Let me elaborate a little. Maybe the following example will help.

Suppose you've 100% modulated a 1 MHz carrier with a 0.1 Hz sine wave.
Our knowledge of frequency domain analysis tells us the spectrum will be
a 1 MHz "carrier", with two sidebands, one at 1,000,000.1 Hz and the
other at 999,999,999.9 Hz. At 100% modulation, the power amplitude of
each sideband will be 1/4 the amplitude of the carrier; the voltage
amplitude of each will be 1/2 the amplitude of the carrier.

Now, imagine that you can draw three sine waves on a long piece of
paper. They would have the frequencies and amplitudes of the three
spectral components above. These are the time domain representations of
the three frequency domain components. (In that sense, you *can* speak
of a carrier or a sideband in the time domain -- so I was perhaps unduly
dogmatic about that point.) But here's the important thing to keep in
mind -- all three of these components have constant amplitudes. They
extend from the beginning of time to the end of time, and don't start,
stop, or change at any time. That's what those spectral lines mean, and
what we get when we transform them back to the time domain.

At each instant of time, look at the values of all three components and
add them. At some times, you'll find that the two sideband sine waves
are both at their maxima at the same time that the carrier sine wave is
at its maximum. At those times, the sum of the three will be twice the
value of the carrier wave alone. At some other times, both sidebands are
hitting their maxima just when the carrier is at its minimum value. At
those instants, the sum will be zero. After you plot enough points,
you'll find you've reconstructed the time waveform of the modulated
signal. You'll also find you need at least ten seconds of these three
waveforms to create one full cycle -- repetition -- of the modulated
wave. During that ten second period, the carrier sine wave doesn't
change amplitude, nor do the sideband sine waves change amplitude. Only
the time waveform, which is not the carrier or the sidebands, but always
the sum of the three, changes. When we speak of a carrier wave, we mean
that sine wave of constant amplitude that never changes -- in other
words, a single component in the frequency domain.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Gary Schafer wrote:

So what you are saying is that the carrier of a modulated signal is
ONLY a frequency domain concept?


Yes.

That would mean that it really does

turn on and off in the time domain at the modulation rate.


"It" only exists in the frequency domain. Talking about the carrier in
the time domain makes no more sense than talking about the sidebands in
the time domain, or the envelope in the frequency domain.

Roy Lewallen, W7EL