Design and experiments on a high-resolution multi-gigahertz sampling rate superconducting A/D converter

Work towards a high-resolution multi-gigahertz sampling rate A/D converter is presented. A brief review of the overall architecture which consists of a coarse section and an interpolator section is given. Experiments on two designs for the coarse sections are discussed. One is a 6-bit A/D converter built with two-leaf phase tree periodic comparators. Asynchronous beat frequency tests at 2.01 GHz sampling rates indicate this circuit is capable of 6 bits of resolution at 2 GHz input bandwidth. The resolution falls off to about 5 bits at 4 GHz and 4 bits at 6 GHz. The other approach involves two related novel single threshold comparators with large dynamic range. For one of the comparators, dynamic range in excess of 60 db is demonstrated by transfer characteristic and input current noise measurements, while the other showed 54 db of dynamic range. A chain of 15 comparators based on one of the designs has been designed and tested. Asynchronous beat frequency tests at 2.01 GHz sampling rates show a monotonic response for input frequencies up to 8 GHz. Threshold offsets due to flux trapping limited the resolution in this set of experiments to about 5 bits. Experiments on a periodic interpolator circuit based on the two-leaf phase tree comparator are also presented. The results suggest that it should be possible to obtain 10-bits of resolution with this approach.<>