Conclusion and Future Work

The problem of further reduction of dc power dissipation in RSFQ devices was studied theoretically, on a simple model of the power supply line, and experimentally, on the main building blocks of the autocorrelator. The study gave the following new results:

• an estimate of all main effects of mutual- and self-influence of junctions via the bias line, including quantitative recommendations for layout design;
• a measurement of nominal bias and frequency dependences of error rates at both lower and upper bias margins of an RSFQ clocked logical gate including a measurement of the minimum error rate of an RSFQ cell at optimal bias at the limiting frequency;
• a demonstration of a 23 nanowatt XOR gate operating at frequencies of up to ;
• a demonstration of an array of 8 30 nanowatt T flip-flops operating at frequencies of up to .

Two different designs of an RSFQ autocorrelator for radio astronomy applications were proposed and studied experimentally with the following main results:

• a demonstration of the largest asynchronous RSFQ circuit including 3 stages of a dual-rail autocorrelator delay line with multiplication (total of 294 Josephson junctions), fully operational at low frequency;
• a demonstration of a 16-stage autocorrelator delay line with XOR multipliers integrated with an array of T flip-flop prescalers (total of 821 Josephson junctions), fully operational at low frequency;
• a demonstration of basic frequency and signal-to-noise resolution of a fully integrated 16-channel autocorrelator complete with on-chip prescalers, clock generator and quantizer (total of 1636 Josephson junctions) at clock speeds of up to .

In the nearest future we plan to focus on demonstrating a completely operational, fully integrated prototype autocorrelator with 16 channels and beyond, complete with the double oversampling quantizer, delay line, and prescalers. Projected parameters of this autocorrelator include 4 GHz input signal bandwidth, 16 GHz clock frequency and 16 Mbps-per-channel output rate. It will be integrated with a specialized room-temperature interface, allowing real-time data acquisition. Currently, physical dimensions of the existing layout permit to place up to 64 channels on a standard chip. Further increase of layout density (for the HYPRES' standard - - Nb-trilayer process) is possible and should allow up to 100 channels on a chip, or up to a 400 channels on a single chip. Straightforward pickup of signals from each channel in this case might cause serious problems with pin count and heat load from the leads. However, multiplexing of the outputs can be readily introduced, slashing the number of pins by a factor of 16 or more. Going beyond channels to achieve even higher frequency resolution would require development of special multi-chip modules [54]. Further improvement in performance of auto- and cross- correlators for radio astronomy can be achieved by changing the bit representation of the sampled data from 2-level to 3- or 4-level. With minor modifications, ultra-wide-band correlators can also be used in spread spectrum communications and Doppler radar systems.

Measurement of error rates in low-power gates carried out at speed further builds our confidence that RSFQ technology offers unparalleled performance, particularly in unique applications such as space-borne missions or Petaflops-scale computing (see, e.g, [39]) where low power dissipation (combined with high clock speed) could be a crucial advantage.