928 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES.VOL.57.NO.4.APRIL 2009 An 18-mW 1.175-2-GHz Frequency Synthesizer With Constant Bandwidth for DVB-T Tuners Lei Lu,Student Member,IEEE,Jinghong Chen,Senior Member,IEEE, Lu Yuan,Hao Min,and Zhangwen Tang,Member;IEEE Abstract-A fully integrated 1.175-2-GHz differentially tuned-87 dBc/Hz at a 10-kHz offset [2].In addition,because of the frequency synthesizer aimed for digital video broadcasting-ter- very wide frequency range,the synthesizer loop bandwidth, restrial tuners is implemented in a 0.18-um CMOS process. which affects the phase-noise optimization and loop stability. To maintain phase-noise optimization and loop stability over the entire output frequency range,techniques of constant loop may vary quite significantly due to two reasons.Firstly,to bandwidth are proposed.The voltage-controlled oscillator gain cover such a wideband frequency range and achieve a rela- Kvco and band step fr are both maintained by simultaneously tively low voltage-controlled oscillator (VCO)gain (Kvco),a adjusting the sizes of switched capacitors and varactors.Charge switched capacitor array is usually employed in the LC VCO. pump current Icp is programmed to compensate the variation of the division ratio N.The measured results show an in-band phase However,even with a switched capacitor bank,the VCO gain noise of-97.6 dBc/Hz at a 10-kHz offset and an integrated phase variation is still huge.The phase noise is degraded as the VCO error of 0.630 from 100 Hz to 10 MHz.The measured variations of gain increases [3].Secondly,a large range of division ratio Kvco and fres are less than 12.5%and 4.5%,respectively.The N is required to obtain the wide tuning range of nearly one variations of the measured phase noise at 10-kHz and 1-MHz fre- octave.The N variation also changes the loop bandwidth,thus quency offsets are less than 1 dB.The measured 3-dB closed-loop bandwidth is 110 kHz and the variation is less than 9%.The chip impacting the phase noise and loop stability. draws 10-mA current from a 1.8-V supply while occupying a To achieve a constant loop bandwidth,several methods have 2.2-mm2 die area. been previously proposed [4],[5].In [4],natural frequency wn Index Terms-Frequency synthesizer,loop bandwidth,loop gain, is used as a definition of the loop bandwidth.To handle the large phase noise,voltage-controlled oscillator(VCO)gain,wideband. division ratio range,a sampled loop filter (LPF)network is con- structed and an inverse-linear charge pump(CP)is used to keep the natural frequency over reference frequency (wn/wref)and I.INTRODUCTION the damping factor constant.Though the natural frequency HE DIGITAL video broadcasting-terrestrial (DVB-T)and damping factor are widely used in the analysis of synthe- standard has ushered in a new era in TV entertainment. sizer loop dynamic,strictly speaking,they are applicable only Often the DVB-T tuners employ a double-conversion zero-IF to second-order loops.The inverse-linear CP also adds design (DZIF)architecture,which demands the use of a wideband complexity.Reference [5]uses open-loop crossover frequency frequency synthesizer as the first local oscillator (LO)(LO)We as the loop bandwidth and adjusts only the CP current(ICp) to cover the 48-862-MHz-wide frequency range and a frac- to compensate the Kvco and N variations.Although the open- tional-N frequency synthesizer as the second LO (LO2)to loop crossover frequency is often used to define a loop band- support several channel bandwidths(6/7/8 MHz)[1],[2]. width,it cannot accurately capture the low-pass corner charac- There are many challenges in designing a wideband fre-teristics of the closed-loop transfer function.In addition,though quency synthesizer LO1.It must achieve a wide frequency adjusting Icp alone can achieve a constant loop bandwidth,a tuning range,while providing low phase noise and low in-multiband VCO with constant and smaller gains is still desired tegrated phase error.The wideband synthesizer LO needs in order to improve the phase-noise performance. to meet a stringent phase-noise requirement over the entire In this paper,the concept of loop gain is used as the loop frequency range,which should be larger than the input range bandwidth to accurately model the low-pass characteristics of of 814 MHz.An adequate target for the overall phase noise is the closed-loop transfer function.Techniques achieving a con- stant and smaller VCO gain Kvco while obtaining a constant Manuscript received July 31,2008:revised December 19,2008.First loop bandwidth are proposed [6].Both a constant Kvco and published March 10,2009:current version published April 08.2009.This an equal band step fres are obtained in a multiband VCO.Au- work was supported in part by the National 863 Program of China under Grant 2007AA01Z282 and the National Science Funding of China under Grant tomatic frequency control (AFC)is used to ensure the opera- 60876019. tion of VCO in the linear region of the tuning range.The VCO L.Lu,L.Yuan,H.Min,and Z.Tang are with the Application Specific In- tegrated Circuit (ASIC)and System State Key Laboratory.Fudan University, with equal band steps greatly helps to simplify the AFC loop de- Shanghai 201203,China (e-mail:zwtang@fudan.edu.cn). sign.Programmable CP current is also adopted to compensate J.Chen is with Analog Devices Inc.,Somerset,NJ 08873 USA (e-mail: the variation of the division ratio N. jinghong@ieee.org). Color versions of one or more of the figures in this paper are available online The remainder of this paper is organized as follows.In at http://ieeexplore.ieee.org. Section II,design issues of the frequency synthesizer are con- Digital Object Identifier 10.1109/TMTT.2009.2014449 sidered and the loop gain is derived.The KVCo variation in a 0018-9480/s25.00©2009EEE928 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 4, APRIL 2009 An 18-mW 1.175–2-GHz Frequency Synthesizer With Constant Bandwidth for DVB-T Tuners Lei Lu, Student Member, IEEE, Jinghong Chen, Senior Member, IEEE, Lu Yuan, Hao Min, and Zhangwen Tang, Member, IEEE Abstract—A fully integrated 1.175–2-GHz differentially tuned frequency synthesizer aimed for digital video broadcasting-ter￾restrial tuners is implemented in a 0.18- m CMOS process. To maintain phase-noise optimization and loop stability over the entire output frequency range, techniques of constant loop bandwidth are proposed. The voltage-controlled oscillator gain ￾ and band step  are both maintained by simultaneously adjusting the sizes of switched capacitors and varactors. Charge pump current  is programmed to compensate the variation of the division ratio . The measured results show an in-band phase noise of 97.6 dBc Hz at a 10-kHz offset and an integrated phase error of 0.63 from 100 Hz to 10 MHz. The measured variations of ￾ and  are less than 12.5% and 4.5%, respectively. The variations of the measured phase noise at 10-kHz and 1-MHz fre￾quency offsets are less than 1 dB. The measured 3-dB closed-loop bandwidth is 110 kHz and the variation is less than 9%. The chip draws 10-mA current from a 1.8-V supply while occupying a 2.2-mm die area. Index Terms—Frequency synthesizer, loop bandwidth, loop gain, phase noise, voltage-controlled oscillator (VCO) gain, wideband. I. INTRODUCTION T HE DIGITAL video broadcasting-terrestrial (DVB-T) standard has ushered in a new era in TV entertainment. Often the DVB-T tuners employ a double-conversion zero-IF (DZIF) architecture, which demands the use of a wideband frequency synthesizer as the first local oscillator (LO) to cover the 48–862-MHz-wide frequency range and a frac￾tional- frequency synthesizer as the second LO to support several channel bandwidths (6/7/8 MHz) [1], [2]. There are many challenges in designing a wideband fre￾quency synthesizer . It must achieve a wide frequency tuning range, while providing low phase noise and low in￾tegrated phase error. The wideband synthesizer needs to meet a stringent phase-noise requirement over the entire frequency range, which should be larger than the input range of 814 MHz. An adequate target for the overall phase noise is Manuscript received July 31, 2008; revised December 19, 2008. First published March 10, 2009; current version published April 08, 2009. This work was supported in part by the National 863 Program of China under Grant 2007AA01Z282 and the National Science Funding of China under Grant 60876019. L. Lu, L. Yuan, H. Min, and Z. Tang are with the Application Specific In￾tegrated Circuit (ASIC) and System State Key Laboratory, Fudan University, Shanghai 201203, China (e-mail: zwtang@fudan.edu.cn). J. Chen is with Analog Devices Inc., Somerset, NJ 08873 USA (e-mail: jinghong@ieee.org). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2009.2014449 87 dBc Hz at a 10-kHz offset [2]. In addition, because of the very wide frequency range, the synthesizer loop bandwidth, which affects the phase-noise optimization and loop stability, may vary quite significantly due to two reasons. Firstly, to cover such a wideband frequency range and achieve a rela￾tively low voltage-controlled oscillator (VCO) gain , a switched capacitor array is usually employed in the VCO. However, even with a switched capacitor bank, the VCO gain variation is still huge. The phase noise is degraded as the VCO gain increases [3]. Secondly, a large range of division ratio is required to obtain the wide tuning range of nearly one octave. The variation also changes the loop bandwidth, thus impacting the phase noise and loop stability. To achieve a constant loop bandwidth, several methods have been previously proposed [4], [5]. In [4], natural frequency is used as a definition of the loop bandwidth. To handle the large division ratio range, a sampled loop filter (LPF) network is con￾structed and an inverse-linear charge pump (CP) is used to keep the natural frequency over reference frequency and the damping factor constant. Though the natural frequency and damping factor are widely used in the analysis of synthe￾sizer loop dynamic, strictly speaking, they are applicable only to second-order loops. The inverse-linear CP also adds design complexity. Reference [5] uses open-loop crossover frequency as the loop bandwidth and adjusts only the CP current to compensate the and variations. Although the open￾loop crossover frequency is often used to define a loop band￾width, it cannot accurately capture the low-pass corner charac￾teristics of the closed-loop transfer function. In addition, though adjusting alone can achieve a constant loop bandwidth, a multiband VCO with constant and smaller gains is still desired in order to improve the phase-noise performance. In this paper, the concept of loop gain is used as the loop bandwidth to accurately model the low-pass characteristics of the closed-loop transfer function. Techniques achieving a con￾stant and smaller VCO gain while obtaining a constant loop bandwidth are proposed [6]. Both a constant and an equal band step are obtained in a multiband VCO. Au￾tomatic frequency control (AFC) is used to ensure the opera￾tion of VCO in the linear region of the tuning range. The VCO with equal band steps greatly helps to simplify the AFC loop de￾sign. Programmable CP current is also adopted to compensate the variation of the division ratio . The remainder of this paper is organized as follows. In Section II, design issues of the frequency synthesizer are con￾sidered and the loop gain is derived. The variation in a 0018-9480/$25.00 © 2009 IEEE