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An electromagnetic instrument transformer is a common device used to measure large current values in high-voltage electrical networks; it has been in use for more than a century. However, the optical current transformer, a promising technology also known as a fiber optic current sensor (FOCS), offers increased safety and ease of operation, as well as the absence of errors caused by the magnetic circuit of legacy transformers. Although the FOCS scheme is well known and has been actively developed for over a quarter century, it has certain disadvantages that limit its use. This paper describes the authors’ efforts to solve these problems in order to make FOCS technology competitive and widely adopted. We upgraded the FOCS optical circuit, expanded the frequency band of the captured current signal, and reduced the solution’s cost. We designed new signal processing algorithms to compensate for errors caused by internal factors in the measurement circuit, as well as those caused by environmental influences. We developed an FOCS computer model based on the Jones matrix formalism to enhance the experimental debugging. It allowed us to define the requirements for elements of the optical circuit and its production accuracy.

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Wide band filter and low loss resonator have been developed using the new piezoelectric material, Langasite. The filter has about three times wider passband compared with the filter utilizing quartz crystal, and the resonator has approximately two to five times smaller equivalent series resistance than that of quartz crystal.We have developed 71MHz BAW filter for GSM base station. Since the filter is made in fundamentals mode, we have to obtain the blank of about 19,um thickness because the frequency constant is 1380kHz.mrn. We tried to obtain the required blank thickness by chemically etching the center area of the blank (thickness: 70,um) from the both sides. Special technology of langasite etching which has big differences compared with usual quartz technology was developed. We have successfully etched the blank to the required thickness using an efficient etchant. The etching area of the blank has a chemically polished surface and it is sufficient for high frequency filters.Two MCF units are used to make a 4-pole langasite filter. The size of the 4-pole langasitefilter is 10.3 X 3.5 mm and it is five times smaller than conventional filter.

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