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 it possible to make cellular direct RF sampling radios up to 2.7 GHz.
These 12-bit ADCs come in five versions with maximum sampling rates of 1.0, 1.6, 2.0, 3.2, and 3.6 Gsamples/s, and they let you sample beyond the seventh Nyquist zone as needed. The 3.6-Gsample/s device boasts third-order intermodulation distortion (IMD3) of -71 dBc at 2.7 GHz with an amazing noise floor of -152.2 dBm/Hz.
These chips are pin-compatible with National's earlier line of ADC101x00 10-bit devices. Each IC is a dual-channel device with two independent ADC signal chains (see the figure). Each channel can be used independently or in an interleaved format to achieve even higher sampling rates. Power consumption depends on sampling rate but ranges from 3 to 4 W for the higher rates.
One of the best features of these new ADCs is that they can completely eliminate multiple intermediate frequency (IF) downconversion stages that include amplifiers, mixers, filters, and other components. This drastically reduces the bill of materials (BOM) cost, board size, weight, and power consumption. Eliminating the local oscillators in IF designs also greatly reduces interference and as well as the need for filters and other components.
These ADCs target cellular basestations and backhaul point-to-point radios, along with radar and military communications. Consumer uses include media servers and set-top boxes, automobile radar, and game systems. Data acquisition and test equipment are additional potential uses.
If you're designing any high-frequency SDR or direct conversion radio, keep these new ADCs in mind. They're available now in a thermally enhanced 292-ball ball-grid array (BGA) package. Check with National Semiconductor for datasheets and prices.
National Semiconductor Corp.