MEMS-based compact dual-band bandpass filters with applications to wireless local area network

초록

This paper reports two types of compact dual-band bandpass filters for wireless local area network (WLAN) applications operating at two frequency bands of 2.4 and 5.2 GHz. The two types of filters (contact-type and capacitive-type) are focused on their digital mode operations in order to alternatively select the operational frequency band of the WLAN. Different types of micromachined frequency-tuning elements having only two states are implemented on each filter to control the center frequencies of the filters. Two digitally operated WLAN filters are fabricated using a surface micromachining technology, which enables us to make simple planar structures of the filters, resulting in easy integration with other planar circuits. Besides, external switching devices for frequency selection are not required, since they are already implemented on the filter's own structures. One of the proposed filters is a contact-type filter using switched inductors with direct-contact MEMS switches. In this filter, a precise and stable frequency-tuning ratio can be obtained because the center frequency is changed by total inductance change due to ON/OFF actuations of the switch. The measured center frequencies of the filter were 2.5 and 5.1 GHz (49% tuning ratio), and the passband insertion loss and return loss were 4.7 dB and 21.5 dB at 2.5 GHz, and 5.2 dB and 21 dB at 5.1 GHz, respectively. The other is a low-loss capacitive-type filter using micromachined variable capacitors. The frequency tunability of this filter is controlled by discrete change of a capacitance according to simultaneous actuations of a total of 12 variable capacitors. This filter shows lower loss compared to the contact-type filter, since loss due to contact resistance of the switch is not included in this filter configuration. The initial center frequency was measured to be 5.4 GHz and it was shifted to 2.6 GHz (48% tuning ratio) with the applied voltage. The total insertion loss and return loss were 2.8 dB and 24.4 dB at 5.4 GHz and 3.3 dB and 20.1 dB at 2.6 GHz, respectively.

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