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AMPLITUDE MODULATION |
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Amplitude modulation uses the instantaneous amplitude of
a modulating signal (voice, music, data, etc.) to
directly vary the amplitude of a carrier signal.
Modulation index, m, is used to describe the
ratio of maximum voltage to minimum voltage in the
modulated signal. If the modulating signal is equal in
magnitude to the carrier, then m = 1 and the
modulated signal varies from a scaled maximum of unity
down to zero (see figure below). When m = 0, no
modulation of the carrier is performed. If m is
greater than 1, the carrier is actually cut off for some
period of time, and unwanted harmonics are created at
the transmitter output.
In
the frequency domain, the carrier frequency is flanked
on both sides by mirror image copies of the modulating
signal. wM1
= wc±wm1,
wM2
= wc±wm2
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AM General Equation |
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Let the carrier be xc(t) = Xc·cos
(wct),
and the modulating signal be
xm(t) = Xm·cos (wmt) |
Then x(t) = Xc·[1+m·cos
(wmt)]·cos
(wct) |
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Modulation Index |
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m
= |
Vmax - Vmin
Vmax + Vmin |
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In the following examples, the carrier frequency
is eleven time the modulation frequency. Red (dashed)
lines represent the modulation envelope. Blue (solid)
lines represent the modulated carrier.
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100% Modulation |
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Here, the maximum voltage (Vmax) is 2 V and the
minimum (Vmin) is 0 V. From the modulation index
formula:
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50% Modulation |
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Here, the maximum voltage (Vmax) is 3 V and the
minimum (Vmin) is 1 V. From the modulation index
formula:
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25% Modulation |
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Here, the maximum voltage (Vmax) is 1.25 V and
the minimum (Vmin) is 0.75 V. From the
modulation index formula:
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m
= |
1.25 - 0.75
1.25 + 0.75 |
= 0.25 |
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150% Modulation |
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Here, the maximum voltage (Vmax) is 2.5 V and
the minimum (Vmin) is -0.5 V. From the
modulation index formula:
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m
= |
2.5 - (-0.5)
2.5 + (-0.5) |
= 1.5 |
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Note:
AM waveforms created with MathCAD 4.0 software.
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Class |
Conduction Angle |
Maximum Efficiency |
Description |
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A |
360º |
25% |
Device is biased in to conduct in the linear region all
the time. |
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AB |
>
180º, << 360º |
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Two
devices connected like a Class B, but biased to conduct
somewhere between Class A and Class B. |
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B |
180º |
78.5% |
Two
devices in series with the output taken at the common
junction. Both devices biased to conduct in the linear
region for opposite half a cycle, i.e., they are never
on at the same time. |
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C |
> 0º,
< 180º |
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Device is biased to turn on after a certain input
threshold voltage is exceeded. Very efficient, but
creates distortion. |
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D |
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100% |
Used
to switch completely on or completely off. |
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E |
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Used
for rectangular input signals |
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