Modulate,
Demodulate, Heterodyne, Mix, Beat, Translate, Multiply...
"a
rose
by any other name would smell as sweet"
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Link
to Some Examples of Types of Modulation
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Modulation/Demodulation
is a Nonlinear Process
where Two Sinusoids are Multiplied
(F1
x F2). |
The
Product of this Multiplication--in the Time Domain--is a wave whose
Amplitude
is:
a(X)
= a(F1) X a(F2). |
However,
in the Frequency Domain there is an Addition and Subtraction of
Frequencies,
i.e.,
F1
+ F2, F1 - F2, F2 + F1, F2 - F1,
etc. |
In
practical terms, the Amplitude of one of the two sinusoids is held to a
Constant Value; therefore, the useful results of this process are only
the Derived Frequencies. |
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Modulation
per se is used to impress a message (voice, image, data, etc.) on to a
carrier wave for transmission. A bandlimited range of frequencies that
comprise the message (baseband) is translated to a higher range of frequencies.
The bandlimited message is preserved, i.e., every frequency in that message
is scaled by a constant value. |
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Contrast
this to the Linear Process
of Algebraically Summing Two Sinusoids: it results in a Sum and
Difference only of the two waves; there are No Products Generated. |
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Intermodulation
is a Special Case where two (or more) sinusoids effect one another to produce
undesired products, i.e., Unwanted Frequencies. Again, this can
only occur when both waves share the same NonLinear device. --See
Cross
Modulation, a form of Intermodulation. |
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To
Clarify: What is a Nonlinear Device? It is Any Active Device[1].
In normal designs,
radio receivers, Stereos, etc., Intermodulation is not a problem. However,
when these systems are subjected to Excessive Signal Level Input
the active devices in the "Front End" are driven
out of their Linear
Operating Regions--into or near--Saturation and/or Cutoff, where they become,
in effect, "Modulators."
[1]
Active Devices: Transistors, Diodes, ICs, etc.
Passive devices: Resistors,
Capacitors, Inductors, etc. |
See also: AM--DSB--SSB--
FM
--QAM
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Cross Modulation
(a form of Intermodulation)
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If you have ever been listening to a distant FM station while driving
by an AM Broadcast station's transmitting tower; you more than likely heard
both stations--one on top of the other. That effect was "Cross Modulation,"
a form of Intermodulation.
Since an FM receiver can only receive--Demodulate--one station at a
time (unlike AM which cannot separate interfering stations), that is, two
stations on the same frequency, the receiver will only demodulate the stronger
of the two [2].
In the scenario above, you heard both the distant FM station and the
very close AM transmitter. The FM station's signal, as it was entering
the receiver's "Front End," was being Modulated by the very strong AM signal.
In effect, the receiver's RF Amplifier was acting as a Detector (rectifier)
varying its bias point, which caused the amplifier's gain to change rapidly
(at the AM transmitter modulation rate). This varying gain is effectively
Modulating (Multiplying) the distant FM station's signal. By the time this
"Mess" reaches the receiver's Demodulator it appears to the Discriminator
as an FM signal having two audio messages superimposed one on the other. |
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[2] Capture Effect
In FM receivers, the Demodulator (Limiter/Discriminator combination)
will only extract Zero Axis Crossings of the strongest of competing signals.
E.g., if two signals have nearly equal strength, the stronger of the two
will be "Captured" while rejecting the other. Depending on the receiver
design, signals as close as <1-dB, the stronger will dominate.
That is why: when listening to a distant FM station and driving away
from it and approaching another FM station on the same frequency, the stations
will "bounce back and forth," hearing one then the other, never both at
the same time. It can happen so rapidly that sometimes it sounds as if
they are both being demodulated simultaneously. |
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Constructive & Destructive Interference--"Multi-Path"
You can see this effect everyday while driving around town listening to
your FM radio. As you pull up to a stop light you may notice that the station
is noisy, but if you roll forward a few feet, the station clears up. It
isn't that the signal is weak where it was noisy, it was the effect of
two or more competing signals (reflections) arriving at your antenna having
common phase angles and amplitudes first canceling, then adding as you
rolled forward for the clearer signal. This Constructive & Destructive
Interference is sometimes referred to as Multi-Path. |
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Some Examples
of Types of Modulation
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