Chapter 3 Theoretical analysis and structure
4.2 Circuit Application
4.2.1 Design flow and simulation results
The schematic of oscillator circuit is depicted Fig. 4.9, which uses a complementary structure to omit a connection to the common-mode point of the inductor. It is possible to use only one symmetric inductor in this design. Besides, the crossed coupled pair formed by NMOS and PMOS transistors is used to generate a negative conductance and the output buffer is realized by a common-drain PMOS transistor. Providing the sufficient bias current for VCO, the current mirror comprises an N+ diffusion resistor (R2) and two PMOS transistors, which have larger channel width to suppress the thermal noise.
The tank of LC-VCO consists of the proposed miniature inductor and group 3(G3) varactor available in TSMC process. The tunable capacitances range from 0.4pF to 1.8pF with different applied voltage at 5GHz, as shown in Fig.4.10.
Accordingly, the single-turn inductor with the nominal inductance 0.7nH is needed to form a 5GHz resonator. Just as the design flow illustrated in Fig.3.4, there are several steps to design a miniature inductor. First, since the Q of resonator has significant effects on phase noise in a LC-tank VCO, the wider strip width (25μm) is adopted to improve the quality factor of the miniature inductor. Next, in order to satisfy the βl product (17o)and the expected length (300 μm), the required phase constant β and real part characteristic impedance R is 1.54 (rad/mm) and 27.9 ohms, respectively.
Then, three MIM capacitors, each of 21.5 fF, are uniformly distributed
M1 M2
M3 M4
M5 M6
M7 M8
Vdd
Vcontrol
M3
C1 C2 R1 R2
Bias-T Bias-T
Fig. 4.9 Schematic of 5GHz LC-VCO circuit utilizing the proposed miniature inductor
within the CPS and the distance between two MIM capacitors can be estimated by the quotient of (300/3). Finally, finely tuning the spacing between the CPS is to have the sufficient effective inductance. The dimensions, effective inductance, and quality factor of the single-turn inductor and miniature inductor, for the same inductance, are shown at Fig.4.11(a),(b),and (c) individually and are summarized in Table 4.3. As expected, the proposed miniature inductor uses 90% of the area of the single turn inductor for the same inductance.
Simulation results of phase noise and tuning range of the voltage-controlled oscillators, which adopt a single-turn and a miniature inductor respectively, are shown in Fig.4.12. According to above simulation results, the VCO with a miniature inductor achieve a 10% area reduction and the same tuning range as the VCO with a single-turn inductor without any process modifications. However, the phase noise of the proposed miniature inductor case is -92.9 dBc/Hz with 1MHz offset and it is higher than the single turn inductor case due to the degradation of quality factor. The performance and bias conditions of oscillator using the miniature inductor are summarized in Table 4.4.
-1 0 1
-2 2
0.5 1.0 1.5
0.0 2.0
5 10 15 20
0 25
Varactor Bias Voltage
C_varactor (pF) Quality factor at 5 GHz
Fig.4.10 C-V and Q-V of the G3 varactor at 5 GHz
(a) 400 um
330 um
25 um 35 um
10.77 um
4.77 um
Proposed miniature inductor 415 um
345 um
25 um 35 um
Single-turn inductor
10 20 30 of the single turn and the proposed miniature inductor
Table 4.3 Simulation results of HS-CPS and single-turn spiral inductor A (HS-CPS) B (Single Turn) MiM Cap /period 22.5 fF/102.5 um NO
Q( 5GHz ) 24 27.6
Qmax 25.6 30
Fsr 30 GHz >40 GHz Area(um2) 330 * 400 345 * 415 A.R. -10 %
( )
1. .
*
sin
−
=
−turn gle
dut
Area reduction Area
Area R A
0.3 0.6 0.9 1.2 1.5
Phase Noise (dBc/Hz) HS-CPSSpiral
1E6
Phase Noise (dBc/Hz) HS-CPSSpiralHS-CPSSpiral
(b)
Fig. 4.12 (a) Tuning range (b) Phase noise of the oscillator utilizing a single-turn inductor and a miniature inductor
VCO parameters This Work
Supply Voltage 1.8 V
Bias Current of VCO core (without output buffer)
7 mA Overall Power Consumption
(with output buffer) 14.3 mW
Tuning Range 4.26 ~ 6.5 GHz
VCO Gain 1244 MHz / V
Phase-Noise
@ 1 MHz
-92.9 dBc/Hz Phase-Noise
@3 MHz -106.5 dBc/Hz
Table 4.4 Performance of LC-VCO utilizing the proposed miniature inductor
4.2.2 Experimental Results
The micrograph of the VCO circuit is depicted in Fig.4.13. The size of the test chip is 0.72 by 0.62 mm2. The RF input and output ports are placed in the opposite direction of the chip horizontally in order to enhance the port to port isolation.
Standard Ground-Signal-Ground (GSG) configuration is used at input and output RF port for on-wafer probing. Besides, the DC and control voltage are supplied at the bottom of the chip with Power-Ground-Power (PGP) formation.
The output of vco is measured by the HP 8563E Spectrum analyzer and Cascade Microtech probe station for on-chip measurement. The measured tuning range together with simulation results and output spectrum at 6.32 GHz is shown in Fig.4.14 (a), (b). The solid line presents the measured data and the curve with circle marker
Fig. 4.13 Micrograph of 5 GHz LC-VCO circuit
0 0.3 0.6 0.9 1.2 1.5 1.8 4
4.5 5 5.5 6 6.5
Control voltage(V)
Oscillation Frequency(GHz)
Measurement Simulation
(a)
(b)
Fig.4.14 (a) Tuning range (b) output spectrum of VCO circuit
0 V to read the lower frequency of 4.26 GHz or to 1.8 V to give the upper frequency of 6.32 GHz and the overall tuning range is 1.96 GHz. However, there is still some discrepancy between the measurement and simulation especially in the boundary of the oscillation frequency. That might result from the inaccuracy of varactor model.
In addition, another figure-of-merit of VCO is the phase noise. Since the gain of VCO is up to 1089 MHz/V, the spectrum with control voltage 0.8V is extremely sensitive to the voltage fluctuations and the measurement results will be not reliable.
Therefore, the control voltage is changed to 1.8 V where gain of VCO is lower. From the output spectrum, the carrier frequency is 6.32 GHz and the output power difference between carrier and noise power at 1MHz offset is 45 dB and the resolution bandwidth (RBW) is 100kHz. The phase noise can be calculated:
( ) ( )
( )
dBcP Hz RBWP noise
Phase out atoffset frequency out carrier /
Above all, the dc condition and important VCO parameters together with the simulation results are summarized in Table 4.5
Table 4.5 Summary of VCO parameters
Simulation Measurement
Supply Voltage 1.8 V
VCO core current (with current mirror)
7.03 mA 7 mA
VCO total current (including buffer)
CHAPTER 5
Summary and Future Works
5.1 Summary
A miniature inductor structure utilizing HS-CPS and two related pattern, crossed and shifted prototype, are presented. The miniature inductor saves about 13 % area, which is required by the conventional symmetric spiral inductor with the same inductance (~0.9 nH).
The effective inductance and quality factor of the miniature inductor are analyzed and optimized with the aid of a transmission line model, especially at 10 GHz in this thesis, which is verified both EM simulation and measurement.
Furthermore, the miniature inductor is compatible with standard silicon IC technology;
thus there is no process modifications required to result in additional cost. The miniature inductor is also applied to 5GHz LC-VCO (Voltage Controlled Oscillator) circuit. With nominal inductance 0.76 nH, it features a smaller size (330*400) with 10% area reduction comparing with conventional single-turn inductor (345*415).
Above all, the proposed inductor shows a strong potential to implement the highly-integrated single-chip circuit applications with smaller area and compatible performance.
5.2 Future Works
The quality factor of the proposed miniature inductor is still lower than that of spiral inductor. How to shrink the more area without sacrificing the quality factor becomes a challenge in the future. The imaginary part of the characteristic impedance
of the proposed miniature inductor is obviously decreased as the MIM capacitors are inserted. It causes side effects on quality factor. Thus, to recover the decrement efficiently might be the solution to achieve a higher Q factor.
The measured quality factor is worse than the simulated results. The measured through parasitics show great influence on the deviation of Q. How to be immune from such severe sensitivity will be an issue in the future measurement of small inductors.
Moreover, the phase noise of VCO circuit is not good enough. Since larger capacitance of varactor was chosen, the more parasitics from capacitors degraded the quality factor of LC tank though the inductor in this design was optimized. Therefore, it needs to be done that to emphasize the feature of the proposed inductor by proper VCO optimization.
REFERENCES
[1] A.Zolfaphari, A.chan, and B.Razavi, “Stacked inductors and transformer in CMOS technology,” IEEE Journal of Solid-State Circuit, vol.36, pp. 620-628, Apr.2001.
[2] C. P. Yue, and S. S. Wong, “On-chip Spiral Inductors with Patterned Ground Shields for Si-Based RF IC’s,” IEEE Journal of Solid-State Circuit, vol. 33, pp.
743-752, May 1998
[3] J. N. Burghartz, and B. Rejaei, “On the Design of RF Spiral Inductors on Silicon,” IEEE Trans. on Electron Devices, vol.50, pp.718~729, Mar. 2003
[4] M. Danesh, J. R. Long, “Differentially Driven Symmetric Microstrip Inductors,” IEEE Trans. Microwave Theory Tech , vol.50 , pp. 332-340 , Jan.
2002.
[5] H. T. Tso, and C. N. Kuo, “On-Chip Interconnect of High Slow Wave Factor CPS Structure in Nano-CMOS Technology,” in Proc. of ISNCGS, Feb 2004.
[6] D. K. Cheng, Field and Wave Electromagnetics 2nd, published by Addison-Wesley publishing company, 1992.
[7] Data sheet of “Infinity probe”
簡歷
姓 名: 馬健嘉
學 歷:
高雄市立高雄高級中學 ( 84 年 9 月 ~ 87 年 6 月) 私立大同大學電機工程學系 ( 87 年 9 月 ~ 88 年 6 月) 國立成功大學電機工程學系 ( 88 年 9 月 ~ 91 年 6 月) 國立交通大學電子工程所碩士班 ( 91 年 9 月 ~ 94 年 1 月)