Abstract
A digital double relaxation oscillation (DROS) superconductingquantum interference device (SQUID) has the potential for a very large dynamicrange and a slew rate that is orders of magnitude larger than that ofconventional SQUID systems. This is, for example, advantageous when using theSQUID in an unshielded environment. An important characteristic of the digitalDROS is that the feedback flux is quantized since this flux is supplied by asuperconducting up-down counter. Together with the clock frequency, thequantization unit of the feedback flux determines the maximum slew rate. In anoptimized design, the quantization unit is adapted to the broadband flux noiseof the DROS, so that the slew rate is maximized without compromising thesensitivity. Simulations on our optimized digital DROS showed proper operationfor slew rates up to 5×106 Φ0 s-1. The relaxationoscillations generated at a frequency of 100 MHz deliver an on-chip clocksignal, so that no external clock is required. Slew rates up to108 Φ0 s-1 can be achieved by increasing the relaxationoscillation frequency to a few GHz. This means that sub-µm2 Josephsonjunctions with high critical current densities are required. The firstexperiments using low-Tc ramp-type Josephson junctions are presented inthis paper.
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