Chapter02-1 Amplitude Modulation
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Transcript of Chapter02-1 Amplitude Modulation
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Chapter 2.1
Continuous Wave Modulation:
Fundamentals and Circuits of Amplitude
Modulation
References:
Communication Electronics: Principles and Applications by Louis E. Frenzel
Communication Systems by S. Haykin
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Topics Covered
AM Concepts
Modulation Index and Percentage of Modulation
Sidebands and the Frequency Domain
AM Power
Classification of AM
Amplitude Modulators
Amplitude Demodulators
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AM Concepts
In the modulation process, the voice, video, or digital signal modifies another signal called the carrier.
In amplitude modulation (AM) the information signal varies the amplitude of the carrier sine wave.
The instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal.
An imaginary line called the envelope connects the positive and negative peaks of the carrier waveform.
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AM Concepts
Figure : Amplitude modulation. (a) The modulating or information signal.
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AM Concepts
Figure : Amplitude modulation. (b) The modulated carrier.
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AM Concepts
Figure: Amplitude modulator showing input and output signals.
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AM Concepts
In AM, it is particularly important that the peak value of
the modulating signal be less than the peak value of
the carrier.
Vm < Vc
Distortion occurs when the amplitude of the
modulating signal is greater than the amplitude of the
carrier.
A modulator is a circuit used to produce AM.
Amplitude modulators compute the product of the
carrier and modulating signals.
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Modulation Index and Percentage
of Modulation
The modulation index (m) is a value that describes
the relationship between the amplitude of the
modulating signal and the amplitude of the carrier
signal.
m = Vm / Vc
This index is also known as the modulating factor or
coefficient, or the degree of modulation.
Multiplying the modulation index by 100 gives the
percentage of modulation.
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Modulation Index and Percentage
of Modulation
Overmodulation and Distortion
The modulation index should be a number between 0
and 1.
If the amplitude of the modulating voltage is higher than
the carrier voltage, m will be greater than 1, causing
distortion.
If the distortion is great enough, the intelligence signal
becomes unintelligible.
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Modulation Index and Percentage
of Modulation
Overmodulation and Distortion
Distortion of voice transmissions produces garbled,
harsh, or unnatural sounds in the speaker.
Distortion of video signals produces a scrambled and
inaccurate picture on a TV screen.
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Modulation Index and Percentage
of Modulation
Figure : Distortion of the envelope caused by overmodulation where the modulating
signal amplitude Vm is greater than the carrier signal Vc.
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Modulation Index and Percentage
of Modulation
Percentage of Modulation
The modulation index is commonly computed from
measurements taken on the composite modulated
waveform.
Using oscilloscope voltage values:
Vm =Vmax − Vmin
2
The amount, or depth, of AM is then expressed as the
percentage of modulation (100 × m) rather than as a
fraction.
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Modulation Index and Percentage
of Modulation
Figure : AM wave showing peaks (Vmax) and troughs (Vmin).
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Sidebands and
the Frequency Domain
Side frequencies, or sidebands are generated as
part of the modulation process and occur in the
frequency spectrum directly above and below the
carrier frequency.
AM signal described previously
v2= Vc sin2π fct + (Vmsin2π fmt )(sin2π fct)
= Vc sin2π fct + (Vm/Vc)Vc (sin2π fmt )(sin2π fct)
= Vc sin2π fct + mVc (sin2π fmt )(sin2π fct)
= Vc sin2π fct + (mVc/2)cos2πt(fc-fm) - (mVc/2)cos2πt(fc+fm)
Carrier LSB USB
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Sidebands and
the Frequency Domain
Sideband Calculations
Single-frequency sine-wave modulation generates
two sidebands.
Complex wave (e.g. voice or video) modulation
generates a range of sidebands.
The upper sideband (fUSB) and the lower sideband
(fLSB) are calculated:
fUSB = fc + fm and fLSB = fc − fm
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Sidebands and
the Frequency Domain
Figure : The AM wave is the
algebraic sum of the carrier and
upper and lower sideband sine
waves. (a) Intelligence or
modulating signal. (b) Lower
sideband. (c ) Carrier. (d ) Upper
sideband. (e ) Composite AM wave.
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Sidebands and
the Frequency Domain
Frequency-Domain Representation of AM
Observing an AM signal on an oscilloscope, you see only amplitude variations of the carrier with respect to time.
A plot of signal amplitude versus frequency is referred to as frequency-domain display.
A spectrum analyzer is used to display the frequency domain as a signal.
Bandwidth is the difference between the upper and lower sideband frequencies.
BW = fUSB−fLSB
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Sidebands and
the Frequency Domain
Figure : The relationship between the time and frequency domains.
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Sidebands and
the Frequency Domain
Frequency-Domain Representation of AM
Example:
A standard AM broadcast station is allowed to transmit modulating frequencies up to 5 kHz. If the AM station is transmitting on a frequency of 980 kHz, what are sideband frequencies and total bandwidth?
fUSB = 980 + 5 = 985 kHz
fLSB = 980 – 5 = 975 kHz
BW = fUSB – fLSB = 985 – 975 = 10 kHz
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Sidebands and
the Frequency Domain
Figure : Frequency spectrum of an AM signal modulated by a square wave.
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Sidebands and
the Frequency Domain
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AM Power
In radio transmission, the AM signal is amplified by a
power amplifier.
A radio antenna has a characteristic impedance that is
ideally almost pure resistance.
The AM signal is a composite of the carrier and
sideband signal voltages.
Each signal produces power in the antenna.
Total transmitted power (PT) is the sum of carrier
power (Pc ) and power of the two sidebands (PUSB and
PLSB).
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AM Power
When the percentage of modulation is less than the
optimum 100, there is much less power in the
sidebands.
Output power can be calculated by using the formula
PT = (IT)2R
where IT is measured RF current and R is antenna
impedance
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AM Power
The greater the percentage of modulation, the higher
the sideband power and the higher the total power
transmitted.
Power in each sideband is calculated
PSB = PLSB = PUSB = Pcm2 / 4
Total AM power , PT= Pc(1+m2/2)
Maximum power appears in the sidebands when the
carrier is 100 percent modulated.
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Classification of Amplitude
Modulation
In amplitude modulation, two-thirds of the transmitted
power is in the carrier, which conveys no information.
Double Side Band Suppressed Carrier:Signal
information is contained within the sidebands.
Single-sideband (SSB) is a form of AM where the
carrier is suppressed and one sideband is eliminated.
In Vestigial Sideband (VSB), one of the sidebands is
partially suppressed and a vestige of the other
sideband is transmitted to compensate for that
suppression.
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DSB Signals
The first step in generating an SSB signal is to suppress
the carrier, leaving the upper and lower sidebands.
This type of signal is called a double-sideband suppressed carrier (DSSC) signal. No power is wasted on the carrier.
A balanced modulator is a circuit used to produce the sum and difference frequencies of a DSSC signal but to cancel or balance out the carrier.
DSB is not widely used because the signal takes large
BW and is difficult to demodulate (recover) at the
receiver.
Classification of Amplitude
Modulation
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Figure : A frequency-domain display of DSB signal.
Classification of Amplitude
Modulation
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SSB Signals
One sideband is all that is necessary to convey
information in a signal.
A single-sideband suppressed carrier (SSSC) signal
is generated by suppressing the carrier and one
sideband.
Classification of Amplitude
Modulation
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Classification of Amplitude
Modulation
SSB Signals
SSB signals offer four major benefits:
1. Spectrum space is conserved and allows more signals to be transmitted in the same frequency range.
2. All power is channeled into a single sideband. This produces a stronger signal that will carry farther and will be more reliably received at greater distances.
3. Occupied bandwidth space is narrower and noise in the signal is reduced.
4. There is less selective fading over long distances.
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Classification of Amplitude
Modulation
A vestigial sideband signal (VSB) is produced by
partially suppressing the lower sideband. This kind of
signal is used in TV transmission.
In commercial TV transmission, the upper side band,
25% of the lower side band and the picture carrier are
transmitted.
Video information typically contains frequencies as high
as 4.2MHz. To reduce the BW to the 6MHz maximum
allowed TV signal, a portion of LSB is suppressed
leaving only a small vestige of the LSB. Video signals
above 0.75MHz are suppressed in LSB and all video
frequencies are transmitted in USB.
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Classification of Amplitude
Modulation
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Amplitude Modulators
There are two types of amplitude modulators. They are low-
level and high-level modulators.
Low-level modulators generate AM with small signals and
must be amplified before transmission.
High-level modulators produce AM at high power levels,
usually in the final amplifier stage of a transmitter. In high-
level modulation, the modulator varies the voltage and power
in the final RF amplifier stage of the transmitter. The result is
high efficiency in the RF amplifier and overall high-quality
performance.
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Amplitude Modulators
Low-Level AM: Diode Modulator
Diode modulation consists of a resistive mixing network, a diode rectifier, and an LC tuned circuit.
The carrier is applied to one input resistor and the modulating signal to another input resistor.
This resistive network causes the two signals to be linearly mixed (i.e. algebraically added).
A diode passes half cycles when forward biased.
The coil and capacitor repeatedly exchange energy, causing an oscillation or ringing at the resonant frequency.
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Amplitude Modulators
Figure : Amplitude modulation with a diode.
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Amplitude Modulators
High-Level AM: Collector Modulator
The collector modulator is a linear power amplifier that takes the low-level modulating signals and amplifies them to a high-power level.
A modulating output signal is coupled through a
modulation transformer to a class C amplifier.
The secondary winding of the modulation transformer is
connected in series with the collector supply voltage of
the class C amplifier.
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Amplitude Modulators
Figure : A high-level collector modulator.
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Amplitude Demodulators
Demodulators, or detectors, are circuits that accept
modulated signals and recover the original modulating
information.
Figure : A diode detector AM demodulator.
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Amplitude Demodulators
Diode Detector
On positive alternations of the AM signal, the capacitor charges quickly to the peak value of pulses passed by the diode.
When the pulse voltage drops to zero, the capacitor discharges into the resistor.
The time constant of the capacitor and resistor is long compared to the period of the carrier.
The capacitor discharges only slightly when the diode is not conducting.
The resulting waveform across the capacitor is a close approximation to the original modulating signal.
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Amplitude Demodulators
Diode Detector
Because the diode detector recovers the envelope of
the AM (modulating) signal, the circuit is sometimes
called an envelope detector.
If the RC time constant in a diode detector is too long,
the capacitor discharge will be too slow to follow the
faster changes in the modulating signal.
This is referred to as diagonal distortion.
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Modulators for DSSC and SSB
For DSSC signal:
Modulator: Balanced modulator
For SSB signal:
Modulator: Balanced modulator with filter.
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Demodulators for DSSC and SSB
DSB and SSB Demodulation
To recover the intelligence in a DSB or SSB signal, the
carrier that was suppressed at the receiver must be
reinserted.
A product detector is a balanced modulator used in a
receiver to recover the modulating signal.
Any balanced modulator can be used as a product
detector to demodulate SSB signals.
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Demodulators for DSSC and SSB
Figure : A balanced modulator used as a product detector to demodulate an SSB
signal.
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Questions?
Confusions!
Thank you.