IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 44, NO. 10, OCTOBER 2009 2655 SiGe Bipolar VCO With Ultra-Wide Tuning Range at 80 GHz Center Frequency Nils Pohl, Student Member, IEEE, Hans-Martin Rein, Fellow, IEEE, Thomas Musch, Member, IEEE, Klaus Aufinger, and Josef Hausner, Member, IEEE Abstract—A SiGe millimeter-wave VCO with a center frequency around 80 GHz and an extremely wide (continuous) tuning range of 24.5 GHz ( 30%) is presented. The phase noise at 1 MHz offset is dBc/Hz at the center frequency (and less than dBc/Hz in a frequency range of 21 GHz). The maximum total output power is about 12 dBm. A cascode buffer improves decoupling from the output load at reasonable VCO power consumption (240 mW at 5 V supply voltage). A low-power frequency divider (operating up to 100 GHz) provides, in addition, a divided-by-four signal. As a further intention of this paper, the basic reasons for the limita- tion of the tuning range in millimeter-wave VCOs are shown and the improvement by using two (instead of one) varactor pairs is demonstrated. Index Terms—Millimeter-wave VCOs, regenerative frequency divider, SiGe bipolar ICs, ultra-wideband, voltage-controlled oscillators. I. INTRODUCTION I N THE LAST few years, SiGe bipolar technologies proved to be well suited for voltage controlled oscillators (VCO) in the millimeter-wave region, e.g., in long-range 77 GHz auto- motive radar systems. Some other applications require a wider tuning range , in which the VCO frequency must be con- tinuously adjustable by a tuning voltage. For example, the spec- ified of future automotive short-range radar systems at 79 GHz is 4 GHz [1]. For other applications, like high-preci- sion radar systems for industrial applications, even higher (e.g., GHz) is aimed at to achieve improved accuracy. The demand for VCOs with wide tuning range in high-resolu- tion radar systems is demonstrated in Fig. 1. In Fig. 1(a), a sim- ulated impulse response of a radar system with GHz is shown. The tuning range is directly related to the target res- olution. The spurious signal, reflected by a disturbing object 10 cm behind the target to be detected, cannot be separated from the signal of interest, thus reducing the measurement ac- curacy. However, with a wider tuning range, e.g., of 10 GHz as in Fig. 1(b), both signals can be separated. It should be mentioned that the typical values of have usually to be substantially higher than the specified ones. This is because design tolerances, production spread, temperature vari- Manuscript received January 09, 2009; revised May 28, 2009. Current ver- sion published September 28, 2009. This paper was approved by Guest Editors William Peatman and Erik Daniel. N. Pohl, H.-M. Rein, T. Musch, and J. Hausner are with the Ruhr-Universität Bochum, D-44780 Bochum, Germany (e-mail: Nils.Pohl@is.rub.de). K. Aufinger is with Infineon Technologies AG, D-85579 Neubiberg, Germany. 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/JSSC.2009.2026822 Fig. 1. Simulations of the impulse response of a radar system using different tuning ranges : (a) 4 GHz and (b) 10 GHz. The reflecting target of interest is about 1 m apart from the antenna. ation, and the degradation of some important VCO parameters (e.g., phase noise and output power) at both edges of the tuning range reduce the useable . While the first two effects can be roughly compensated by cutting some metal links (fuses) on the chip after fabrication (cp. [2], [3]), the other ones require continuous frequency variation by a single tuning voltage (e.g., in a PLL). VCOs with center frequencies around 75–80 GHz, typical for applications in automotive radar systems, are mostly real- ized in SiGe bipolar or BICMOS technologies (e.g., [3]–[6] ) since low phase noise and higher output power can be obtained more easily compared to CMOS technologies (cp. [7]). Rela- tive tuning ranges up to 6–9% of center frequency are achieved by use of collector-base junctions or MOS structures as a var- actor. A much wider tuning range of 21% is claimed in [8] for a push-push VCO with a MOS varactor, however at very low output power. The phase noise is given only at center frequency dBc/Hz , so that the usable tuning range, which can be phase noise limited, is not evident. With an InP DHBT based VCO, a tuning range of 13% is achieved at a center frequency of 94 GHz and moderate phase noise ( to dBc/Hz) [9]. Up to now, the SiGe bipolar VCO presented in [10] shows the best overall performance with a wide tuning range at high output 0018-9200/$26.00 © 2009 IEEE Authorized licensed use limited to: Ruhr Universitat Bochum. Downloaded on September 29, 2009 at 09:16 from IEEE Xplore. Restrictions apply.