Vol.30,No.11 Journal of Semiconductors November 2009 An eighth order channel selection filter for low-IF and zero-IF DVB tuner applications* Zou Liang(邹亮)'，Liao Youchun(廖友春)2，and Tang Zhangwent(唐长文)l,t (I ASIC System State Key Laboratory,Fudan University,Shanghai 201203,China) (2 Ratio Microelectronics Technology Co.Ltd,Shanghai 200433,China) Abstract:An eighth order active-RC filter for low-IF and zero-IF DVB tuner applications is presented,which is implemented in Butterworth biquad structure.An automatic frequency tuning circuit is introduced to compensate the cut-off frequency variation using a 6-bit switched-capacitor array.Switched-resistor arrays are adopted to cover different cut-off frequencies in low-IF and zero-IF modes.Measurement results show that precise cut-off frequencies at 2.5,3.3.5 and 4 MHz in zero-IF mode,5,6,7 and 8 MHz in low-IF mode can be achieved,60 dB frequency attenuation can be obtained at 20 MHz,and the in-band group delay agrees well with the simulation.Two-tone testing shows the in-band IM3 achieves-52 dB and the out-band IM3 achieves-55 dB with-11 dBm input power. This proposed filter circuit,fabricated in a SMIC 0.18 um CMOS process,consumes 4 mA current with 1.8 V power supply. Key words:active-RC filter;Butterworth;frequency tuning;group delay;noise;linearity D0:10.1088/16744926/30/11/115002 EEACC:1270 1.Introduction 2.System requirements The specification of an analog filter can be summarized In digital video broadcasting (DVB)tuner systems, as two parts:the first part is the ACR,which includes the cut- shown in Fig.1,wide bandwidth and high linearity require- off frequency and frequency attenuation;the second part is ments make analog-to-digital converters (ADC)difficult to im- the error vector magnitude(EVM)loss,which includes the in- plement.To obtain a good adjacent channel rejection(ACR) before ADC,a high-order analog filter is adopted to achieve band ripple,group delay,noise and linearity.The in-band rip- good attenuation and an active-RC architecture is selected to ple and group delay determine the quality of the signal transfer function,and the noise and linearity determine the EVM loss achieve high linearity.The cut-off frequency of an integrated caused by the analog filter itself.Here,the critical system re- active-RC filter is determined by on-chip resistors and capaci- tors which may vary greatly with the process,voltage and tem- quirements will be discussed. perature (PVT).Thus,an automatic frequency tuning (AFT) 2.1.Noise and linearity circuit should be engaged to calibrate the cut-off frequency variation. In DVB tuner receivers,noise figure (NF)and linearity are critical performance parameters,which affect the system For DVB-T/H protocols,different signal bandwidths, signal-to-noise ratio(SNR).The definition of sensitivity is: such as 5,6.7 and 8 MHz,have been defined.To cover all these signal bandwidths,a programmable channel selection Pin.min KT +NF 101g B+SNRmin. (1) filter with switched-capacitor arrays and switched-resistor ar- rays is proposed in this paper.In zero-IF mode,the channel To meet the minimum SNR requirement, selection filter covers the cut-off frequencies of 2.5.3,3.5 and 4 MHz.Halving the switched-capacitor value automatically, NF Pin.min KT -101g B-SNRmin. (2) this filter can also be switched to low-IF mode,in which the Linearity can be defined in different forms,such as IIP3, corresponding cut-off frequencies are 5,6,7 and 8 MHz re- PidB,composite second order distortion(CSO)and compos- spectively. ite triple beat distortion(CTB).Because CSO,CTB and PidB This paper also illustrates the system requirements of have a direct relationship with IIP,only IIP will be con- noise and linearity in detail,shows the circuit design,including sidered here.In DVB tuner systems,the adjacent interferences the selection of biquad structures,amplifier design and AFT become a bottleneck of the linearity requirement;DVB-T/H tuning circuit,and proposes critical design insights to mini- protocols clearly show that adjacent channels may be 40 dB mize the non-ideal factors which will affect tuning precision. larger than the desired channel.To assure enough SNR,the Project supported by the National High Technology Research and Development Program of China (No.2007AA01Z282). Corresponding author.Email:zwtang@fudan.edu.cn Received 5 May 2009,revised manuscript received 26 May 2009 2009 Chinese Institute of Electronics 115002-1Vol. 30, No. 11 Journal of Semiconductors November 2009 An eighth order channel selection filter for low-IF and zero-IF DVB tuner applications∗ Zou Liang(邹亮) 1 , Liao Youchun(廖友春) 2 , and Tang Zhangwen(唐长文) 1, † (1 ASIC & System State Key Laboratory, Fudan University, Shanghai 201203, China) (2 Ratio Microelectronics Technology Co, Ltd, Shanghai 200433, China) Abstract: An eighth order active-RC filter for low-IF and zero-IF DVB tuner applications is presented, which is implemented in Butterworth biquad structure. An automatic frequency tuning circuit is introduced to compensate the cut-off frequency variation using a 6-bit switched-capacitor array. Switched-resistor arrays are adopted to cover different cut-off frequencies in low-IF and zero-IF modes. Measurement results show that precise cut-off frequencies at 2.5, 3, 3.5 and 4 MHz in zero-IF mode, 5, 6, 7 and 8 MHz in low-IF mode can be achieved, 60 dB frequency attenuation can be obtained at 20 MHz, and the in-band group delay agrees well with the simulation. Two-tone testing shows the in-band IM3 achieves –52 dB and the out-band IM3 achieves –55 dB with –11 dBm input power. This proposed filter circuit, fabricated in a SMIC 0.18 µm CMOS process, consumes 4 mA current with 1.8 V power supply. Key words: active-RC filter; Butterworth; frequency tuning; group delay; noise; linearity DOI: 10.1088/1674-4926/30/11/115002 EEACC: 1270 1. Introduction In digital video broadcasting (DVB) tuner systems, shown in Fig. 1, wide bandwidth and high linearity require￾ments make analog-to-digital converters (ADC) difficult to im￾plement. To obtain a good adjacent channel rejection (ACR) before ADC, a high-order analog filter is adopted to achieve good attenuation and an active-RC architecture is selected to achieve high linearity. The cut-off frequency of an integrated active-RC filter is determined by on-chip resistors and capaci￾tors which may vary greatly with the process, voltage and tem￾perature (PVT). Thus, an automatic frequency tuning (AFT) circuit should be engaged to calibrate the cut-off frequency variation. For DVB-T/H protocols, different signal bandwidths, such as 5, 6, 7 and 8 MHz, have been defined. To cover all these signal bandwidths, a programmable channel selection filter with switched-capacitor arrays and switched-resistor ar￾rays is proposed in this paper. In zero-IF mode, the channel selection filter covers the cut-off frequencies of 2.5, 3, 3.5 and 4 MHz. Halving the switched-capacitor value automatically, this filter can also be switched to low-IF mode, in which the corresponding cut-off frequencies are 5, 6, 7 and 8 MHz re￾spectively. This paper also illustrates the system requirements of noise and linearity in detail, shows the circuit design, including the selection of biquad structures, amplifier design and AFT tuning circuit, and proposes critical design insights to mini￾mize the non-ideal factors which will affect tuning precision. 2. System requirements The specification of an analog filter can be summarized as two parts: the first part is the ACR, which includes the cut￾off frequency and frequency attenuation; the second part is the error vector magnitude (EVM) loss, which includes the in￾band ripple, group delay, noise and linearity. The in-band rip￾ple and group delay determine the quality of the signal transfer function, and the noise and linearity determine the EVM loss caused by the analog filter itself. Here, the critical system re￾quirements will be discussed. 2.1. Noise and linearity In DVB tuner receivers, noise figure (NF) and linearity are critical performance parameters, which affect the system signal-to-noise ratio (SNR). The definition of sensitivity is: Pin, min = KT + NF + 10 lg B + SNRmin. (1) To meet the minimum SNR requirement, NF < Pin, min − KT − 10 lg B − SNRmin. (2) Linearity can be defined in different forms, such as IIP3, P1dB, composite second order distortion (CSO) and compos￾ite triple beat distortion (CTB). Because CSO, CTB and P1dB have a direct relationship with IIP3 [1], only IIP3 will be con￾sidered here. In DVB tuner systems, the adjacent interferences become a bottleneck of the linearity requirement; DVB-T/H protocols clearly show that adjacent channels may be 40 dB larger than the desired channel. To assure enough SNR, the ∗ Project supported by the National High Technology Research and Development Program of China (No. 2007AA01Z282). † Corresponding author. Email: zwtang@fudan.edu.cn Received 5 May 2009, revised manuscript received 26 May 2009 ⃝c 2009 Chinese Institute of Electronics 115002-1