AM electromagnetic waves: 20 KHz modulation frequencyonanastronomically-low carrier frequency
"isw" wrote in message
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In article ,
"Ron Baker, Pluralitas!" wrote:
"John Fields" wrote in message
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On Thu, 5 Jul 2007 00:00:45 -0700, "Ron Baker, Pluralitas!"
snip
When AM is correctly accomplished (a single voiceband signal is
modulated
The questions I posed were not about AM. The
subject could have been viewed as DSB but that
wasn't the specific intent either.
What was the subject of your question?
Copying from my original post:
Suppose you have a 1 MHz sine wave whose amplitude
is multiplied by a 0.1 MHz sine wave.
What would it look like on an oscilloscope?
What would it look like on a spectrum analyzer?
Then suppose you have a 1.1 MHz sine wave added
to a 0.9 MHz sine wave.
What would that look like on an oscilloscope?
What would that look like on a spectrum analyzer?
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The first example is amplitude modulation precisely _because_ of the
Is there multiplication in DSB? (double sideband)
Yes, and in fact, that multiplication referred to above creates a
DSB-suppressed-carrier signal. To get "real" AM, you need to add back
the carrier *at the proper phase*.
So does the multiplication in the first example really make
it amplitude modulation?
FWIW, if you do the multiplication and then add back a carrier which is
in quadrature (90 degrees) to the one you started with, what you get is
phase modulation, a "close relative" of FM, and indistinguishable from
it for the most part.
A true DSB-suppressed carrier signal is rather difficult to receive
precisely because of the absolute phase requirement; tuning a receiver
to the right frequency isn't sufficient -- the phase has to match, too,
and that's really difficult without some sort of reference.
A SSB-suppressed carrier signal is a lot simpler to detect because an
error in the frequency of the regenerated carrier merely produces a
similar error in the frequency of the detected audio (the well-known
"Donald Duck" effect).
Isaac
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