Design And Research Of Novel Microstrip Bandstop Filters

In a radio communication system, the stop band brought by single bandpass filter is insufficient if a strong disturbance exists in a frequency band or high attenuation in multiple bands is desired. Applying one or more band-stop filters can be a solution to such problems. Traditional microstrip band-stop filters are microstrip lines with resonance length of approximately one fourth of the working wavelength. As a result, the physical size of such a device expands if the working frequency drops. The design method of a traditional microstrip band-stop filter is a synthesis method based on a low-pass prototype. That method employs a transformation from a low-pass prototype to band-stop situations. By using the center frequency as the design parameter for each component in the system, it is appropriate only for narrow band applications but is less accurate for adaptive broadband and multiple stop-bands designs.In this work, we propose a design method for novel microstrip bandstop filters through recent studies-bandstop filters can be achieved by direct transformation from bandpass filter to bandstop filter because of their corresponding relations. As a result, we are able to apply topology transform form existed bandpass structures to design bandstop filters. Subsequently, we demonstrate two novel microstrip bandstop structures-stub-loaded ring step impedance resonator (SIR) and double-digital coupling hairpin type half wavelength bandstop filter. Through ADS and HESS simulation and optimization, the former design is capable to operate at the centre frequency of3.3GHz with a relative bandwidth of60%. It has an insertion loss of0.4dB and a transmission peek attenuation of28dB. It can be switched to filters with multiple stopbands by simple adjustment. The later design operates at the centre frequency of8.0GHz with a relative bandwidth of76.7%. It has an insertion loss of0.6dB approximately and a transmission peek attenuation of50dB. Multiple transmission zeros can be realized within its stopband. These designs both have the advantage for their compact sizes, simple fabrication processes, low insertion loss, broad stopband frequency ranges, and high inhibition in their stop-bands.