Software-defined radio (SDR) replaces traditional radio circuitry like mixers, filters, and demodulators with software that runs on a DSP or with FPGAs. The secret to its success is its ability to sample the radio signal quickly enough so the DSP can do its job.
However, the upper sampling speed of analog-to-digital converters (ADCs) has inhibited its capabilities. Modern chips can easily sample in the hundreds of megahertz and lower gigahertz ranges, but there are still limitations in dynamic range and resolution. That’s the reason why most SDRs are implemented at the intermediate frequency (IF) level rather than directly at RF.
A traditional analog mixer down converts the signal to a frequency within the x2 Nyquist range of the available ADC. The DSP takes it from there to perform filtering, demodulation, and other baseband functions. As the common operating frequencies for wireless systems today have continued to increase well into the microwave region, most SDR continues at the IF level. Hypres Inc. has been working to solve that problem, though, with its digital RF direct conversion to baseband technology.
Hypres has applied its expertise in superconductors to create ICs that operate under cryogenic conditions to produce exceptional switching speeds (see the figure). When cooled to temperatures near absolute zero (zero degrees Kelvin or about –453°F/–269°C), superconductors lose all electrical resistance.
A wide range of such materials has been identified and created over the years with a loss of all resistance occurring in the 4K to 30K range. At those temperatures, the loss of resistance allows semiconductors to operate at very high frequencies with low noise and to switch at speeds not possible with traditional devices. A key application is ADCs for use in SDR.
The company has created ADCs that can sample at rates in the 40- to 60-GHz range in practical devices and to 100 GHz in lab research devices. With such ADCs, true SDR that samples the antenna input and performs all other receiver functions in software is no longer a dream. The problem is that such high sampling speeds mean that the DSP isn't fast enough to process the samples.
Hypres is solving that problem with cryo-cooled digital logic that can perform some of the digital signal processing. The basis of the digital switches and logic isn’t a transistor. It’s a Josephson junction, a two-terminal semiconductor device that can switch in the picosecond to femtosecond range when supercooled to about 4.5K.
To achieve such low temperatures and superconductivity, researchers used to rely on liquid helium (4K) or liquid nitrogen (77K). Today, refrigeration technology makes it possible to build small cryo coolers capable of getting the temperature down to 4.5K. For small circuits such as a sampler, the cooler is only about the size of beer can. For a complete SDR front end, the cryo cooler may only be the size of a small microwave oven, making it practical for wireless infrastructure and military applications.
Hypres is currently developing an SDR front end for wireless infrastructure—specifically cellular basestations. By being able the digitize the whole antenna bandwidth (about 200 MHz) at gigahertz cellular frequencies, carriers can accommodate not only the forthcoming faster cellular radio standards like Long-Term Evolution (LTE) but also handle older legacy standards as they fade away.
Studies by the company have predicted major savings in capital expenditures and operating expenditures, as fewer basestations will be needed, and each of those basestations will be able to multiply its capacity. Look for a future standard product related to this application.
Of course, there are other applications for this superconducting technology. Instrumentation is an obvious one. Advanced computing is another target application, where it will enable supercomputers capable of speeds well into the petaFLOP range. Cryo electronics isn't an easy technology, but it is finally coming into its own and finding affordable real world applications.
LOUIS E. FRENZEL