Abstract
We have made a modified type of superconducting double-loop lowpass sigma-delta modulator with feedback gain of 64 operated at the sampling frequency of 4 GHz. The feedback driver consists of multi-stage SFQ splitters and a series array of 64 3J-SQUID gates. A simulation showed that a current margin of the 3J-SQUID feedback driver was enlarged by a factor of 1.5, compared to our previous 2J-SQUID feedback driver. A new high voltage gate with an SFQ storage inductor was used to increase a timing margin between externally applied high frequency sampling pulses and AC bias currents. The high voltage gate needs a smaller AC bias current, thus reducing ground ripple noise. Also, a flip-chip type probe was introduced to reduce ground ripples. The fabricated modulator was tested at 4 GHz. The noise floor in the power spectrum was reduced by almost 20 dB.
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