So what you are saying is that the carrier of a modulated signal is
ONLY a frequency domain concept? That would mean that it really does
turn on and off in the time domain at the modulation rate.

73
Gary K4FMX


On Thu, 23 Oct 2003 10:49:46 -0700, Roy Lewallen
wrote:

You have to be careful in what you call the "carrier". As soon as you
start modulating the "carrier", you have more than one frequency
component. At that time, only the component at the frequency of the
original unmodulated signal is called the "carrier". So you have a
modulated RF signal, part of which is the "carrier", and part of which
is sidebands.

General frequency domain analysis makes the assumption that each
frequency component has existed forever and will exist forever. So under
conditions of modulation with a periodic signal, you have three
components: A "carrier", which is not modulated, but a steady, single
frequency, constant amplitude signal; and two sidebands, each of which
is a frequency shifted (and, for the LSB, reversed) replica of the
modulating waveform.

You can take each of these waveforms, add them together in the time
domain, and get the familiar modulated envelope.

So, the short answer is that the carrier, which is a frequency domain
concept, is there even if you're modulating at 0.001 Hz. But to observe
it, you've got to watch for much longer than 1000 seconds. You simply
can't do a meaningful spectrum analysis of a signal in a time that's not
a lot longer than the modulation period.

Roy Lewallen, W7EL

Gary Schafer wrote:
Speaking of AM modulation,, we all know that the carrier amplitude
does not change with modulation. Or does it?

Here is a question that has plagued many for years:
If you have a plate modulated transmitter, the plate voltage will
swing down to zero and up to two times the plate voltage with 100%
modulation. At 100% negative modulation the plate voltage is cutoff
for the instant of the modulation negative peak.

How is the carrier still transmitted during the time there is zero
plate voltage?

If we lower the modulation frequency to say 1 cps or even lower, 1
cycle per minute, then wouldn't the transmitter final be completely
off for half that time and unable to produce any carrier output??

Question is, at what point does the carrier start to be effected?


73
Gary K4FMX