Transcript AMPLITUDE MODULATION - Welcome to EEE3223 Wikidot site

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Amplitude modulation (AM) radio is a commonplace
technology today, and is standard in any type of
commercial stereo device. Because of the low cost of the
parts necessary to implement AM transmission and the
simplicity of the underlying technology, using amplitude
modulations is a cheap and effective way to perform many
tasks that require wireless communication.
The most well-known application of an AM transmitter is in
radio. Am radio receivers are available in numerous
devices, from automobile stereos to clock radios. However,
the usage of AM transmitters is not restricted to
professional radio stations
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Transmit information bearing (message) or baseband signal
(voice music) through a communication channel
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Baseband = is a range of frequency signal to be transmitted.
eg: Audio (0 - 4 kHz), Video (0 - 6 MHz).
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Communication channel:
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Transmission without frequency shifting.
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Transmission through twisted pair cable, coaxial cable and fiber
optic cable.
Significant power for whole range of frequencies.
 Not suitable for radio/microwave and satellite communication.
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Carrier communication
Use technique of modulation to shift the frequency.
 Change the carrier signal characteristics (amplitude,
frequency and phase) following the modulating
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signal amplitude.
Suitable for radio/microwave and satellite
communication.
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the instantaneous amplitude of a carrier wave is varied in
accordance with the instantaneous amplitude of the
modulating signal. Main advantages of AM are small
bandwidth and simple transmitter and receiver designs.
Amplitude modulation is implemented by mixing the
carrier wave in a nonlinear device with the modulating
signal. This produces upper and lower sidebands, which are
the sum and difference frequencies of the carrier wave
and modulating signal.
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The carrier signal is represented by
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The modulating signal is represented by
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c(t) = A cos(wct)
m(t) = B sin(wmt)
Then the final modulated signal is
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[1 + m(t)] c(t)
= A [1 + m(t)] cos(wct)
= A [1 + B sin(wmt)] cos(wct)
= A cos(wct) + A m/2 (cos((wc+wm)t)) + A m/2 (cos((wc-wm)t))
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Form of amplitude modulation(AM); carrier is
suppressed (typically 40 – 60dB below carrier)
For short we call it SSB since the carrier is usually not
transmitted
Advantages
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Conservation of spectrum space (SSB signal occupies only half
the band of DSB signal)
The useful power output of the transmitter is greater since the
carrier is not amplified
SSB receiver is quieter due to the narrower bandwidth (receiver
noise is a function of bandwidth)
Disadvantages
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The carrier must be reinserted the receiver, to down convert
the sideband back to the original modulating audio
Carrier must be reinserted with the same frequency and phase
it would have if it were still present
This is why clarity control on a CB radio is so fiddly, if carrier
not inserted correctly the received output will be like donald
duck voice.
SSB-LSB SIGNAL
SSB-USB SIGNAL
 SSB
signal frequency spectrum
SSB-LSB signal
SSB-USB signal
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Hartley modulator
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A direct approach for creating a single sideband AM signal (SSB-AM)
is to remove either the upper or lower SB by filtering the DSBSCAM signal (frequency discriminator method)
SSB modulator using the frequency discrimination approach
Magnitude spectra : (a) baseband ; (b) DSBSC-AM; (c) upper SSB; (d) lower SSB
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Quadrature modulator can be used to create a SSB-AM
signal by selecting the quadrature signal to coherently
cancel either the upper /lower SB from the inphase
channel
SSB-AM signal given:
(1)
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Using the minus sign in equation (1) results in upper
SSB,whereas selection of the plus sign yields lower SSB
(2)
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The hilbert transform is a wideband -90° phase shifter
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Hartley modulator for SSB; -sign gives upper SSB, +sign
gives lower SSB