A Low Power Fully Integrated Analog Baseband Circuit with Variable Bandwidth for 802.11 a/b/g WLAN Kai-Yin Liu, Chun-Hao Chen, Yu-Che Yang, Hsiao-Chin Chen, Shih-An Yu and Shey-Shi Lu Graduate institute of Electrical Engineering, National Taiwan University Email: r92943118@ntu.edu.tw Abstract: This paper presents experimental results of an analog baseband circuit with variable bandwidth for WLAN direct conversion receiver in UMC 0.18um CMOS process. A seventh order chebyshev lowpass filter with triple bandwidth is used in the analog baseband circuit. The bandwidth is selectable from 7.56MHz, 19.5MHz, or 26.5MHz. The circuit adopts the servo loop for dc offset cancellation. It also has a gain range from 20dB to 60 dB with 10 dB steps while only dissipating 22.248mW. In addition, an automatic frequency tuning loop (ATL) is reported to achieve the bandwidth accuracy of the filter. Key Words: WLAN, chebyshev, lowpass filter, tuning. I. INTRODUCTION In the recent year, the demand of wireless data communication grows rapidly. The wireless local area network (WLAN) standard with high data-rate becomes the popular solution, such as 802.11a in the 5GHz UNII-band and 802.11b/g in the 2.4GHz ISM-band. In this paper, a fully integrated analog baseband with variable bandwidth is presented for 802.11a/b/g WLAN direct conversion receiver. Besides, Master-Slave method is applied to solve the non-accuracy of the filter bandwidth due to the process variation. II. FILTER AND FREQUENCY TUNING The seventh order chebyshev filter is used in the analog baseband and is implemented by the active-RC method. The gain, quality factor, and pole locations of the filter are all determined by the MIM caps and the poly resistances. The active-RC method achieves better linearity due to the passive components. The real pole is implemented by the first order passive RC, and is placed in the first stage of the whole filter to pre-filter the down converted signals. The other complex pole pairs are implemented by the Two-Thomas biquad. In order to maximize the dynamic range of the filter without violating the SNR limit, the biquads are sectioned in the order of increasing values of Q. The whole filter is shown in the Fig.1. A transconductance amplifier is places before the first biquad to 0-7803-9433-X/05/$20.00 ©2005 IEEE. APMC2005 Proceedings Authorized licensed use limited to: IEEE Xplore. Downloaded on November 7, 2008 at 19:57 from IEEE Xplore. Restrictions apply.