Superconducting Technology Assessment - nitrd

Each class will utilize RSFQ logic’s extreme clock speed to allow manipulation of the true time dependence of the
signal in the digital domain. In addition, a significant source of error and noise in the operation of mixed signal
components is removed by the exact quantum mechanical reproducibility of each RSFQ pulse. The >10x lower thermal
noise in the circuits (due to their 4 K operating temperature and lower characteristic impedance) also facilitates
utilizing the extreme sensitivity to magnetic fields in setting the minimum detectable signal. The high speed system
clock allows time averaging between base band changes in the signal information content.
The simultaneous scan-and-stare receivers could significantly improve our battlespace (and homeland) situational
awareness by both accessing more simultaneous signals by parallelizing only the digital filters and by processing
the signals in closer to real time. Especially in single antenna systems, this is enabled by being able to digitize
accurately wide swaths of frequency and then digitally selecting the specific sub-band and bandwidth desired for
each signal without losing accuracy. This compares favorably to selecting the signals of interest in the analog
domain (via heterodyning techniques) prior to digitization. Only RSFQ logic has demonstrated this ability. Cross-correlation
techniques on the RF waveform turn out to be a straightforward way of implementing matched filtering under real
time software control and harvesting additional processing gain in comparison to base band approaches.
Software radios are exemplified by the Joint Tactical Radio System (JTRS) program. The goal is to unify the
hardware required to receive and transmit all legacy waveforms and facilitate the introduction of new, higher-data-rate
waveforms. The idea is that the software running at a given time will determine how the hardware functions.
Inter-banding (the essential enabler of interoperability) is achieved by using different waveform software on receive
and transmit. So far, the conventional approaches implemented in semiconductor technologies have not been
highly successful. For example, JTRS cluster 1 faces a major re-evaluation and potential termination after EOA in
the spring of 2005. They are having trouble breaking away from simply co-locating multiple radios in a federated
design. The above discussed uniquely demonstrated ability of RSFQ logic to implement true digital reception allows
one to change waveforms by changing the control parameters in the digital filters that operate directly on the RF
signals. In receivers, this direct reception eliminates many expensive analog components and their associated
spurious signals and drastically simplifies the processing.
In transmitters, the RSFQ clock speed allows linearization to be done straightforwardly using the carrier and its
harmonics, not some deep subharmonics that cannot capture the subtleties of the actual signal. Indeed, many
systems, including JTRS, want one transmitter to do multiple tasks, ideally simultaneously. However, this is very
difficult to do with reasonable energy efficiency with today’s current hardware power amplifiers – they are typically
highly non-linear when operated in their highest energy efficiency mode (often below 40% even for a single tone).
To avoid transmitting large numbers of substantial amplitude spurious signals due to the non-linearity, separate
amplifiers for each signal and power combiners that throw away 50% of energy at each combine are used. By
enabling predistortion in the digital domain to compensate for the amplifier non-linearity, these combiners can be
eliminated. Better signal quality and substantially better energy efficiency – especially important whenever a limited
fuel supply or lifetime of battery are concerns – are expected to result from the use of RSFQ logic in such systems.
Commercial Potential
Three applications with commercial potential were identified at the 2001 Workshop on Superconducting Electronics:
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A digital signal processor for use in CDMA base stations for cellular telephone networks.
A scanning firewall to catch malicious content.
Low-power information servers.
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