstopband.
•In some application this can be achieved e.g. voice signals
Voicesignalsspectrumshowslittlepowercontentatthe
origin.Thusfilteringtheunwantedsidebandisrelatively
easy.
Tests have shown that frequency components
below 300Hz are notimportant.
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-ShiftMethod:
The basis of this method is the followingequation

SSB(t)m(t)cosw
ctm
h(t)sinw
ct
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41. Generation of SSBSignals(cont…)
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42. Amplitude Modulation:Vestigial
Sideband(VSB)
•VSB is a compromise between DSB andSSB
•It combines the advantages of DSB and SSB
while avoid disadvantages at smallcost
•Its generation is relatively easy and bandwidth
requirement is 25% greater thanSSB

43. 3/18/2014
Amplitude Modulation:Vestigial
Sideband(VSB)
•If vestigial shaping filter produce VBS from DSB is
H
i(f), Its spectrum willbe
•VBS filter allows transmission of one sideband but
suppress other side band gradually, NOTcompletely
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44. 3/18/2014
Amplitude Modulation:Vestigial
Sideband(VSB)
•Fordemodulation,
85
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