I've done some reading on software defined radios and while the concept is clear I have yet to grasp how the signaling is actually done. I've researched the Analog to Digital IC's that are involved in the process and they say that the support sampling rates of 500Mhz or more (is that even correct?) My question is: How is the modulation and demodulation acheived in a software deined radio?
EDIT: I was assuming that there was some sort of voltage controlled oscilator involved, however I can't find the literature.
Best Answer
There is a four part series "Software radio for the masses", linked from the ARRL, which explains all this basic information, including the different ways of doing software defined radio.
Once the signal is digitized, it's processed using DSP techniques. You can find more details about them in "Signals, Samples and Stuff: A DSP Tutorial" articles, also linked from above mentioned page.
Oscillators in SDR can be of any kind, from crystal, to PLL controlled VCOs, to direct digital synthesis (DDS), to numerically controlled oscillators (NCO, CORDIC) done entirely in software or FPGA.