Development of a High-precision Ac-dc Transfer Standard Using the Fast-reversed Dc Method

The ac-dc transfer standard is one of the basic electrical standards, by which the ac voltage and ac current are deduced from their dc counterparts in the frequency range between 10 Hz and 1 MHz [1,2]. The dc voltage standards are established using a Josephson voltage standard with uncertainty better than 10-7 [3-5]. The ac voltage standard in the frequency range 10 Hz to 1 MHz are derived from the dc voltage standard by the following two methods, as illustrated in figure 1.1. (a) Direct synthesizing of ac (sine) waveform by the use of high-precision D/A converter. (b) Comparison of electric power between acand dcvoltage by converting the power to force or heat. In the latter case, converters may be recognized as a reference standard, and the system of the standard based on this principle is called the “ac-dc transfer standard”. The most accurate ac-dc transfer standards are realized by the use of “thermal converters”. The thermal converter is capable of comparing the joule heating between ac and dc modes at 0.1 ppm level, and are widely employed as the primary standard in the most of the national standard laboratories [6-8]. As shown in figure 1.2, the thermal converters were developed in the 1950s and are still widely used in the field of ac-dc transfer standards [9-11]. Four types of thermal converters have been developed as ac-dc transfer standards, that is, Single-Junction thermal converters (SJTCs), Multijunction thermal converters (MJTC), thin-film (planar) MJTC, and semiconductor rms sensors. The detailed descriptions on the four types of the thermal converters are given in section 1.2.2. On the other hand, the accuracy of the waveform-synthesizing methods has been drastically increased with the improvement of the high-speed analog switches. The most accurate waveform-synthesizing source presently available can produce sinusoidal waveform with a precision of 1 ppm level up to 100 Hz [21]. Recently, D/A converters based on the Josephson devices are under development [22-24]. The pre§