Keith Dysart wrote:
On May 10, 6:29 am, "Jimmie D" wrote:
No, Its an illusion. The same thing happens when you view an AM signal. On
an oscilloscope the pattern you see may give you the impression that the
carrier is changing in amplitude with the modulation. Perhaps standing waves
are this same type of illusion.

I am unsure why you would call this an illusion.

The modulated waveform can be accurately described by
(f(t)+1)*cos(2*pi*fc*t) where f(t) is the modulating signal

from which it is easy to discern that the amplitude is changing
with the modulation.

There is often more than one way to describe an observation and
the existence of this description in no way detracts from the
alternative which has a carrier plus and minus the modulating
signal.

Many of the arguments here do seem to be of the form "You
say tomatoe and I say tomatoe", but the important point is
that the appropriate description be used for the problem at
hand. Filter design is probably better done with the latter,
while modulators and envelope detectors are likely better
analyzed with the former.

But I find no reason to declare one to be less of an illusion than
the other.

You are correct though; this is exactly like the arguments
about "standing waves" and "travelling waves". The mathematical
expressions for each accurately describe the voltage and
current distribution on the line, yet some wish to argue that one
description is more real than the other.

They are equally real and equally illusions. The important point
is to choose the one that best helps solve whatever problem is
at hand and not to get carried away with a belief that one is more
real than the other.

...Keith


Thank you for nicely elucidating the
distinctions in emphasis between
"science" and engineering, Keith.

I believe a perfect (just to keep this
at an abstract level) SA reveals the
underlying reality of the modulated AM
carrier.

An oscilloscope displays a waveform that
can be mathematically derived from the
underlying reality. On the scope, it is
produced by electronically combining
three (assumed) sine waves. Without the
mathematical or electronic operations, I
suggest the waveform displayed by the
scope does not exist.

Mathematical equivalence between time
and frequency domains does not
demonstrate (in my humble opinion) a
duality in the underlying reality.

In reality, there are only the original
three frequencies which can be
demonstrated by selective filtering.
Whether the oscilloscope waveform is an
illusion is perhaps a semantic issue
since it is an artifact constructed
from, and convertible at will back into
the three continuously existing sine
waves which never surrender their
independent qualities.

Quite a bit of difference from
transmission line standing waves, no?

My $02.

Chuck


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