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ISIC 2009 A 330μW, 1.7GHz locking range LC Injection Locked Frequency Divider in CMOS Technology

Xuepo Ma, Wei Zhang, Xu Zhang School of Electronic Information Engineering, Tianjin University Tianjin, China PR Email: tjuzhangwei@tju.edu.cn

Abstract—An LC injection locked frequency divider (ILFD) in CMOS technology is designed. By adopting the direct injection of signal into the resonation tank, the locking range of the ILFD is improved. For the purpose of reducing the power consumption, the complimentary topology is adopted in this work and also the voltage swing of the ILFD is optimized. The simulation results indicate that the ILFD has a wide locking range from 9.3GHz to 11GHz, and the core of the ILFD consumes 100μA under 3.3V power supply.

Keywords—ILFD, high speed divider, low power consumption.

I. INTRODUCTION High speed frequency dividers are key components in telecommunication systems, since the speed of these systems continues to increase [1]. Also, driven by the market of portable devices, the frequency divider with low power dissipation draws more and more research attention. To fulfill the demands of circuit performance and to achieve higher speed as well as lower power consumption, CML circuit with low voltage swing was proposed. However, as the operation frequency continues to increase, especially in multi-gigahertz applications, CML divider consumes several or tens of mini watts power which can not be ignored [2]. In this case, many investigators pay their attention to the injection locked frequency divider which is in nature a kind of power-saving and high speed circuit [3-6]. In this paper, a 10GHz, 330μW, 1.7GHz locking range LC ILFD based on CMOS technology is presented which aims to work in high speed and low power consumption applications.

(a) Direct injection

(b) Conventional Fig.1. Topology of LC ILFD

II. DESIGN CONSIDERATIONS The ILFD is based on a complimentary CMOS LC injection oscillator, as shown in Fig.1 (a). The cross couple pairs providing negative resistance and maintaining oscillation consist of four transistors, M1, M2 and M4, M3. Transistor M6 is the current mirror. For the purpose of preventing frequency drifting caused by supply voltage variation, the current mirror is placed at the supply end. Because the bias current is used by both PMOS cross couple pair and NMOS cross couple pair at same time, the cross couple pairs provide more transconductance to maintain the oscillation, compared with NMOS or PMOS only topology, and thus the power consumption is reduced. Also, the transistor gate parasitic capacitor is used in the tank instead of other on-chip capacitors.

In high frequency applications, the designer wishes that parasitic capacitance of the input stage of a circuit to be as small as possible, especially in the ILFD. In the conventional topology as shown in Fig.1 (b), the injection signal is injected into the tank through the tail transistor which is usually laid out with large width to provide large bias current for the circuit. Therefore, the parasitic capacitance of this transistor is very large. The injection signal vanishes through the parasitic capacitor and the injection efficiency is reduced leading to

609 decrease of the locking range. By using the shunt peaking inductor, designer can reduce the effect of parasitic capacitance at the cost of more chip area. In Reference [7], the direct injection technology is proposed, as shown in Fig.2. Because the added transistor M5 does not have the function of providing bias current, it is often laid out small and the parasitic capacitance is reduced. And the locking range is improved.

Fig.2. Equivalent circuit of the resonation tank Power Consumption: Without the injection signal, the ILFD shown in Fig.1 (a) is essentially an LC oscillator. Assuming the cross couple pairs are hard switches and the LC tank of the oscillator can be simplified as an RLC network, the current varies with the time as shown in Fig.2. The higher harmonics would get filtered out by the resonation circuit, and then the amplitude of the tank would be (4/3)πIR, in which I represents the bias current and R represents the parallel equivalent resistor of the tank. When the operation frequency is high, the current would become more sinusoid-like, and the amplitude of the tank will be simply IR.

As for an LC oscillator, the power consumption is dominated by the losses in the tank [8]: