FIRST MEASUREMENTS OF AC-COUPLED JOSEPHSON WAVEFORM SYNTHESIZER.
NIST researchers have completed preliminary rms voltage measurements of the Josephson waveform synthesizer when it is biased using a new ac-coupled technique. AC coupling allows us to use distributed arrays of more than 4000 junctions that have larger output voltage as compared to the previously used lumped arrays with only 250 junctions. More importantly, this technique allows researchers to ground the Josephson array output voltage so that it can be directly measured using low impedance instruments, such as spectrum analyzers and thermal voltage converters. Direct connection to measurement instrumentation is required for optimum performance of calibrations because it avoids the possible gain and distortion errors that can be induced by intermediate amplifiers. NIST researchers have made the first spectral measurements of a directly coupled array and found that harmonic distortion was more than 80 dB below the amplitude of synthesized fundamental frequencies of I kHz and 50 kHz. This measurement was likely limited by distortion in the spectrum analyzer. They also used a thermal transfer standard to compare the rms voltages of synthesized sine waves at 3 kflz, 10 kHz, 20 kHz, and 50 kHz with a synthesized 1 kHz sine wave. The measured Type A uncertainty for each comparison was no more than one-tenth that of previous comparisons using lower voltage lumped arrays.