Re: am transmitter - vlsi project


Michael Black wrote:

Well no.

The audio signal is mixed with the carrier, and the output of that mixer
is the original carrier, unmodified, and two sidebands on either side
of that carrier. Taken as a whole, the amplitude varies, but the carrier
itself stays constant.

This is likely a useful hint, because it's far more common to see a balanced
mixer in ICs than something called an "AM modulator".


Well, your explanation is mighty confusing, and partially incorrdct,
IMHO but it's not your fault.

Originally AM modulation was done in a mighty crude but effective way--
by putting a regular old telephone's carbon microphone in series with
the transmiting antenna!

That setup should give you a clue as to what "AM" really is, in the
time-domain that is-- The carbon mike's resistance varies up and down
as sound waves hit it-- resistance goes DOWN as a high pressure wave
hits, then goes UP as the lower pressure hits, each happens once each
audio cycle.

Later in "grid modulation" was figured out. here things get a bit
fuzzy, but you could look at it as the audio moves the tube's bias
point around and effects it's efficiency. You can also look at it as
a "mixing" action is going on, which leads you to think a
frequency-conversion is going on. Both viewpoints are correct, and of
course both are misleading.


Later on "plate modulation" came in, where you put the audio signal in
series with the plate DC supply. Again it's obvious what's happening
in the time-domain: the plate voltage goes up and down with the audio,
hitting twice the voltage on the positive peaks, and zero voltage on
the negative peaks. Looked at PURELY in the time-domain, it looks like
the carrier is going UP and down in amplitude. In the frequency
domain of course it looks like sidebands are popping up, which they
are.

Much later on, the theory of sidebands, "mixers", and "multipliers"
was cleared up. mathematically, the old "mixers" were revealed to be
kinda like a poor non-linear multiplier, which multipled the two input
voltages (not terribly linealy, but good enough). And the math said
when you multipled two sie waves you end up, in the frequency domain,
with their sums and difference frequencies.

But people kept calling capacitors "condensers", and multipliers
"mixers", so the confusion continues.

You can look at it as "multiplication" or "mixing"-- both are at least
partially correct.

But if you look at the output waveform on a scope, you have this
waveform where the RF amplitude gores UP and DOWN, all the way down to
ZERO, so in some sense the carrier sure LOOKS like it's going down to
zero. And if you think of the B+ voltage going to zero, it's hard to
imagine how the carrier can still be going out when the plate voltage
is zero.

So it's at least partially incorrect to say the carrier "stays the
same" and "sidebands pop up".

.

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