- Buyers Guide
Design of a Low Phase Noise Ku-band Oscillator Using a SiGe HBT
Myrianne Regis, Hugues Lafontaine and Steve Kovacic
Ottawa, ON, Canada
Gilles Cibiel and Olivier Llopis
In the past decade, the demand for microwave, millimeter-wave and wireless communication systems has rapidly increased. Over the same period, the semiconductor technology for such systems has provided a progressive improvement in cost/performance ratio for the most important subsystems, including amplifiers, oscillators and mixers. For example, local multipoint distribution service (LMDS), or local multipoint communication systems (LMCS), as the technology is known in Canada, is positioned to allow network integrators and communication service providers to quickly bring high value, quality data services to homes and businesses using two-way radio technology. LMDS technology includes high speed Internet access, real-time multimedia file transfer, remote access to corporate local area networks, interactive video, video-on-demand, video conferencing and telephony among other potential applications.
In the United States, LMDS uses 1.3 GHz of RF spectrum and this roughly translates to a 1 Gbps digital data transmission. Effectively, this technology can provide broadband access to customers without the need to dig trenches and lay cable. In many respects, LMDS is a low cost and rapid deployment alternative to other broadband data communication systems.
Running in broadband applications, these LMDS and other radios using a high frequency carrier require very low noise microwave sources. Indeed, for LMDS, the oscillator phase noise must be lower than -105 dBc/Hz at 10 kHz offset frequency in Ku-band. Dielectric resonator oscillators (DRO) or coaxial resonator oscillators (CRO) can exceed this performance when a low flicker noise active device is used in conjunction with a microphonic free design.
Among the different existing technologies, experimental SiGe bipolar devices have already demonstrated attractive characteristics for the design of very low phase noise oscillators.1-6 The low flicker noise performance of the silicon-based bipolar device combined with the high frequency gain properties of SiGe technology allows the limits of oscillator performance and innovate design principles to be explored. In this article, the characteristics of a commercially available SiGe heterojunction bipolar transistor (HBT) used in an oscillator operating at Ku-band are presented. It describes the application of the SiGe HBT to DROs in the 12 to 16 GHz frequency range.
First, the validation of the nonlinear model of the SiGe transistor is presented through a comparison between the simulated and the measured S-parameters. Next, the residual phase noise measurement data at 10 GHz is reported. Then the design of a DRO at 12.9 GHz is described. This oscillator is based on a series feedback configuration. The phase noise measurement on the oscillator is also given. Finally, a conclusion is reached by comparing this design against others described in the research literature.
The Nonlinear Transistor Model
Fig. 1 Comparison between simulated and measured S-parameters of a SiGe HBT from 1 to 40 GHz.
The LPT16ED SiGe transistor is a NPN device with gold bonding pads suitable for wirebond or flip-chip. The device is constructed in a technology featuring a unity-current gain cut-off frequency (fT ) greater than 30 GHz and a low base impedance. At 16 GHz, the device can provide a maximum stable gain (MSG) greater than 5 dB with a low current consumption (20 mA) from a single 3V power supply.
The large-signal device model is based on the SPICE-Gummel Poon model. The core model is augmented with external resistive and capacitive elements and distributed components to simulate the high frequency characteristics of the transistor. The complexity of the model, of course, increases accordingly, as do the number of required circuit nodes. For small circuits with low transistor count, however, the improvement in accuracy is both warranted and desirable.
The validation of this model is shown in Figure 1 where the S-parameter measurements are compared with simulation results. The small-signal measurements have been performed at the bias point Ic = 17 mA and Vce = 3 V. Excellent accuracy is achieved up to 40 GHz. For best RF performance, the active device can be biased with Vce between 0.8 and 3.3 V and Ic between 1 mA and 20 mA. Of course, the RF gain is dependent upon the bias point.
The classical approach to optimize the phase noise in a DRO consists of minimizing the dielectric resonator coupling in order to obtain high Q values.7,8 The relation between the phase noise and resonator Q is given by
DfT = residual phase fluctuation of the active device (rad/√Hz)
f0 = oscillation frequency
QL = resonator loaded quality factor
fm = noise offset frequency
Clearly, from Equation 1, increasing QL and minimizing DfT will reduce the phase noise.
LF noise measurement
depends only on DC bias and LF load
No information on the conversion mechanism is provided by this measurement
Oscillator phase noise measurement
Provides more insight on the final result
Results dependent on many other parameters: DC and LF load/bias, but also RF load, compression level (input power) and loop phase shift
Residual phase noise measurement
Similar to the oscillator phase noise measurement but with better control possibilities, particularly on the input power (no loop requirements, either in phase or amplitude)
SiGe HBT Residual Phase Noise
Fig. 2 Residual phase noise measurement.
Fig. 3 The 12.9 GHz DRO circuit.
To evaluate the suitability of the transistor for designing a very low noise microwave oscillator, it must be understood that there are three distinct but correlated types of noise measurements. These are low frequency noise, residual phase noise near the carrier frequency in an open loop configuration and oscillator phase noise near the carrier. These three measurements present advantages and drawbacks, which are shown in Table 1 . For these reasons, the transistor's residual phase noise is the more appropriate parameter to characterize the device's performance when used to design an oscillator.
The measurement has been performed at 10 GHz with a commercial DRO as the reference source.9,10 The measurement system noise floor is -175 dBc/Hz at 10 kHz offset frequency. Note that the device is in an open-loop configuration terminated with 50 W loads both at the input and at the output. Furthermore, the shape of the curve is influenced by the bias tee which is responsible for the resonance peak near 15 kHz offset. The measurement result is plotted in Figure 2 and the measured residual phase noise at 10 kHz offset is -166 dBc/Hz (equal to -163 dBrad/Hz). Note that the best residual phase noise that can be achieved with a FET in the same conditions is typically -155 dBc/Hz at 10 kHz frequency offset.
Design of a 12.9 GHz DRO
Fig. 4 S11 magnitude and phase as a function of frequency at (a) 1 to 15 GHz and (b) 10 to 15 GHz.
There are different topologies available to realize an oscillator. The two main topologies used are parallel feedback and series feedback. The parallel feedback is based on a transmission amplifier (S21 > 0) and the series one is based on a reflection amplifier (S11 > 0). In the case of the parallel feedback, the transistor must be matched to achieve a sufficient margin of gain in the 12 to 16 GHz frequency range. Previous work has shown that by increasing the open loop gain, the open loop phase fluctuations that degrade the final phase noise performance of the oscillator increase.7 In addition, it has been previously reported that the best trade-off between high gain and low phase noise can be achieved with the series feedback configuration.7
In this work, a series feedback has been investigated where the transistor is used in a common emitter configuration. To create the negative resistance, a short stub is placed on the emitter. The location of the gain peak is easily controlled by the stub length with proper consideration of the parasitics.
The 12.9 GHz DRO module is shown in Figure 3 . It uses an alumina substrate and a common emitter configuration with Vce = 2 V, Ic = 22.7 mA. The dielectric resonator is made of a standard Murata R material, separated from the alumina substrate using a 40 mil thick Teflon pad.
Fig. 5 Output signal of the DRO.
Fig. 6 Measured phase noise of the DRO.
Fig. 7 Phase noise comparison with other state-of-the-art oscillators (data points are referenced in Table 2).
The negative resistance measurement has been performed using a network analyzer and the result is plotted in Figure 4 . The first measurement spans a very large frequency range (1 to 15 GHz) to verify that there is only one area with a negative resistance (S11 > 0). Another span, over the 10 to 15 GHz frequency range, provides an accurate value of 4 dB at 12.9 GHz.
The oscillator output signal is plotted in Figure 5 . The output power is approximately -7.8 dBm due to a mismatch at the output. The oscillator phase noise is characterized in the free running mode using a test bench based on a frequency discriminator.11 A cross-correlation technique is used that allows a phase noise floor of -125 dBc/Hz at 10 kHz offset from a 10 GHz carrier12 to be achieved. Faraday shielding reduces the electrical, low frequency perturbations that are one of the major problems in the characterization of this type of oscillator. The measured phase noise is plotted in Figure 6 . A phase noise level of -110 dBc/Hz at 10 kHz offset has been achieved. This observed performance is close to state-of-the-art for a ceramic dielectric resonator and is compared in Figure 7 against others described in the research literature.13 The reference data is compiled in Table 2 .
In this article, the performance of a 12.9 GHz DRO realized with a commercially available discrete SiGe transistor (LPT16ED from SiGe Semiconductor Inc.) has been reported. A phase noise level of -110 dBc/Hz at 10 kHz offset frequency from a 12.9 GHz carrier has been measured.
The authors would like to acknowledge C. Glaser from the Communication Research Center of Canada (CRC) and J. Rayssac from the Laboratory of Analysis and Architecture of Systems to the CNRS (LAAS-CNRS) for their technical help.
1. A. Ghrule, C. Manher and K. Weidmann, "Low Phase Noise 10 GHz DRO With Low 1/f Noise SiGe HBTs," 28th European Microwave Conference , Amsterdam, 1998, pp. 391-394.
2. J.F. Luy, K.M. Strohm and E. Sassa, "Si/SiGe MMIC Technology," IEEE MTT Symposium Digest , 1994, pp. 1755-1757.
3. C.N. Rheinfelder, K.M. Strohm, L. Metzger, H. Kibbel, J.F. Luy and W. Heinrich, "47 GHz SiGe MMIC Oscillator," IEEE MTT Symposium Digest , 1999, pp. 5-8.
4. M. Regis, O. Llopis, B. Van Haaren, R. Plana, A. Grhule, J. Rayssac and J. Graffeuil, "Ultra Low Phase Noise C- and X-band Bipolar Transistor Dielectric Resonator Oscillators," Proceedings of the 1998 IEEE Frequency Control Symposium , Pasadena, June 1998, pp. 507-511.
5. P. Abale, E. Sonmez, K.B. Schad and H. Schumacher, "24 GHz SiGe MMIC Oscillator Realized with Lumped Elements in a Production Line," 30th European Microwave Conference , Paris, Vol. 3, October 2000, pp. 1-3.
6. S.P. Voinigescu, D. Marchesan and M.A. Copeland, "A Family of Monolithic Inductor-varactor SiGe HBT VCOs for 20 GHz to 30 GHz LMDS and Fiber-optic Receiver Applications," Proceedings of the RFIC Symposium , Boston, June 2000, pp. 173-176.
7. M. Regis, O. Llopis and J. Graffeuil, "Nonlinear Modeling and Design of Bipolar Transistors Ultra Low Phase Noise Dielectric Resonator Oscillator," IEEE Transactions on Microwave Theory and Techniques , Vol. 46, No. 10, October 1998, pp. 1589-1593.
8. D.B. Leeson, "A Simple Model of Feedback Oscillator Noise Spectrum," Proceedings of the IEEE , February 1966, pp. 329-330.
9. T.R. Faulkner and R.E. Temple, "Residual Phase Noise and AM Noise Measurements and Techniques," Hewlett Packard Application Note , Reference 03048-90011.
10. G. Cibiel, M. Regis, E. Tournier and O. Llopis, "AM Noise Impact on Low Level Phase Noise Measurements," Submitted to IEEE-UFFC, June 2001.
11. O. Llopis, J.B. Juraver, M. Regis, M. Chaubet and J. Graffeuil, "Evaluation of Two Non-standard Techniques for the Phase Noise Characterization at Microwave Frequencies," Proceedings of the 2000 IEEE International Frequency Control Symposium , June 2000, pp. 511-515.
12. A. Lance, W. Seal, N. Hudson and F. Mendoza, "Phase Noise Measurements Using Cross-spectrum Analysis," Conference on Precision Electromagnetic Measurements , Ottawa, June 1978, pp. 94-96.
13. 1999 and 2000 European Microwave Week Short Course on Low Phase Noise Oscillators organized by O. Llopis.
Myrianne Regis is an R&D design engineer at SiGe Semiconductor. She received her MS degree from the University Paul Sabatier, Toulouse, in 1995, and her PhD degree in electronics from the University Paul Sabatier, Toulouse, in 1999. Since 2000 she has been with SiGe Semiconductor, Ottawa, Ontario, Canada, where her work includes research and development of high performance integrated circuits based on SiGe technology.
Hugues Lafontaine serves as research engineer at SiGe Semiconductor. He obtained his BEng and his MScA degrees in physics engineering in 1989 and 1991, respectively, from the Ecole Polytechnique of Montreal. He later pursued PhD work on high electron mobility transistors (HEMTs) at the CNRS in Bagneux (France) and at the INRS in Varennes (Quebec, Canada). After obtaining his degree in 1994, he spent three years at the National Research Council of Canada, developing silicon-germanium technology for both electronic and optical devices. Since joining SiGe Semiconductor in 1997, Lafontaine has been working on SiGe HBTs as well as on integrated, high Q, passive elements.
Steve Kovacic is director, research and development, and is responsible for technology innovation at SiGe Semiconductor. Kovacic has more than nine years of experience in silicon-germanium device modeling, transistor and process design, and high frequency circuit design. Prior to joining SiGe Semiconductor he was the SiGe project leader at Nortel's Advanced Technology Group. There, he worked with IBM to bring SiGe HBT technology into Nortel's wireless and broadband operations. He also served as a technical liaison between IBM's process integrators and Nortel's IC design group. He holds six patents in the area of semiconductor devices and circuits, and has published extensively in the field of SiGe process technology. He holds a doctorate in engineering physics from McMaster University, and a bachelor of science degree in physics from the University of Waterloo.
Gilles Cibiel is a student working toward his PhD degree in electronics at the Laboratory of Analysis and Architecture of Systems of the National Center of Scientific Research (LAAS-CNRS), Toulouse, France. His main field of interest is in the study of oscillators phase noise modeling and the development of specific phase noise measurement techniques. Cibiel also holds an MS degree in physics from the University Paul Sabatier, Toulouse.
Olivier Llopis is currently a researcher working with the French National Center for Scientific Research (CNRS) in the Laboratory of Analysis and Architecture of Systems (LAAS) in Toulouse. His main field of interest is in the study of noise in microwave nonlinear circuits, including the oscillators phase noise modeling and the development of specific phase noise measurement techniques. He received the diploma of telecommunications engineer from ENSTB, Brest, in 1987, and his PhD degree in electronics from the University Paul Sabatier, Toulouse, in 1991.