A 6-BIT JOSEPHSON FLASH A/D CONVERTER WITH GHz INPUT BANDWIDTH

We have combined edge-triggered periodic comparators based on low inductance SQUIDs with a 5mV step generator used as a clocked power supply to produce the highest input bandwidth A/D converter of which we are aware. We have measured the noise and linearity to be more than adequate for 6 bits. Asynchronous beat frequency tests indicate the current design is capable of about 4 effective bits resolution at 5GHz input bandwidth and 3 bits at 10 GHz. The aperture jitter at 6GHz clock rates has been measured to be 100 femtoseconds 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.