Modulation and demodulation of 2 GHz pseudo random binary sequence using SFQ digital circuits

The high speed and low power needed for Spread Spectrum communications leads to a desirable application of digital superconductivity. Low temperature Nb/AlOx/Nb digital Single Flux Quantum (SFQ) circuits have been designed and fabricated in our facility, which compose critical subcircuits of the superconductive Spread Spectrum Modem. Four- and 6-bit Pseudo Random Binary Sequence (PRES) generators with output latching amplifiers have been produced and operated at up to 5.4 GHz. On chip modulation and de-modulation of these PRES codes have been performed with time resolution of less than one chip of the code. High-speed off-chip data was XORed with the on-chip PRES at up to 3.7 GHz to produce the modulated pattern. Rapid synchronization allows the code to start within one clock cycle of being fully reset, and the latching output amplifier gives enough power to do real time room temperature measurements of high-speed SFQ circuits.