Parts-Per-Million Level Power Supply Quantum Voltage Standard Application
The pursuit of metrological certainty in voltage measurement has reached the quantum era with the widespread adoption of the Josephson Junction Array (JJA) voltage standard. This standard generates an ultraprecise DC voltage defined by the Josephson constant (K_J = 2e/h) and an applied microwave frequency: V = n f / K_J, where n is the number of junctions. This quantum-accurate voltage is used to calibrate digital voltmeters (DVMs) and precision voltage sources. However, the quantum standard itself requires a specialized, ultra-stable environment to operate, and its output (typically 1 V or 10 V) is used to characterize and calibrate other instruments. The application of a parts-per-million (ppm) level precision high-voltage power supply in this context is not to generate the quantum standard, but to act as a transfer standard amplifier or as a critical component in systems that verify and disseminate the quantum-derived voltage to higher potential levels with minimal degradation of accuracy.
The primary application is in building programmable Josephson voltage standard (PJVS) or pulse-driven Josephson arbitrary waveform synthesizer (JAWS) systems. While these systems can directly synthesize precise voltages up to 10V, many calibration laboratories require stable DC or AC signals at higher voltages—100V, 1000V, or even 10kV—for calibrating industrial meters, high-voltage dividers, and specialized instrumentation. To obtain a precise high voltage traceable to the quantum standard, a precision high-voltage amplifier is used. This amplifier takes the stable 1V or 10V reference from the JJA and scales it by a precise gain factor (e.g., x100 or x1000). The performance of this high-voltage amplifier is the limiting factor in transferring the quantum accuracy to the higher voltage.
The requirements for such a ppm-level high-voltage power supply are extraordinarily stringent. Its gain stability must be at the ppm level over time scales of minutes to hours. This means its internal scaling resistors and feedback networks must be made of low-temperature-coefficient materials (e.g., special alloys or hermetically sealed wire-wound resistors) and be housed in a temperature-stabilized oven. Any drift in the resistance ratio directly introduces error. The amplifier's linearity must also be at the ppm level across its output range. Non-linearity means the gain is different at 1% of full scale versus 100% of full scale, preventing accurate scaling. This necessitates ultra-low distortion, precision operational amplifiers or discrete transistor stages with meticulous trimming and characterization.
Long-term stability and low noise are non-negotiable. The output voltage must be free from drift and have negligible AC ripple. Any low-frequency (flicker) noise or power-line related ripple (50/60 Hz and harmonics) adds uncertainty to the measurement when a DVM is being calibrated against it. The power supply's output stage must be exceptionally clean, often employing linear regulation despite the inefficiency, because switching regulators introduce unacceptable high-frequency noise. The power supply must also have an extremely low output impedance to ensure the voltage remains stable regardless of the load presented by the device under test (DUT), within its current rating.
Another critical application is in the calibration of high-voltage dividers themselves. A precision high-voltage supply is used to apply a stable, known voltage (e.g., 10 kV) across a divider. The divider's output ratio (e.g., 10000:1) is measured by comparing its reduced output (e.g., 1V) directly against the quantum voltage standard using a null meter or a precision DVM. In this setup, the absolute accuracy of the high-voltage supply's output is less critical than its stability during the measurement period. However, if the supply's output has any non-linearity or load-dependent regulation, it can introduce error. Therefore, even in this comparative role, the supply's short-term stability, low noise, and low output impedance are paramount.
Environmental control is integral. The entire system—quantum standard, null meter, and the precision high-voltage amplifier—is typically housed in a temperature-controlled laboratory. The amplifier may have its own internal temperature stabilization. Vibration isolation is also considered, as microphonic effects in components can introduce noise.
Finally, the interface and control of such a supply are designed for metrology. It features remote sensing terminals to compensate for lead resistance, guarded outputs to minimize leakage currents, and programmable interfaces (GPIB, Ethernet) that allow it to be integrated into automated calibration sequences controlled by metrology software. The software can implement measurement protocols like reverse polarity measurements to cancel out thermal EMFs in the connecting wires, a process that requires the high-voltage supply to provide stable outputs of both polarities.
In essence, a ppm-level high-voltage power supply in a quantum voltage standard application serves as a precision scaling link between the microscopic world of quantum physics and the macroscopic world of practical electrical engineering. It is a transfer instrument of the highest order, whose design prioritizes gain stability, linearity, low noise, and environmental insensitivity over raw power or efficiency. By faithfully amplifying the quantum-defined voltage with negligible added uncertainty, it enables the traceable calibration of high-voltage measuring equipment across industries, from power transmission and aerospace to advanced materials research, ensuring that measurements anywhere can be confidently linked back to a fundamental constant of nature.
