A TRANSFORMER-BASED X-BAND CMOS QUADRATURE VCO WITHOUT CURRENT SOURCES Sangsoo Ko, Taeksang Song, Euisik Yoon, and Songcheol Hong Department of Electrical Engineering Korea Advanced Institute of Science and Technology 373-1 Guseong-Dong, Yuseong-Gu Daejeon, 305-701, Korea Received 3 August 2004 ABSTRACT: An X-band quadrature voltage-controlled oscillator (QVCO) is fabricated using 0.18-m CMOS technology. The high im- pedance of a current source allows a highly effective Q-factor of the LC tank [1]. The proposed X-band CMOS quadrature VCO has low phase noise of -117.5 dB/Hz at 1-MHz offset frequency, despite the absence of a current source, because a transformer can separate the gate bias of series-connected transistors from cross-coupled RF feedback. The VCO can operate under 1.8 V of power supply. The ﬁgure of merit (FOM) is 187.5 dB at a 10.3-GHz oscillation frequency, which is the best value among X-band VCOs. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 305–307, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20618 Key words: transformer; CMOS; VCO; quadrature; current source 1. INTRODUCTION An accurate quadrature signal source is a prerequisite for image- rejection transceivers. The part is implemented with a single- output voltage-controlled oscillator (VCO) followed by quadrature generators, such as a frequency divider, and a polyphase ﬁlter. The former exhibits high-power consumption due to its circuit com- plexity. The latter needs additional buffers, since the passive network of the ﬁlter attenuates the VCO’s signals signiﬁcantly. In order to overcome the limitation of the above quadrature generators, many VCOs that can generate quadrature signals with- out any additional circuits have been proposed. One is a ring oscillator, which is implemented only in some optical-communi- cation circuits due to its poor noise property. Another topology, more suitable for high FOM, is the coupling of two symmetric LC-tank VCOs to each other. This topology is implemented suc- cessfully at less than 5 GHz [2]. However, above the X-band, this topology has been unavailable in CMOS integrated circuits due to the poor property of an integrated inductor. Although a distributed VCO (DVCO) is one attractive solution for the CMOS process, it demands a larger area than a VCO with an LC tank. To improve the Q-factor of an integrated inductor for the CMOS process, a transformer is used in a QVCO [3]. Two coupled LC tanks in a transformer can improve the Q-factor of a resonator. Moreover, the transformer separates the gate biases of series-connected tran- sistors from cross-coupled RF signals [3]. Generally, a differential VCO uses a current source to start up reliably and to prevent the large output swing from increasing phase noise [1]. On the other hand, a VCO without a current source can operate under low power supply. The current source in the proposed QVCO can be removed while the VCO maintains low noise under low power supply, due to the aforementioned trans- former characteristics. In section 2, we explain the topology and noise-reduction mechanism of the proposed transformer-based QVCO. In section 3, the measurement results are presented. 2. DESIGN OF THE VCO In general, a quadrature VCO is composed of two differential LC-tank VCOs, as shown in Figure 1. Because the G m cell of the differential VCOs are derived from both in-phase and quadrature signals [2], each differential VCO needs at least four transistors. In Figure 1(a), Q SW and Q CPL are derived from the in-phase and quadrature signals, respectively. The current of transistors I TR is almost controlled by the in-phase signal. The quadrature signal is used only to couple two differential VCOs. Thus, it is not neces- sary for Q CPL to have high gain. A transformer-based VCO has two LC tanks. One is composed of L 1 and C 1 in the drain side, and the other of L 2 and C 2 in the gate side. When the each LC tank is coupled by coupling-coefﬁ- cient k , the resonator in the VCO improves the Q-factor by a factor of 1 + k [3]. Moreover, the DC bias of each LC tank can be separated, while RF signals are coupled. The Vg can tune the gate of Q SW without changing V CC through L 2 . Although V g connected to only gate of Q SW , the bias of Q CPL is more sensitive than that of Q SW for variations of V g . The gain, current, and noise of Q CPL are decreased as V g is decreased. If the tail current is ﬁxed by a current source, the bias is not highly affected by the V g variations. Flicker noise in a transistor is a key noise source of the VCO phase noise. Since it is proportional to the drain current, the phase noise is degraded as the number of transistors is increased at a ﬁxed bias current. Generally, the quadrature VCO, having more transistors, was thought to have worse phase noise than a differ- ential VCO. However, this is valid only when all transistors remain in the saturation region. In the triode region, the drain is directly connected to source without a high E-ﬁeld region in the channel. This reduces electrons to be scattered on the oxide surface and accordingly decrease ﬂicker noise. On the other hand, the gain in the triode region is too small to be used as that of an ampliﬁer. Figure 1 Schematic of the proposed transformer-based quadrature VCO Figure 2 Photograph of the fabricated quadrature VCO (1100  680 m 2 ) MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 44, No. 4, February 20 2005 305