A ultra-compact 77-GHz CMOS bandpass filter using grounded pedestal stepped-impedance stubs

This paper presents a compact 77-GHz CMOS bandpass filter suitable for single chip integration with an RF transceiver. The CMOS bandpass filter is mainly composed of an open resonator loaded with two grounded pedestal stepped-impedance stubs to enhance the band selectivity and reduce the chip size. The grounded pedestal is constructed from the vertical stack of the multiple metal layers of the CMOS structure such that the slow-wave effect is present when inserting it beneath the microstrip signal trace. Hence the effective guided-wavelength is shortened. By using this technique, a bandpass filter at 77 GHz was designed. The chip size is as small as 0.31×0.26 mm², the measured insertion loss is 3.9 dB and the return loss is better than 20 dB in 75 to 84 GHz. Two transmission zeros at 61 GHz and 109 GHz dramatically enhance the band selectivity.