460 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 6, JUNE 2007 A Wide Band Injection Locked Frequency Divider With Variable Inductor Load Yun-Hsueh Chuang, Sheng-Lyang Jang, Senior Member, IEEE, and Shao-Hwa Lee Abstract—This letter proposes a new wide band CMOS injection locked frequency divider (ILFD). The circuit is made of a two-stage differential CMOS ring oscillator and is based on MOS switches directly coupled to the differential outputs of the ring oscillator. A tuning circuit composed of inductors in series with a metal oxide semiconductor field effect transistor is used to extend the locking range. The divide-by-two ILFD can provide wide locking range and the measured results show that at the supply voltage of 1.8 V, the free-running frequency of the ILFD is operating from 0.92 to 3.6 GHz while the Vtune is tuned from 0 to 1.8 V. At the incident power of 0 dBm, this ILFD has a wide locking range from 1.15 to 7.4 GHz. Index Terms—CMOS, direct injection-locked frequency divider (ILFD), ring oscillator, transformer. I. INTRODUCTION A FREQUENCY divider is widely used in high-speed sys- tems such as frequency synthesizers and used for quadra- ture signal generation. The harmonic-injection dividers basi- cally consist of an oscillator, whose harmonic component of order synchronizes with the input signal. The resonator based injection-locked frequency divider (ILFD) [1] is often used because the operating frequency can be very high. How- ever, it usually has limited locking bandwidth. In this letter, we present a divide-by-2 ILFD with a wide-band locking range. This circuit is made up of a two-stage differen- tial ring oscillator with the direct injection topology. The load of the ring oscillator consists of an inherent inductorless load and a tunable load composed of two inductors in series with a MOSFET, the latter is used to vary the net load of ring oscil- lator and increase the locking range of ILFD. In the following sections, the concept of the proposed circuit is explained and the measured results are also presented. II. CIRCUIT DESIGN The circuit is designed using a two-stage differential ring os- cillator [2], which consists of two delay cells for power con- sumption and phase noise minimization. The diagram of the pro- posed circuit is shown in Fig. 1(a) where two amplifier stages are connected in a ring with a crossover to implement a phase shift of 180 for oscillation. To achieve a wide band function, a tuning circuit is used to change the oscillation frequency. The Manuscript received November 20, 2006; revised February 26, 2007. This work was supported by the National Science Council of Taiwan, R.O.C., under Contract NSC 95-2221-E-011-173. The authors are with the Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, R.O.C. (e-mail: sljjj@mail.ntust.edu.tw; d9002302@mail.ntust.edu.tw). Digital Object Identifier 10.1109/LMWC.2007.897848 Fig. 1. (a) Diagram of the proposed dual band ILFD and (b) complete schematic of the proposed dual band ILFD. tuning circuit consists of NMOS transistors ( ) and induc- tors ( – ) which can be formed by a transformer to occupy less chip area. Fig. 1(b) shows the detailed schematic of the proposed ILFD. The complementary cross-coupled MOS pairs are used as the delay cell. The delay cell consists of one NMOS input pair ( ), one PMOS input pair (Mp1), one NMOS positive feed- back pair ( ) and one PMOS positive feedback pair ( ) for maintaining oscillating. The Vinj is the gate voltage of the injector. The tuning circuit is placed between the differential output nodes of two differential delay cells to tune the oscilla- tion frequency. When the Vtune is biased at 0 V, the transistors are at off state, and the inductors are floating. Therefore, the frequency of the ring oscillator is independent on the induc- tors. But as the Vtune is biased higher than the threshold voltage of the transistors , the FETs are operated at linear re- gion, and they can be treated as variable resistors controlled by the Vtune. Thus, the frequency of the ring oscillator changes as the loads at the output nodes vary with the Vtune. The phase relationships between the outputs of the ILFD are unique, be- cause the four complementary MOS pairs ( ) form a ring oscillator [3] and provide a determined phase relationship between outputs. To understand the operation function of wide band ring os- cillator, the half circuit of Fig. 1(b) is used to explain the circuit operating principle. As shown in Fig. 2(a), the symbols In and 1531-1309/$20.00 © 2007 IEEE