A 6-bit Josephson flash A/D converter with GHz input bandwidth

Edge-triggered periodic comparators based on low-inductance superconducting quantum interference devices (SQUIDs) have been combined with a 5-mV step generator used as a clocked power supply to produce an analog-to-digital (A/D) converter with the widest input bandwidth of which the author is aware. The noise and linearity have been measured to be more than adequate for 6 b. Asynchronous beat frequency tests indicate that the current design is capable of about four effective bits resolution at 5-GHz input bandwidth and 3 b at 10 GHz. The aperture jitter at 6-GHz clock rates has been measured to be 100 fs rms indicating that the intrinsic noise is low enough to permit subpicosecond aperture times in faster designs. Further advantages include tolerance of large critical current variations and a sinewave clock as the only high-speed waveform necessary to drive the circuit. The amplitude and spectral purity of the sinewave are not critical as long as it reaches a threshold value, since it is regulated by the step generator.<>