EENG 3810 Chapter 4

```EENG 3810 Chapter 4
Amplitude Modulation
(AM)
1
Chapter 4 Homework
1. For an AM DSBFC modulator with a carrier frequency
fc = 200KHz and a maximum modulating signal
frequency fm(max) = 10 KHz, determine :
a. Frequency limits for the upper and lower sidebands.
b. Bandwidth.
b. Upper and lower side frequencies produced when
the modulating signal is a single-frequency 6 KHz tone.
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Homework Continued
2. For the AM wave form above determine:
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Homework Continued
3.
400
25
5
6
4
Homework Continued
4. Repeat steps (a) through (d) in Example
4 in these lecture slides for a modulation
coefficient of 0.5.
5. For an AM DSBFC wave with a peak
unmodulated carrier voltage Vc = 20 Vp,
a load resistance RL = 20 , and a
modulation coefficient m = 0.8, determine
the power of the modulated wave
5
Homework Continued
6.Determine the noise improvement for a
receiver with an RF bandwidth equal to
100 KHz and an IF bandwidth equal to 20
KHz.
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Amplitude Modulation Transmission
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AM Generation
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Frequency Spectrum of An AM Double Sideband
Full Carrier (DSBFC) Wave
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Example 1
For an AM DSBFC modulator with a carrier frequency
fc = 100KHz and a maximum modulating signal
frequency fm(max) = 5 KHz, determine :
a. Frequency limits for the upper and lower sidebands.
b. Bandwidth.
c. Upper and lower side frequencies produced when the
modulating signal is a single-frequency 3 KHz tone.
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Example 1 Solution
a.
b.
c.
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Example 1 d. The Output Spectrum
For An AM DSBFC Wave
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Phasor addition in an AM DSBFC envelope
• For a single-frequency modulating signal, am AM
envelop is produced from the vector addition of the
carrier and upper and lower side frequencies.
Phasors of the carrier,
• The upper and lower frequencies combine and
produce a resultant component that combines with
the carrier component.
• Phasors for the carrier, upper and lower
frequencies all rotate in the counterclockwise
direction.
• The upper sideband frequency rotates faster than
the carrier. (usf > c)
• The lower sideband frequency rotes slower than
the carrier. (usf < c)
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Phasor addition in an AM DSBFC envelope
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Modulation Coefficient
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If the modulating signal is pure, single frequency sine wave and the modulation
process is symmetrical, the % modulation can be derived as follows:
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Peak Amplitudes of Upper and Lower Sidebands
The peak change in amplitude of the output wave
(Em) is equal to the sum of the voltages from the
upper and lower sideband frequencies. Therefore,
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Percent Modulation of An AM DSBFC Envelope
(
a) modulating signal; (b) unmodulated carrier; (c) 50% modulated wave;
(d) 100% modulated wave
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Example 2
For the AM wave form above determine:
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Example 2
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Voltage Spectrum for an AM DSBFC Wave
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Generation of an AM DSBFC Envelope
Shown in The Time Domain
–½ cos(230t)
sin(225t)
+ ½ cos(220t)
summation of (a), (b), and (c)
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Voltage of an AM DSBFC Envelope
In The Time Domain
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Example 3
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Example 3 Continued
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Output Spectrum for Example 3
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AM envelope for Example 3
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Power for Upper and Lower Sideband
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Total Power for an AM DSBFC Envelop
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Power Spectrum for an AM DSBFC Wave with a
Single-frequency Modulating Signal
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Example 4
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Power Spectrum for Example 4
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Single Transistor, Emitter Modulator
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Single Transistor, Emitter Modulator
(output waveforms )
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Medium-power Transistor AM DSBFC Modulator
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High-power AM DSBFC Transistor Modulator
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Linear Integrated-circuit AM Modulator
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Block Diagram of a Low-level AM DSBFC Transmitter
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Block Diagram of a High-level AM DSBFC Transmitter
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Single-Sideband
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Conventional DSFC-AM
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Single-side Band Full Carrier
(SSBFC)
The carrier is transmitted at full
power and only one sideband is
transmitted.
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SSBFC waveform, 100% modulation
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Single-Sideband Suppressed Carrier
(SSBSC)
The carrier is suppressed 100% and
one sideband is removed. Only one
sideband is transmitted.
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SSBSC waveform
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Single-Sideband Reduced Carrier
(SSBRC)
One sideband is removed and the
carrier voltage is reduced to 10%
of its un-modulated amplitude.
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Independent Sideband
(ISB)
A single carrier is independently modulated
by two different modulating signals.
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ISB waveform
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Vestigial Sideband
(VSB)
The carrier and one complete sideband
are transmitted, but only part of the
other sideband is transmitted.
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Single-Sideband Generation
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Balanced modulator waveforms
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FET Balanced Modulator
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AM DSBSC modulator using the
LM1496/1596 linear integrated circuit
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Amplitude Modulation Reception
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Simplified Block Diagram of an AM Receiver
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Simplified Block Diagram of an AM Receiver
• Receiver front end = RF section
– Detecting the signal
– Band-limiting the signal
– Amplifying the Band-limited signal
• Mixer/converter
– Down converts the RF signal to an IF signal
• Intermediate frequency (IF) signal
– Amplification
– Selectivity
• Ability of a receiver to accept assigned frequency
• Ability of a receiver to reject other frequencies
• AM detector demodulates the IF signal to the original signal
• Audio section amplifies the recovered signal.
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Bandwidth Improvement (BI)
•
•
•
•
•
Noise reduction ratio
BI = BRF / BIF
Noise figure improvement
NFIMP = 10 log BI
Determine the noise improvement for a receiver with an
RF bandwidth equal to 200 KHz and an IF bandwidth
equal to 10 KHz.
– BI = 200 KHz / 10 KHZ = 20
– NFImp = 10 log 20 = 13 dB
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Sensitivity
• Sensitivity: minimum RF signal level that the
receiver can detect at the RF input.
– 10 dB signal to noise ratio
– ½ watt (27 dBm) of power at the audio output
– 50 uV Sensitivity
– 40 dB signal to noise ratio
– 5 mw (7 dBm) of power at the output
• Aa
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Dynamic Range
• Dynamic Range
– Difference in dB between the minimum input level and
the level that will over drive the receiver (produce
distortion).
– Input power range that the receiver is useful.
– 100 dB is about the highest posible.
• Low Dynamic Range
– Causes desensitizing of the RF amplifiers
– Results in sever inter-modulation distortion of weaker
signals
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Fidelity
• Ability to produce an exact replica of the original signal.
• Forms of distortion
– Amplitude
• Results from non-uniform gain in amplifiers and filters.
• Output signal differs from the original signal
– Frequency: frequencies are in the output that were
not in the orginal signal
– Phase
• Not important for voice transmission
• Devastating for digital transmission
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