Experimental demonstration of superconducting bandpass delta-sigma modulator

Superconducting bandpass delta-sigma (δσ) modulators are being developed for direct analog-to-digital conversion of radio frequency signals in the GHz range. Recently we demonstrated operation of a superconducting bandpass δσ modulator with the highest center frequency (2.23 GHz) and sampling rate (up to 45 GHz) reported to date. The test chip used in that experiment includes an integrated acquisition memory, which eliminates the need for very high speed interfacing. This paper describes key implementation details of this complex test chip, which contains 4065 Josephson junctions. The paper also explains how magnetic coupling from large power supply currents can be reduced through the use of optically-isolated current sources with independent grounds. The measured output spectra of the modulator at a 45 GHz sampling rate are presented.