Superconductor Components for Direct Digital Synthesizer

We are developing a Direct Digital Synthesizer (DDS) based on a novel digital-to-analog converter (DAC) technology capable of directly generating wideband signals at RF with large signal-to-noise ratio (SNR) and spur-free dynamic range (SFDR). The key parts of this oversampling interpolating DAC are a digital interpolation filter (DIF), a digital sigma-delta modulator, a phase rotator, and an output amplifier array. We have developed first and second order DIF circuits to perform digital interpolation—accurate up-conversion of baseband data to RF (125 MS/s to 2 GS/s) followed by digital filtering using a Hogenauer Cascaded integrator comb filter. The digital sigma-delta modulator performs sigma-delta encoding of the interpolated data and comprises circuits similar to the DIF circuits. The phase rotator provides the phase information for the output amplifier array to create an analog waveform from a digital time-varying signal. The phase rotator provides the reference phases as four quadrature outputs. Depending on the digital input the rotator is capable of shifting phase by either one or two clock periods in any direction. The output driver is a differential digital amplifier based on SQUID arrays. It also can be assembled using voltage multipliers or superconducting quantum interference filters (SQIFs). Design and experimental results of these DDS components are presented and discussed.