  The Boltzmann constant was measured by comparing the Johnson noise of a
resistor at the triple point of water with a quantum-based voltage reference
signal generated with a superconducting Josephson-junction waveform
synthesizer. The measured value of k = 1.380651(18) \times 10^-23 J/K is
consistent with the current CODATA value and the combined uncertainties. This
is our first measurement of k with this electronic technique, and the first
noise thermometry measurement to achieve a relative combined uncertainty of 13
parts in 10^6. We describe the most recent improvements to our Johnson Noise
Thermometer that enabled the statistical uncertainty contribution to be reduced
to seven parts in 10^6, as well as the further reduction of spurious systematic
errors and EMI effects. The uncertainty budget for this measurement is
discussed in detail.
