Radium hath wroth:

On Jun 30, 3:46 pm, Jeff Liebermann wrote:

With AM, it's ALWAYS the high frequency
that acts as the carrier
and the lower that acts as the modulation.

In AM, isn't the carrier the signal which always maintains a constant
frequency and only varies by amplitude?

You really are clueless. The carrier does NOT vary in amplitude. If
it did, that would be modulating the carrier, which is the job of the
modulator, not whatever is producing the carrier. You could have two
modulators in series, that would make the circuit overly complicated.
Please re-read my highly simplified previous explanation about the
symmetry of the AM multiplier (mixer) input ports until it's absorbed
and understood by your porous brain.

Incidentally, the reason I keep using the term "multiplier (mixer)" is
to avoid confusion with a harmonic multiplier. An AM modulator is a
mixer, not a harmonic multiplier.

Also, the carrier might remain constant frequency, for a given FCC
channel assignment, but the modulation is all over the place. For
example, your voice goes from 300 to 3000Hz, all of which is fed to
the modulator for digestion.

Conventional TV is VSB (visidual side band) which is a form of AM with
one of the two side bands partially removed, usually by filtering.
There's a carrier 1.25MHz offset for the video, another carrier 4.5Mhz
offset for the audio, and whatever else they can throw in for low
speed data. Two more more carriers are required for TV+audio.

If you want to get really high-techy, the new digital modes (DRM,
iBiquity, HD Radio, etc) all have multiple carriers, each of which is
modulated individually. Same with various OFDM modes, which have
multiple carriers, individually modulated and positioned orthogonally
from each other to prevent mutual interference from adjacent modulated
carriers.

If a carrier signal varies by anything other than just amplitude, then
it isn't AM. Right?

Wrong. The carrier can also vary, such as in a sweep generator or
jammer. It's not commonly done but it's possible. Want to obliterate
the entire AM broadcast band? No problem. Just sweep the carrier
from 530KHz to 1650KHz, while modulating the 300 to 3000Hz audio with
a rendition of your incoherent ranting.

By the way, you're welcome.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558