Frequency of Modulated Signal: Carrier vs Message

[Ocata]

If a carrier signal is 1000 Hz, and my message signal has a bandwidth of 100 Hz.

Is there any change in frequency of the modulated signal? Why is it said that the message signal is now operating between 1000hz +/- 100Hz? I thought the original message signal frequency remains unchanged when superimposed on the carrier signal. Does the carrier signal portion of the A.M. wave have a varying frequency after the message signal is mixed into it?

 

[osilmag]

The message signal adds sidebands to the carrier frequency. That is where the message information resides.https://www.electronics-notes.com/a...odulation-am-bandwidth-spectrum-sidebands.php

 

[jim hardy]

Does the carrier signal portion of the A.M. wave have a varying frequency after the message signal is mixed into it?

Do you believe in Fourier representation of sinewaves?
AM is a multiplication.
for starters look at your trig identities for product of sines

and that's why sidebands appear at sum and difference frequencies
The original doesn't disappear though

The math is interesting
see https://web.ma.utexas.edu/users/davis/reu/ch2/ch2/AM/am.pdf

However, by adjusting the degree of modulation, ie ratio of carrier to signal that's handed to the multiplier, one can cause some the carrier to remain ,
and they do that in AM radio.
Overmodulation, ie applying too much signal , makes harmonics that show up in other radio bands and that's frowned on.

old jim

 

[sophiecentaur]

AM is the simplest form of modulating an RF 'carrier wave'. From the well known pictures of an AM signal, you can recognise the 'Envelope' (slow variation) sits on top of the 'Carrier' (fast variation) and it is easy to make sense of that waveform. The early simple Crystal Sets could easily receive AM signals, which was a good reason for using AM, in the early days, In terms of the frequency spectrum, you can say that the carrier itself carries no information. In fact, you do not actually need to transmit the carrier at all and, more than that, you only need to transmit just one of the sidebands. It uses less transmitter power and occupies less bandwidth but the waveform on a 'scope makes no sense at all. A receiver for that signal is much more complicated.