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NICT REPORT 33successfully generated an optical frequen-cy comb signal with high frequency stabil-ity and a 300 GHz bandwidth.Fundamental technologies for terahertz spectrum measure-mentsIn FY2007, we designed, prototyped and evaluated a 400 GHz band mixer that can input frequency bands equally divided by a filter bank and simultaneously downcon-vert them to an intermediate frequency (IF). By making this mixer’s IF band as wide as possible, we were able to reduce the number of filter banks needed to cover the measurement frequency band. By re-ducing the number of mixers, we ended up with a simpler system. In a conventional mixer, an isolator is used to eliminate the impedance mismatch with the IF amplifier connected to the mixer, but this restricts the IF band. In our study, as shown in Fig.3(a), we integrated a 3–21 GHz band IF amplifier into a 400 GHz band mixer block, and we coupled the mixer chip and a monolithic microwave integrated circuit (MMIC) with a wideband matching circuit. Figure 3 (b) shows the results of measur-ing how the mixer’s noise temperature and gain vary across the IF band, together with the simulated results obtained from an equivalent circuit model. We successfully achieved a bandwidth much broader than the 4–12 GHz band caused by the limita-tions of conventional isolators, and the de-vice performance also matched the simu-lated behavior very closely.International standardizationIn WP5A (Land mobile service) and WP5C (Fixed wireless systems), two new reports have been completed on technical operation characteristics and spectrum re-quirements of LMS and FS systems oper-ating at 275–450 GHz, and these reports have been published as Report ITU-R M.2417-0 and Report ITU-R F.2416-0. In WP3K (Point-to-area propagation), a revi-sion of Recommendation P.1238 has been proposed regarding terrestrial mobile in-put to 300 GHz band terrestrial mobile ap-plication systems. Meanwhile, at an APT preparatory meeting for WRC-19 (APG19-3), an NICT official was put in charge of the DG (drafting group) for agenda item 1.15, and compiled an APT draft provisional opinion on this item.Furthermore, the IEEE (Institute of Elec-trical and Electronic Engineers) standard-ization task group 802.15.3d has been studying the standardization of 300 GHz band short-range WPAN (Wireless Per-sonal Area Network) systems, and Direc-tor General Iwao Hosako of the Terahertz Technology Research Center has taken part as vice chairperson of the working group. Due to the strong leadership of the standardization study, the study of this group is way ahead of schedule, and on September 28, 2017, its system specifica-tions received final approval and were published as IEEE std 802.15.3d-2017.Fig.2 : Improvement of oscillation and phase noise characteristics of an optoelectronic oscillator incorporating an optical frequency comb oscillatorFig.3 : (a) Photograph of the integrated parts of the mixer chip and IF amplier, (b) Com-parison of simulated and measured noise / gain characteristicsResearch and Development