Introduction of single-flux-quantum circuits to software-defined radio

Abhishek Arun, Hans G. Kerkhoff, Marcel H.H. Weusthof

Software-Defined Radio (SDR), as explained in [1], is a promising concept for the wireless communication industry since the entire transceiver function can be implemented in software, avoiding the replacement of hardware each time the system has to be upgraded. However, the practical implementation of such a system is depended on the hardware, which consists of precise, ultra highspeed electronic devices such as Analog-to-Digital Converters (ADCs). The present state-of-the-art semiconductor ADCs cannot be used for the implementation of SDR in the near future [2]. Some of the factors that hamper the implementation are the requirements of very high speed of a few tens of GHz and the high resolution (e.g. 16 bits) at these speeds. On the other hand, using the Low-Temperature Superconducting (LTS) technology [3], which relays on the quantum mechanical property of superconductors, very high-speed electronic devices with the accuracy of a flux quantum, can be constructed. In a closed superconducting loop, the magnetic flux can exist only in discrete quantized amounts equal to the multiples of the magnetic flux quantum, φ0 = h/2e ~ 2.07x10 Wb. The theoretical speed limit of such a Rapid Single Flux Quantum (RSFQ) device is close to 1 THz [4]. These RSFQ circuits are the fastest of all devices built in any known technology. The basic building block of an RSFQ circuit is the Josephson Junction (JJ), which is formed when two superconductors are separated by an interface. Since RSFQ circuits work at extremely high speeds, testing and verification of such a device is a difficult and challenging task. Direct testing at these speeds is not possible at this moment. Furthermore, the costs of the equipment required for such tests would be extremely high. Hence methods have to be developed to reduce the requirements of external

Fujimaki Akira

Kiyotaka Nakajima

Akira Fujimaki

We describe three types of software-defined-radio (SDR) receivers based on superconducting technologies. The superconducting analog bandpass filters are essential for all types of the receivers. Another key component is an analog-to-digital converters (ADCs), which are required to have high resolution with a broad band width. The complementary A ADC based on the single-flux-quantum circuit is a promising candidate for the SDR receivers because it has a practical nature together with above-mentioned requirements. The experimentally obtained signal-to-noise ratio (SNR) and sensitivity, which are closely related to the resolution, are 34 dB and 20 μA for a quarter of the full-scale input with a band width of about 20 MHz. If we use the optimum decimation filter, the ADC is expected to have the SNR of 82 dB and the sensitivity of 300 nA. These values meet the requirements of the easiest type of the SDR receiver. After new fabrication process has been introduced and the architecture of the ADC has been improved, all types of recievers could be realized based on suoerconductors.

Kiyotaka Nakajima

Akira Fujimaki

Advancement of superconductor digital electronics, especially Rapid Single Flux Quantum (RSFQ) logic circuit is described. Ultra short pulse of a voltage generated across a Josephson junction and release from charging/discharging process for signal transmission in RSFQ circuits enable us to reduce power consumption and gate delay. The power-delay products (PDPs) of RSFQ integrated circuits (ICs) are 4 or 5 orders of magnitude smaller than those of semiconductor ICs. The fabrication process technology and the related designing technology have been advanced, and RSFQ ICs have been applied to software-defined radio receivers, superconductor detector array systems, and supercomputers. Recently, several kinds of energy-efficient single flux quantum circuit have been proposed to obtain further advantage to semiconductor devices. The PDPs of these circuits become at least 1 order smaller than those of conventional RSFQ circuits.