Keith Dysart wrote:
On May 10, 11:25 am, Chuck wrote:
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.

Let me offer two examples.

I turn on my RF signal generator. I turn up the RF Level,
then I turn it down, then up, then down, ....
I can see this varying RF level on my oscilloscope (slow
sweep), and even on my RF voltmeter.
I know I am varying the level of the RF.
But I also know that I could produce exactly the same
output by adding 3 signals of slightly different frequency
together. I am not at all comfortable with saying the latter
is 'real' while the former isn't. I know I was varying the RF
Level.

Or,
I turn on my RF signal generator with some level for 1
minute. I turn it off for a week. I turn it on for one minute.
I turn it off. I compute the Fourier transform. I can create
exactly the same signal by adding all the Fourier terms,
extending forward and backwards in time, forever.
But is this more real than: I turn it on, then off, then
on, then off?

Using these examples, I can find no reason why the
multiple signal explanation is more real than the
varying amplitude explanation. And I suggest, that for
these two cases, the varying amplitude explanation
is probably more useful.

...Keith


Well, if I understand, and I often
don't, you are saying that the spectrum
produced by method 1 is
indistinguishable from the spectrum
produced by method 2 and THEREFORE,
neither spectrum alone can be considered
true reality.

I grant immediately that it doesn't
matter how you produce the spectrum.

What is at issue, if I am not mistaken,
is whether the reality is that which is
observed on the scope, vs. that which is
observed on the SA (in the case of an
amplitude modulated carrier, of course).

Abstracting, there are three (by
assumption) coherent sinusoids in the AM
modulation case. Each can be directly
measured and characterized.

A composite of these sinusoids can be
displayed on a scope. Any number of
mathematical or electronic operations
can be performed on the sinusoids, and
the results displayed on a scope.

Usefulness, like convenience, may share
a bed with veracity, but its intentions
should be suspect.

I guess I can continue to assert that
mathematical equivalence between
frequency and time domains is not
evidence to me of an underlying duality
in reality, and you can continue to
assert that to you, it is. And I guess
we could still be friends. ;-)

73,

Chuck





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