IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 40, NO. 7, JULY 2005 1443 A – Low-pass Filter for Zero-IF Mobile Applications With a Very Wide Tuning Range David Chamla, Andreas Kaiser, Member, IEEE, Andreia Cathelin, and Didier Belot Abstract—A third-order – Butterworth low-pass filter implementing -tuning and -switching to maximize the tuning range is described. This filter is intended to be used as a channel-selection/anti-aliasing filter in the analog baseband part of a zero-IF radio receiver architecture for multimode mobile communications. Its -switching feature allows extending the tuning range and adapting the power consumption. The filter’s cutoff frequency ranges from 50 kHz to 2.2 MHz. An Input IP3 of up to 18 dBV is achieved, for a total worst-case power con- sumption of 7.3 mW for both and paths, and an effective area of less than 0.5 mm in a 0.25- m SiGe BiCMOS process. A new figure of merit is introduced for comparison of published low-pass tunable filters including noise, linearity, and tuning range. Index Terms—Active third-order Butterworth filter, analog baseband filter, CMOS analog integrated circuits, continuous-time filter, figure of merit, – filter, intermodulation-free dynamic range, large tuning range, linear transconductor, low-pass filter, multimode transceiver, reconfigurable filter, SiGe BiCMOS process, total harmonic distortion, triode transconductor, tunable low-pass filter, zero-IF transceiver, 3G standards. I. INTRODUCTION R EQUIREMENTS for analog baseband signal processing in cellular communications are highly dependant on the receiver architecture. Recent demand for multistandard trans- ceivers has been leading to frequent use of direct conversion architectures, notably because of their high integration poten- tial and their relative system design easiness. In a design re-use strategy, analog baseband filtering must strive for meeting sev- eral standards requirements by being able to focus on a limited number of important parameters. The point of this work is to provide a low-pass filter with continuous tuning over a very wide range suitable to be used in a zero-IF receiver (see Fig. 1) between the down-conversion mixer and the analog-to-digital (A/D) converter. A large con- tinuous tuning range is aimed at covering bandwidth require- ments of the various communication standards in use today. Several Communication standards may be addressed by this work, such as GSM, BlueTooth, CDMA2000, and W-CDMA. The largest cutoff frequency variation is given by GSM and W-CDMA ( 115 kHz and 2.1 MHz cutoff frequency respec- tively), hence imposing a cutoff frequency tuning ratio of around Manuscript received November 15, 2004; revised January 27, 2005. D. Chamla is with IEMN/ISEN, 59046 Lille Cedex, France, and also with STMicroelectronics, 38926 Crolles Cedex, France (e-mail: david.chamla@st.com). A. Kaiser is with IEMN/ISEN, 59046 Lille Cedex, France. A. Cathelin and D. Belot are with STMicroelectronics, 38926 Crolles Cedex, France. Digital Object Identifier 10.1109/JSSC.2005.847274 20. If process and temperature variations influences are taken into account, an ratio of around 30 is needed. The design concepts, the used transconductor, and the system tuning scheme will first be presented in Section II. Section III will present and comment measurement results, and a new figure of merit will be introduced in Section IV, attempting to compare this work to other state-of-the-art realizations. II. PROPOSED FILTER DESCRIPTION A – implementation of an ladder filter has been chosen, mainly because it provides low sensitivity to process variations, temperature drifts, and aging when associated with an on-chip automatic tuning system. Moreover, -tuning allows a perfectly continuous tuning over a wide frequency range. To extend the tuning range beyond the transconductor’s intrinsic tuning range, -switching or capacitor-switching could be implemented. -switching presents some important advantages compared to capacitor-switching. Indeed, with -switching the maximum capacitance value is always used, thus maximizing the signal-to-noise ratio. Moreover, no switch has to be implemented on the signal path, so any issue relative to series resistance or switch linearity is avoided. Moreover, the proposed -switching scheme keeps the parasitic capacitance constant on the signal nodes, making the layout implementation straightforward. A. Design Concepts A third-order Butterworth filter transfer function is imple- mented using an ladder network. If no capacitor switching is being used, the simple structure’s tuning ratio would be equal to that of a single transconductor and hence would have to be around 30. Unfortunately, one single transconductor can hardly provide this tuning range with satisfactory performances, espe- cially in terms of linearity, noise, or power consumption. This led us to propose a multiple- configuration structure (see Fig. 2), allowing a significant increase in the ratio. In our case, two transconductor banks are connected in parallel to one single integration capacitor bank, so that the effective transconductance value is the sum of each of the transconductance cells. Within one transconductor bank, all of the transconductors present the same value. The proposed pattern consists of two transconductor banks (namely and ) connected in parallel, one of them being switched on or off. Let be the maximum reachable ratio for a single transconductor. We can then define the effective transconductance of this transconductor as . The first (always 0018-9200/$20.00 © 2005 IEEE