26.10.2022
At PTB, a digital generator has been developed which generates a sinusoidal ac voltage at only a few discrete frequencies, but with an outstandingly high precision and stability. This generator is an essential component for the precise realisation of the capacitance unit “farad” from the quantum Hall resistance.
This quantum-based realisation of the capacitance unit, developed and applied at PTB, is carried out by a quadrature bridge which measures the impedance of a capacitance standard in terms of the quantum Hall resistance. The quantum Hall resistance whose numerical value is exactly defined in the revised International System of Units (SI) is operated at alternating current. Because the quadrature bridge is restricted to discrete impedance ratios like 1:1, 2:1 or 1:2, and only decade capacitance values are needed to set up the capacitance scale, the quadrature bridge can only be operated at a few discrete frequencies, namely at 1233.15 Hz or 2466.29 Hz, and in the future at 616.57 Hz as well. These frequencies must have a relative precision and stability of better than 6·10‑10 and the spectrum must be very pure.
Since there is no commercial sinewave generator suitable for this application, a digital sinewave generator was developed by PTB. Its output signal is directly derived from the 10‑MHz reference signal of an atomic clock. The generator is based on simple proven methods, namely a digital sinewave synthesis triggered by the frequency-converted reference signal. It does neither need a “phase-locked loop” nor any technique to continuously cover a wide frequency range at the expense of precision and accuracy. As a result, the output frequency is strictly synchronous with the atomic clock, even for many years, and the relative frequency stability is better than 2·10‑11, which is distinctly better than the minimum requirement. The spectrum of the sinusoidal signal is very pure and shows only a very small content of harmonics (‑90 dB for the second harmonic and at least ‑100 dB for all higher harmonics). Due to this, the quadrature bridge does not require a very elaborate detector filter, which is a great practical advantage.
The sinewave generator offers compact dimensions and consists only of commercially available electronic components (i.e., not specially fabricated or selected parts). A prototype has been reliably used at PTB for many years in the quantum-based capacitance realisation. More details can be found in the reference given below.
The innovative synchronous generator was recently included in PTB's technology transfer offer, and interested parties can find out about possible uses on PTB's website under “Patent and Technology Offers”, under “PTB reference 7097".
Figure: View of the digital sinewave generator.
Contact person:
Jürgen Schurr
Department 2.6 „Electrical Quantum Metrology"
Reference:
J. Schurr and J. Lee, „Realization of the farad from the ac quantum Hall resistance – 14 years of experience “, IEEE Trans. Instrum. Meas., vol. 71, pp. 1502008, 2022.