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Amplitude Modulation
By:Sanjay JangraAssistant ProfessorDeptt. Of Electronics & Communication
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Introduction
• Modulation– In the modulation process, some characteristic of a high-
frequency carrier signal (bandpass), is changed according to the instantaneous amplitude of the information (baseband) signal.
• Why Modulation– Suitable for signal transmission (distance…etc)– Multiple signals transmitted on the same channel– Capacitive or inductive devices require high frequency AC
input (carrier) to operate.– Stability and noise rejection
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About Modulation
• Application Examples– broadcasting of both audio and
video signals. – Mobile radio communications, such
as cell phone.
• Basic Modulation Types– Amplitude Modulation: changes the amplitude.– Frequency Modulation: changes the frequency.– Phase Modulation: changes the phase.
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AM Modulation/Demodulation
Modulator Demodulator
Baseband Signalwith frequency
fm(Modulating Signal)
Bandpass Signalwith frequency
fc(Modulated Signal)
Channel
Original Signalwith frequency
fm
Source Sink
fc >> fm Voice: 300-3400Hz GSM Cell phone: 900/1800MHz
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Amplitude Modulation• The amplitude of high-carrier signal is varied
according to the instantaneous amplitude of the modulating message signal m(t).
Carrier Signal: or
Modulating Message Signal: or
The AM Signal:
cos(2 ) cos( ) ( ) : cos(2 ) cos( )
( ) [ ( )]cos(2 )
c c
m m
AM c c
f t tm t f t t
s t A m t f t
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* AM Signal Math Expression*• Mathematical expression for AM: time domain
• expanding this produces:
• In the frequency domain this gives:
( ) (1 cos )cosAM m cS t k t t
( ) cos cos cosc cAM mS t t k t t
)cos()cos(coscos :using 21 BABABA
2 2( ) cos cos( ) cos( )c c ck k
AM m mS t t t t
frequencyk/2k/2
Carrier, A=1.
upper sidebandlower sideband
Amplitude
fcfc-fm fc+fm
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AM Power Frequency Spectrum
• AM Power frequency spectrum obtained by squaring the amplitude:
• Total power for AM:
.
2 22
2
4 4
12
k kA
k
freq
k2/4k2/4
Carrier, A2=12 = 1Power
fcfc-fm fc+fm
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Amplitude Modulation
• The AM signal is generated using a multiplier.• All info is carried in the amplitude of the
carrier, AM carrier signal has time-varying envelope.
• In frequency domain the AM waveform are the lower-side frequency/band (fc - fm), the carrier frequency fc, the upper-side frequency/band (fc + fm).
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AM Modulation – Example
• The information signal is usually not a single frequency but a range of frequencies (band). For example, frequencies from 20Hz to 15KHz. If we use a carrier of 1.4MHz, what will be the AM spectrum?
• In frequency domain the AM waveform are the lower-side frequency/band (fc - fm), the carrier frequency fc, the upper-side frequency/band (fc + fm). Bandwidth: 2x(25K-20)Hz.
frequency
1.4 MHz
1,385,000Hz to 1,399,980Hz
1,400,020Hz to 1,415,000Hz
fc
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Modulation Index of AM Signal
m
c
AkA
)2cos()( tfAtm mm Carrier Signal: cos(2 ) DC: c Cf t A
For a sinusoidal message signal
Modulation Index is defined as:
Modulated Signal:
( ) [ cos(2 )]cos(2 )[1 cos(2 )]cos(2 )
AM c m m c
c m c
S t A A f t f tA k f t f t
Modulation index k is a measure of the extent to which a carrier voltage is varied by the modulating signal. When k=0 no modulation, when k=1 100% modulation, when k>1 over modulation.
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Modulation Index of AM Signal
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Modulation Index of AM Signal
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Modulation Index of AM Signal
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High Percentage Modulation• It is important to use as high percentage of modulation as
possible (k=1) while ensuring that over modulation (k>1) does not occur.
• The sidebands contain the information and have maximum power at 100% modulation.
• Useful equation
Pt = Pc(1 + k2/2)
Pt =Total transmitted power (sidebands and carrier)Pc = Carrier power
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Demodulation of AM Signals
Demodulation extracting the baseband message from the carrier.
•There are 2 main methods of AM Demodulation:
• Envelope or non-coherent detection or demodulation.• Synchronised or coherent demodulation.
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Envelope/Diode AM Detector
If the modulation depth is > 1, the distortion below occurs
K>1
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Synchronous or Coherent Demodulation
This is relatively more complex and more expensive. The Local Oscillator (LO) must be synchronised or coherent, i.e. at the same frequency and in phase with the carrier in the AM input signal.
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Synchronous or Coherent Demodulation
If the AM input contains carrier frequency, the LO or synchronous carrier may be derived from the AM input.
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Synchronous or Coherent Demodulation
If we assume zero path delay between the modulator and demodulator, then the ideal LO signal is cos(ct).
Analysing this for a AM input = tωtm+V cDC cos
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Coherent DetectionAssume zero path delay between the modulator and demodulator:
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Coherent Detection
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AM Transmitter and Receiver
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AM Transmitter and Receiver
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Summary
• Modulation, Amplitude Modulation• Modulation Index, Modulation Depth• Demodulation of AM signals
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SANJAY JANGRAASSISTANT PROFESSOR
sanjay_439@yahoo.co.inAdvanced Educational Institutions
70 km Milestone,Delhi-Mathura Road, Dist. Palwal, Haryana-121105
Enquiry No: +91–1275–398400, 302222Tele Fax: +91-1275-398406
E-mail: info@advancedinstitutions.comWebsite: www.advanced.edu.in
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