Vol.31,No.7 Journal of Semiconductors July 2010 A low-phase-noise digitally controlled crystal oscillator for DVB TV tuners* Zhao Wei(赵薇)，Lu Lei(卢磊)，and Tang Zhangwen(唐长文)f (State Key Laboratory of ASIC System,Fudan University.Shanghai 201203,China) Abstract:This paper presents a 25-MHz fully-integrated digitally controlled crystal oscillator(DCXO)with automatic amplitude control(AAC).The DCXO is based on Colpitts topology for one-pin solution.The AAC circuit is introduced to optimize the phase noise performance.The automatic frequency control is realized by a 10-bit thermometer-code segmental tapered MOS capacitor array,ensuring a~35 ppm tuning range and~0.04 ppm frequency step.The measured phase noise results are-139 dBc/Hz at 1 kHz and-151 dBc/Hz at 10 kHz frequency offset,respectively.The chip consumes 1 mA at 1.8V supply and occupies 0.4 mm2 in a 0.18-um CMOS process. Key words:crystal oscillator,DCXO;phase noise;supply pushing;VCTCXO;VCXO D0:10.1088/1674-4926/31/7/075003 EEACC:1270 published in Refs.[2,3].But the bias current in Ref.[2]is gen- 1.Introduction erated by a bandgap voltage divided by a resistor and the ampli- tude cannot be controlled.Reference [3]has designed a clock Crystal oscillator plays an important role in modern com- munication systems for its precise oscillation frequency,low buffer to make the output swing changeable.However,the am- plitude at the oscillation node which is directly connected to the phase noise and low power.However,the oscillation frequency will drift with temperature as well as with the crystal aging.So crystal is not concerned. In this paper,a low-phase-noise DCXO with an automatic a frequency tuning circuit is necessary to adjust the frequency amplitude control loop implemented in a 0.18-um CMOS pro- to sub-ppm accuracy to cover a wide range of working tem- peratures over several years.Besides,low phase noise is also cess is presented.The AAC circuit is introduced to optimize the phase noise performance.An impedance boost technique is an important concern in tuner systems since the close-in phase noise of a PLL is dominated by the up-converted flicker noise employed to suppress the supply pushing effect and 1/f noise from the power supply.The AFC frequency tuning is achieved (1/f)of the crystal oscillator. by digitally switching a 10-bit thermometer-code MOS capac- The popular way to make the frequency of a crystal oscil- itor array to guarantee a monotonic frequency tuning charac- lator (XO)adjustable is to use a varactor embedded into an XO teristic and tune its voltage.However,this needs an extra ADC to con- vert the digital signal from the baseband to an analog voltage. 2.Basic crystal oscillator design It is a costly and inefficient way for power consumption and pin limitation concerns.A more efficient approach is to design One of the best known oscillator structures is the so called an XO that can be controlled by digital input and so is called three-point oscillator.Depending on which of the three points is DCXO ac grounded,the circuit is known as Colpitts,Pierce and Clapp A simplified scheme of a DCXO is shown in Fig.1.The oscillators.A Colpitts topology is preferred for it only requires DCXO is based on Colpitts topology for one-pin solution.Fre- one connection pin which is suitable with the pin-limited low- quency tuning is achieved by a digitally-controlled capacitance cost package[4] C2.To obtain a reasonable tuning range,the value of C2 may vary in a range of 10-50 pF.This large variation will impact the start-up constraints and bias current must be large to insure oscillation when all the capacitance of C2 is connected to the tank.Moreover,due to the PVT(Process,Voltage and Temper- ature)variation the oscillation amplitude will change a lot for a fixed bias current,thus degrading the phase noise performance. So a well defined amplitude of oscillation is desired to balance the phase noise and power consumption.The other reason why a well controlled amplitude is needed is that the power dissi- pated in the crystal is determined by the amplitude at the crystal node.A very high amplitude will speed up the crystal aging. DCXOs with a good temperature frequency stability are Fig.1.Basic DCXO scheme Project supported by the National Natural Science Foundation of China (No.60876019),the National S&T Major Project of China(No. 2009ZX0131-002-003-02),the Shanghai Rising-Star Program,China (No.09QA1400300),and the National Scientists and Engineers Service for Enterprise Program,China(No.2009GJC00046). Corresponding author.Email:zwtang@fudan.edu.cn Received 24 November 2009,revised manuscript received 2 March 2010 C2010 Chinese Institute of Electronics 075003-1Vol. 31, No. 7 Journal of Semiconductors July 2010 A low-phase-noise digitally controlled crystal oscillator for DVB TV tuners Zhao Wei(赵薇), Lu Lei(卢磊), and Tang Zhangwen(唐长文) Ž (State Key Laboratory of ASIC & System, Fudan University, Shanghai 201203, China) Abstract: This paper presents a 25-MHz fully-integrated digitally controlled crystal oscillator (DCXO) with automatic amplitude control (AAC). The DCXO is based on Colpitts topology for one-pin solution. The AAC circuit is introduced to optimize the phase noise performance. The automatic frequency control is realized by a 10-bit thermometer-code segmental tapered MOS capacitor array, ensuring a  35 ppm tuning range and  0:04 ppm frequency step. The measured phase noise results are –139 dBc/Hz at 1 kHz and –151 dBc/Hz at 10 kHz frequency offset, respectively. The chip consumes 1 mA at 1.8V supply and occupies 0.4 mm2 in a 0.18-m CMOS process. Key words: crystal oscillator; DCXO; phase noise; supply pushing; VCTCXO; VCXO DOI: 10.1088/1674-4926/31/7/075003 EEACC: 1270 1. Introduction Crystal oscillator plays an important role in modern com￾munication systems for its precise oscillation frequency, low phase noise and low power. However, the oscillation frequency will drift with temperature as well as with the crystal aging. So a frequency tuning circuit is necessary to adjust the frequency to sub-ppm accuracy to cover a wide range of working tem￾peratures over several years. Besides, low phase noise is also an important concern in tuner systems since the close-in phase noise of a PLL is dominated by the up-converted flicker noise (1/f / of the crystal oscillator. The popular way to make the frequency of a crystal oscil￾lator (XO) adjustable is to use a varactor embedded into an XO and tune its voltage. However, this needs an extra ADC to con￾vert the digital signal from the baseband to an analog voltage. It is a costly and inefficient way for power consumption and pin limitation concerns. A more efficient approach is to design an XO that can be controlled by digital input and so is called DCXOŒ1 . A simplified scheme of a DCXO is shown in Fig. 1. The DCXO is based on Colpitts topology for one-pin solution. Fre￾quency tuning is achieved by a digitally-controlled capacitance C2. To obtain a reasonable tuning range, the value of C2 may vary in a range of 10–50 pF. This large variation will impact the start-up constraints and bias current must be large to insure oscillation when all the capacitance of C2 is connected to the tank. Moreover, due to the PVT (Process, Voltage and Temper￾ature) variation the oscillation amplitude will change a lot for a fixed bias current, thus degrading the phase noise performance. So a well defined amplitude of oscillation is desired to balance the phase noise and power consumption. The other reason why a well controlled amplitude is needed is that the power dissi￾pated in the crystal is determined by the amplitude at the crystal node. A very high amplitude will speed up the crystal aging. DCXOs with a good temperature frequency stability are published in Refs. [2, 3]. But the bias current in Ref. [2] is gen￾erated by a bandgap voltage divided by a resistor and the ampli￾tude cannot be controlled. Reference [3] has designed a clock buffer to make the output swing changeable. However, the am￾plitude at the oscillation node which is directly connected to the crystal is not concerned. In this paper, a low-phase-noise DCXO with an automatic amplitude control loop implemented in a 0.18-m CMOS pro￾cess is presented. The AAC circuit is introduced to optimize the phase noise performance. An impedance boost technique is employed to suppress the supply pushing effect and 1/f noise from the power supply. The AFC frequency tuning is achieved by digitally switching a 10-bit thermometer-code MOS capac￾itor array to guarantee a monotonic frequency tuning charac￾teristic. 2. Basic crystal oscillator design One of the best known oscillator structures is the so called three-point oscillator. Depending on which of the three points is ac grounded, the circuit is known as Colpitts, Pierce and Clapp oscillators. A Colpitts topology is preferred for it only requires one connection pin which is suitable with the pin-limited low￾cost packageŒ4 . Fig. 1. Basic DCXO scheme. * Project supported by the National Natural Science Foundation of China (No. 60876019), the National S&T Major Project of China (No. 2009ZX0131-002-003-02), the Shanghai Rising-Star Program, China (No. 09QA1400300), and the National Scientists and Engineers Service for Enterprise Program, China (No. 2009GJC00046). Ž Corresponding author. Email: zwtang@fudan.edu.cn Received 24 November 2009, revised manuscript received 2 March 2010 c 2010 Chinese Institute of Electronics 075003-1