Notch Filter

Why notch filter

Magnetic fusion plasmas are often heated by high power (multi-MW) millimeter wave sources. Consequently, a notch filter is required to protect the imaging arrays from spurious ECRH (Electron Cyclotron Resonant Heating) power. The ECRH power has peak power levels of several megawatts at the source, and the leakage level to the imaging array can be several mW or even more. Consequently, the ECRH power can damage the imaging array or saturate the diode, or distort the output signal at somewhat reduced levels. To solve this problem, quasi-optical filters are required on plasma devices such as TEXTOR, KSTAR and LHD. To address this need, a collaborative agreement has been made with Kyushu University and Kyushu Hitachi Maxell, Ltd. to fabricate high precision notch filters.

The ideal notch filter has a high notch attenuation that rapidly falls off away from the notch resonance so as to achieve a wide passband bandwidth.

FSS Theory

Frequency Selective Surface (FSS) is used as a notch filter. FSS has been widely used in radar systems, broadband communications and antenna technology. A Frequency Selective Surface (FSS) can be viewed as a filter for plane waves at any angle of incidence. FSS is often composed of a conducting sheet periodically perforated with apertures or an array of periodic metallic patches. Our notch filter is composed of periodic metal patches on the dielectric.

Frequency selective surface can be interpreted by equivalent circuit theory. Three structure and their equivalent circuits and responses are shown below.