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In the ultimate software-defined radio (SDR), the input from the antenna is connected directly to the analog-to-digital converter (ADC). All the filtering, demodulation, and other processing then are done in DSP. Such radios already exist for the lower frequencies.
For example, some commercial amateur radios and shortwave radios with a frequency range to 30 MHz use the direct sampling technique. Now you can do this with 3G and 4G cellular radios in the low microwave region. National Semiconductor's ADC12Dxx00RF ADCs sample as quickly as 3.6 Gsamples/s (GSPS), making...