[AMRadio] Physical Reality of Sidebands


Bob Bruhns bbruhns at erols.com
Mon Jan 17 00:14:37 EST 2005


Indeed, 'tis a puzzlement.  I can draw three continuous sine waves of
apppropriate amplitude, frequency and phase, corresponding to carrier, upper
sideband and lower sideband, and I can add them point by point, and I can
duplicate a 100% modulated AM signal waveform as we understand it.  I have
to believe that this works backwards; if I make this waveform by modulating
a carrier in an AM transmitter, then I am synthesizing the same continuous
sine waves.  It seems to me that an on-off sequence would only differ from
the sine wave modulated condition in the complexity of the sideband
spectrum.

But in the real world, sometimes theory does not produce results.  For
example, if there was some FM audio subchannel modulating the on-off
carrier, I would simply not be able to receive it while the transmitter was
off, even if I was positive that the carrier was theoretically still
present.  So as Dennis asked on the amradio mailing list - if a tree falls
in the forest, and nobody sees it or hears it... did it really fall?

When I am receiving zero, I can only know that I am receiving zero.  And if
there is interference,  I may not even be sure I am receiving zero!

And as Don pointed out, there is the question of reality, and the appearance
of reality. If I took the time scale to infinity, then if a carrier was EVER
transmitted, even for only a second, I would have to say that it existed for
all of time.  That's ridiculous!  I only think that because I am integrating
over all time, and then generalizing.

I guess it's off when it's off, and it's signalling us when it's on.  So the
question becomes, at what time scale do we stop seeing beat notes, and start
seeing variations and on-off switching?

  Bacon, WA3WDR





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