On Nov 15, 2011, at 10:30 PM, V G wrote:

>> 2) channel spacing/selectivity - the faster you modulate, the more
>> bandwidth you consume
>>=20
> This is exactly what I'm having a hard time visualizing.

TI had a seminar that explained  this really well, covering such material a=
s why we have all these fancy modulation schemes (PSK) instead of just some=
thing simple like VG first described.  I think it was  "RF Basics"; and the=
 slides are here: http://www.ti.com/lit/swrp082 but they lose something wit=
hout the speaker.  Perhaps there is a video somewhere...


Basically, Fourier said that any periodic function can be decomposed into a=
 sum of sine waves.
The bandwidth consumed by a signal is the difference between the maximum an=
d minimum frequencies of those sine waves.
Modulation modifies a pure sine in ways that are not simply adding another =
sine wave, so the sum of frequencies that results is "complicated" (and sho=
wn by FFT/etc)  However, the min and max (signficant) frequencies in the de=
composition are clear, and you don't want them to overlap into other channe=
ls that are trying to communicate as well.

Your 2011-11-16_202307.png plot shows this really well for AM.  Your modula=
tion is a multiplication, but the resulting Fourier shows that the frequenc=
ies "used" have "spread out" by a significant bit (at least 20 percent?  Th=
at's "awful"!  (AM Radio has 9 kHz channel separation of ~200 kHz signal, f=
or example.))

BillW

--=20
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