A low-if 2.4 GHz Integrated RF Receiver for Bluetooth Applications Lai Jiang a, Shaohua Liu b, Hang Yu c and Yan Li d

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vol. 329, pp doi: / Trans Tech Publications, Switzerland A low-if 2.4 GHz Integrated RF Receiver for Bluetooth Applications Lai Jiang a, Shaohua Liu b, Hang Yu c and Yan Li d Shenzhen City Key Laboratory of Embedded System Design, College of Computer Science and Software Engineering, Shenzhen University, Shenzhen, Guangdong, China a jianglai@szu.edu.cn, b liushaohua116@gmail.com, c yuhang@szu.edu.cn, d liyan@szu.edu.cn Keywords: Bluetooth; Gaussian frequency-shift keying demodulator; Complex band-pass Filter Abstract. In this paper, a low-if 2.4 GHz integrated RF receiver for Bluetooth is presented. Designed in a 0.18 µm CMOS technology, the receiver consists of LNA, mixer, complex band-pass filter, and GFSK demodulator. A received signal strength indicator is also employed in the receiver to auto adjust the receiver gain. In this paper, the structures of the major modules were analyzed, and the simulation results are presented and discussed. Introduction The Bluetooth standard uses the unlicensed 2.4-GHz ISM band, and supports 1 Mb/s data connection modulated using Gaussian frequency shift keying (GFSK) [1]. Since Bluetooth is primarily used for short range communication, the development of consumer electronics require to reduce the system power for long time operation, and meanwhile to lower devices cost. Thus, single-chip solutions with a minimum number of external components are proposed [2]. This paper presents a 2 MHz low intermediate frequency (IF) RF receiver designed for Bluetooth applications. The architecture of the receiver is given in Fig. 1. Input GFSK modulated signal is first boosted by the low noise amplifier, and the mixers driven by quadrature clock signals down-convert it to the 2 MHz IF frequency. Before converted into digital signal for GFSK demodulation and further processing, the desired channel is selected by an integrated band pass complex filter (BPF). In addition, a received signal strength indicator is integrated, which allows auto-adjusting the receiver gain and therefore increases system dynamic range. This paper is divided into four sections. In section II, detailed implementations of the key modules were discussed. The complete receiver is designed in a standard CMOS 0.18um process, and the validation results are presented in section III. Finally, conclusions are given in section IV. Low-IF Receiver Design Fig. 1. Block diagram of the receiver. Radio frequency frontend The radio frequency (RF) frontend interfaces between the wireless media and the baseband signal processing. Therefore its characteristic plays a major role in the receiver overall performance. The RF frontend is composed of two components: the low noise amplifier (LNA) and the quadrature image rejection mixer (IRM). All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-19/02/16,15:15:40)

2 Applied Mechanics and Materials Vol As the first active element in the receiving chain, the LNA must be impedance- matched to 50 Ω-termination, and provide proper amplification of input signal within the desired frequency channel [3]. As presented in Fig.2, The LNA employs a cascode topology with source degenerated inductor. Such a topology can not only improve the LNA linearity, but provides isolation between its input and output, and therefore allows better matching property. At the LNA input, L1 (L2), C1 (C2) and C3 (C4) forms the input matching network, and capacitors (C5 and C6) at the output function as the DC blocks, which allows the following IRM to be biased separately. In addition, the tail current I b is designed to be tunable, allowing the gain and bandwidth of the LNA to be adjusted. The IRM implemented in this design is based on double-balanced Gilbert cells topology, as shown in Fig. 2. This topology can provide positive conversion gain, and when compared with its passive counterparts, requires smaller power from the local oscillator (LO). This design does not include the source degenerated components [4], and therefore it is optimized for conversion gain and noise figure, rather than the linearity. Resistive loading is chosen to generate a stale DC biase for the mixer. The completed IRM contains two identical mixer cells with four LO inputs separated by 90 º phase difference at the desired frequency. With the structure, the IRM is able to suppress the input image signal [5], and provides the quadrature outputs for the following stages to process. The simulation results of the RF frontend are presented in Fig. 3. (a) Fig. 2. Simplified schematic of the RF frontend: (b) (a) LNA, and (b) IRM. (a) Frequency and time response of the LNA. (b) Frequency and time response of the IRM. Fig. 3 Simulation results of the RF frontend: (a) LNA, and (b) IRM.

3 418 Advanced Technologies on Measure and Diagnosis, Manufacturing Systems and Environment Engineering Complex band-pass Filter A 4 th ordered complex band-pass filter with 2 MHz center frequency and tunable bandwidth is used in this design to select the desired channel for input data, as shown in Fig. 4. This complex filter is derived from an active-rc Butterworth low pass filter by introducing a frequency shift, which equals to 2 MHz, the filter center frequency, to the transfer function of the prototype low pass filter [6]. Totally eight identical operational amplifiers (OP_amp) are included in this filter design. In order to relief the gain-bandwidth product requirement in such a topology, error compensation technique is used by adding an additional zero to cancel the non-dominate pole of the OP-amp, and therefore reduces the overall filter consumption [7]. Fig. 4. Schematics of the complex BPF. To prevent the unwanted DC offset, which is generated from the LO leakage and self-mixing, to deviate the filter ideal operating conditions, at the filter inputs (I i+, I i-, Q i+, Q i-,), active inductance by positive feedback is also added to form a simplified high pass filter. The frequency response of the BPF, including the complex Butterworth, and the high pass filter, is shown in Fig. 5. Fig. 5 Frequency response of the 4 th order complex BPF.

4 Applied Mechanics and Materials Vol Analog-to-digital convertor In Bluetooth applications, data are modulated using GFSK. In this design, the input RF signal is down-converted, filtered, and converted into the digital domain. Received data can then be recovered by digital signal processing. In this design, a 8-bit successive approximation analog-to-digital convertor is utilized to complete this conversion, as shown in Fig.6. The transient response with the gaussian modulated input is shown in Fig.7. Fig. 6 Block diagram of the 8-bit ADC Design Validation Fig. 7 ADC transient response with the gaussian modulated input. The receiver was designed in a standard CMOS 0.18 µm process, occupies a 3.5 mm 2 die area. Validated using cadence and Spectre simulators, the main parameters are summarized in Table 1. The receiver consumption is about 15 mw with 1.8 V supply voltage. Table 1. The characteristics of the receiver Module Performance Gain Power Consumption LNA 9.04~15.59 db (Tunable) 1.31~2.49 mw mixer 14.7 db 4.23 mw BPF 15.53~31.42 db (Tunable) 3.12~5.34 mw ADC mw Conclusion This paper presents a low power receiver for Bluetooth applications. The low-if receiver down-converts the input 2.4 GHz RF signal into a 2 MHz intermediate frequency, and converts the resulting signal into digital signal for demodulation and further processing. The receiver is designed in a standard CMOS 0.18 µm process. The function of the receiver was validated using Spectre simulator. The receiver consumption is only 15 mw.

5 420 Advanced Technologies on Measure and Diagnosis, Manufacturing Systems and Environment Engineering Acknowledgement The authors would like to thank the National Science Foundation of China (NSFC) under grant No The work is also partly supported by the project S and S of Guangdong R/D Foundation, and the project KQC A, JCYJ and JCYJ of Shenzhen city. Reference [1] Bluetooth Specifications Version 1.0b. Information on [2] Weber, W.W. Si, S. Abdollahi-Alibeik, M. Lee, R. Chang, H. Dogan, S. Luschas and P. Husted: A single-chip CMOS radio SoC for v2.1 Bluetooth applications, Proc. IEEE Int. Solid-State Circuits Conf. Tech. Dig., pp (2008). [3] Ho Kwon Yoon: Multi-Standard Receiver for Bluetooth and WLAN Applications, MS. Thesis Dissetation, School of The Ohio State University (2004). [4] B. X. P.J. Sullivan and W. Ku: Low voltage performance of a microwave CMOS gilbert cell mixer, IEEE Journal of Solid-State Circuits, vol. 32, no. 7, pp (1997). [5] M. Pavio: Double balanced mixers using active and passive techniques, IEEE Trans. Microwave Theory & Tech., vol. 36, no. 12, pp (1988). [6] J. Crols and M. J. Steyaert: Low-IF Topologies for High-Performance Analog Front Ends of Fully Integrated Receivers, IEEE Transactions on Circuits an System-II: Analog and Digital Signal Processing, vol. 45, No. 3, pp (1998). [7] Gang Chen, Zhiqun Li, HaiYong Su, et al.: A 5 th -order Chebyshev active RC complex filter with automatic frequency tuning for wireless sensor networks application, International Symposium on Signal Systems and Electronics, pp.1-4 (2010).

6 Advanced Technologies on Measure and Diagnosis, Manufacturing Systems and Environment Engineering / A Low-IF 2.4 GHz Integrated RF Receiver for Bluetooth Applications /