Radio wave questions.

[Jimbz]

Greetings everybody, two questions popped out of my head regarding transmitting radio waves.
1) In FM you add together a carrier wave (fixed frequency) and a signal, their sum is now a wave that has varied frequency ? But how will the receiver know the correct frequencies that it should pick up the transmitted waves ?
2) How do you transmit DC ? As in DC modulated into carrier waves ?

[Kremmen]

In a nutshell (it is a bit more complext than this, but the idea is here):
1) You have a local oscillator running at the "station frequency". From the amplified antenna signal you subtract the local frequency, leaving the modulation frequency as the result. That is further bandpass filtered and amplified to get the final audio signal. Varuing the local oscillator frequency tunes the stations.
2) You have a static signal frequency offset from the nominal station "carrier" frequency. This is normally not practical however.

[xwarp]

Here;

http://www.egr.msu.edu/~aviyente/AM-FM.pdf

[rob77]

a very simplified explanation of FM modulation - the carrier is swept by the input signal - 0 in the input signal corresponds to center frequency of the carrier. (actually it's more complex than that, but to understand the principle it's enough )

[babysitter]

Its simple: You have a oscillator which you can de-tune with your modulation signal. If there is no modulation at all, just 0V, it will transmit only on a carrier frequency. your positive or negative part of the modulation signal pulls this transmitted frequency either up or down. The receiver needs to know the carrier frequency and uses e.g. filters to detect this variation.

[katzohki]

I'll throw in my explanation of FM:

Say you have a 10kHz sine wave that you want to transmit. And say you have a radio station 93.7 MHz. Ok, so your signal (you could call it a message) "modulates" the carrier (the 93.7 MHz base frequency) by shifting it in frequency at a rate of 10kHz (10000 times per second). The AMOUNT that the frequency varies is based on your "modulation index" which can be based on a standard for the type of communication.


Here's a really bad wording: The FREQUENCY that the frequency varies is based on the frequency of the input signal (sorry).

Demodulation is opposite of modulation.