A 0.6V Quadrature VCO With Optimized Capacitive Coupling for ...
A 0.6V Quadrature VCO With Optimized Capacitive Coupling for Phase Noise Reduction Feng Zhao and Fa Foster Dai, Fellow, IEEE Department of Electrical and Computer Engineering, Auburn University, Auburn, AL 36849, USA Abstract — This paper presents a 0.6V quadrature voltagecontrolled oscillator (QVCO) with a novel capacitive coupling technique, which is employed not only for quadrature signal coupling, but also for noise reduction. As a result, the proposed QVCO can even achieve 3 to 5dB lower phase noise than a singlephase VCO of the same kind. Optimized capacitive coupling combined with inductive enhance-swing technique enables lowpower consumption and low phase noise simultaneously. The QVCO achieves a measured phase noise of -132.3dBc/Hz @ 3MHz offset with a center frequency of 5.6GHz and consumes 4.2mW from a 0.6V supply. This performance corresponds to a Figure-of-Merit (FoM) of 191.5dB. The QVCO RFIC is implemented in a 0.13 µm CMOS technology with core area of 0.6x0.8mm2.
highly desirable to design a QVCO with excellent phase noise performance under very low supply voltage.
Index Terms — quadrature VCO (QVCO), phase noise, Colpitts oscillator, capacitive coupling, enhance-swing, ISF.
For a conventional QVCO shown in Fig. 1, the iVCO and qVCO couple with each other at the gate of the coupling transistor and the largest energy injection happens to be at the zero crossing of the VCO output swing. According to the impulse sensitivity function (ISF) theory [6], the VCO phase noise is most sensitive to disturbance near the zero crossings of the oscillation. Thus, the phase noise of the quadrature outputs is degraded due to the fact that the amplitude-to-phase noise conversion in this topology is maximum at their zerocrossings. It’s for this reason that a QVCO can only end up with worse phase noise than that of single-phase VCO.
I. INTRODUCTION Quadrature signals are widely used in radio-frequency (RF) transceiver to drive complex mixers in order to distinguish positive and negative frequency components. Several techniques can be employed to produce quadrature signals, i.e., (i) a frequency doubled voltage-controlled oscillator (VCO) followed by divide-by-two circuit, assuming the VCO can generate accurate 50% duty cycle output; (ii) poly-phase filter, which is narrow-band, followed by a power hungry amplifier to regain the signal strength and a limiting amplifier to reduce the amplitude mismatch; (iii) using two VCOs with variety of coupling mechanisms to inject-lock each other to generate quadrature outputs. An LC tuned quadrature VCO (QVCO) using parallel transistors for coupling is illustrated in Fig. 1 [1]. This type of QVCO has poor phase noise performance and large power consumption. Series transistor coupling scheme [2] can reduce the current consumption, yet it requires larger voltage headroom. QVCO using back-gate coupling is reported in [3], which dissipates low power, however fails to achieve high figure-of-merit (FoM). Other QVCO structures based on capacitive coupling [4] and super harmonic coupling [5] have been proposed with improved FOMs. Comparing to its single-phase counterpart, the source-coupled QVCO [4] improves its phase noise utilizing the tail-current shaping through its coupling components. The current trend of technology scaling presents challenges for circuit designs. Feature size shrinking forces the power supply drop below 1V. Lowered supply voltage limits the output swing that can be generated, which further limits the phase noise that an oscillator can achieve. Therefore, it’s
978-1-4577-0223-5/11/$26.00 ©2011 IEEE
Fig. 1 Conventional quadrature VCO.
Fig. 2 Proposed QVCO with optimized capacitive coupling.
This paper presents a novel QVCO as shown in Fig. 2. It utilizes an optimized capacitive coupling to improve the phase noise. Instead of using noisy transistors for coupling, capacitive coupling provides the quadrature signal coupling needed for QVCO. By using Cqc and Ccc to couple the two VCO cores, signals at the gates of the oscillation transistors are shaped such that the amplitude of the gate voltage is no longer the maximum during zero crossings of the VCO output swing. As a result, the amplitude-to-phase noise conversion between the two VCOs is reduced and the phase noise
Yin
Admittance for Different Colpitts VCO
Yin
Vb
25
Vb
Rb
20
C1
Real[Yin] (mS)
Rb C1
M1
M2 Ccc
Ccc Vc
15
ES VCO
10
C2
C2
5.2 5.4 5.6 5.8
6
5 ESEGm VCO
0
Tank 2 L2
-1 -1.5 -2 -2.5 -3 -3.5 -4 5
L2 -5 1
3 4 5 6 7 Frequency (GHz) (a) (b) Fig. 3 (a) VCO core with Gm enhancement; (b) admittance for ES and enhanced-Gm Colpitts differential VCO.
performance of the QVCO can be improved beyond what can be achieved by a single-phase VCO. Phase noise is further improved by placing diode junction varactors with reference to ground. To maintain large output swing required for good phase noise performance under a low supply voltage around 0.5V, an enhance-swing (ES) Colpitts VCO structure is adopted [7]. A combination of these techniques enables the proposed QVCO with low phase noise (-122dBc/Hz @ 1MHz offset) and low power consumption (4.2mW). Moreover, the proposed QVCO accomplishes 3~5dB noise improvement when compared with the phase noise of the single-phase VCO core. Therefore, the proposed QVCO outperforms the most of QVCOs published so far with good phase noise, low power consumption and small area. The proposed optimized capacitive coupling technique can also be employed for implementing multi-phase VCOs for phase array applications. II. QVCO WITH REDUCED NOISE-COUPLING A. Enhanced-Gm Colpitts VCO Conventional Colpitts VCO requires large transconductance to meet the start-up conditions in the presence of PVT variations. As a result, high power dissipation is necessary to ensure reliable start-up. The VCO core adopts a differential enhance-swing Colpitts VCO topology with crosscoupled positive feedback. The small-signal admittance looking into the drain of an ES enhanced-Gm (ESEGm) VCO as shown in Fig. 3(a) can be derived as Re[Yin ]ESEGm = −
2 g mω 2 L2C1 (ω 2 L2C2 − 1)
(1)
2
1 − ω 2 L2 ( C1 + C2 ) + 4 g m2 ω 2 L22
while that of an ES Colpitts VCO without cross-coupled feedback is given by Re[Yin ]ES = −
g mω 2 L2C1 (ω 2 L2C2 − 1) 2
(2)
1 − ω L2 ( C1 + C2 ) + g ω L 2
2 m
2 2 2
The difference between the two admittances is plotted in Fig. 3(b). As shown in the frequency range of 5~6GHz, the positive feedback almost doubles the admittance of ES Colpitts VCO and thus relaxes the start-up requirement. As a result, the power consumption can be reduced compared with that of an ES Colpitts VCO. The magnitude of negative Gm
2
decreases as frequency is lowered, i.e., it becomes more difficult for the VCO to meet the start-up condition. As frequency is further reduced, the Gm becomes positive and peaks at the resonant frequency of Tank 2 as shown in Fig. 3(a). The quality factor of Tank 2 has to be decreased in order to improve the start-up. However, the enhanced-Gm VCO doesn’t need to lower the Q of the tank since it inherently has smaller quality factor ES VCO and thus improved start-up. In addition, the resonant frequency of Tank 2 should be placed far below the VCO oscillation frequency to maintain a sufficient margin for start up. B. Noise-Coupling Reduction for Colpitts QVCO Theoretically, the phase noise of a QVCO can be reduced by 3dB compared to a single-phase VCO that draw half of the current of the QVCO. However, the strong coupling forces the QVCO to operate away from the resonant frequency in conventional QVCO topologies. The reason for the degraded phase noise of conventional QVCOs is due to the fact that the coupled signal is maximum when the QVCO is most susceptible to noise, i.e., the two VCO cores inject noise to each other during the zero-crossing point of their output swings. QVCOs using transistors for coupling will suffer additional noise from the coupling active devices. Nevertheless, regardless of the coupling schemes used, all the QVCOs reported so far have worse phase noise than their single-phase VCO counterparts. In order to lower the phase noise, it is beneficial to reduce the voltage swings of the coupled signals at their zero-crossing time. Furthermore, instead of coupling from QVCO outputs, the cross-coupling capacitor Ccc and quadrature cross-coupling capacitor Cqc are connected to source terminals of the oscillation transistors for the following reasons: (i) the voltage swing at the gate will be smaller than that at the QVCO output; (ii) the QVCO output frequency is less sensitive to the parasitics at the source terminals. For the QVCO circuit shown in Fig. 2, we define the coupling-strength factor m between the iVCO and qVCO as
m=
Cqc Cqc + Ccc
(3)
Vg , M 1 ( t ) = miVQsn ( t ) + (1 − m )iVIsn ( t )
(4)
By properly shaping the coupling signals with optimum coupling strength, the impulse sensitivity function (ISF) of the QVCO has been reduced as well, achieving better phase noise performance than a single-phase VCO. Moreover, the diode junction varactors are placed with reference to ground to reduce the noise picked up from substrate since the signal swing on n-type anode is isolated from substrate by connecting the p-type cathode to ac common control voltage.
Figure of Merit (dB)
As shown in Fig. 4(a), the transient voltages at the gates of oscillation transistors have been reduced during zerocrossing. Therefore, the noise contributed by amplitude-tophase conversion is decreased. The noise improvement for different coupling factor m is presented in Fig. 4(b). As the coupling strength increases, the phase noise improvement varies for different frequency offset. For instance, at 1MHz offset, the phase noise improvement is the best when the coupling factor falls from 0.3 to 0.4. Usually, the VCO design cares more about the out-of-band noise at large offset frequency since the close-in noise can be filtered by the phaselocked-loop (PLL).
noise performance and phase error of the proposed QVCO for different coupling-strength factor. After careful trade-off between the noise and phase error, coupling strength of 0.4 is chosen to implement the QVCO. The corresponding FoM and phase error in pre-layout simulation are 198.1dB and 0.04°, respectively.
199
1
198
0.8
197
0.6
FoM
196
0.4 Phase Error
195
194 0
Phase Error (Degree)
Assuming the transient voltages of the quadrature output signals as Isp=V0cos(ω0t) and Qsp= V0sin(ω0t), the voltage swing at the gate of M1 can be written as
0.2
0.2
0.4 0.6 Coupling Factor
0.8
0 1
Fig. 5 Simulated FoM and phase error with coupling strength factor.
III. IMPLEMENTATION AND MEASURED RESULTS
(a)
The QVCO was implemented in a 0.13µm CMOS technology and the die photo of the chip is shown in Fig. 6. The QVCO core including the pads and testing output buffers occupies an area of 1.2x1.2mm2, while the core of QVCO takes only 0.6x0.8mm2. The extracted parasitic capacitance at each source terminal is around 300fF mainly caused by the cross-coupled wiring and diode wiring. Therefore, the locations of the cross-coupled connections and the varactors have a large impact on the QVCO output frequency variation due to parasitic wiring capacitance.
Phase Noise Improvement
Noise Improvement (dB)
5
@ 1MHz Offset
0
@ 100kHz Offset -5 @ 10kHz Offset
-10 0
0.2
0.4 0.6 Coupling Factor m
0.8
1
(b) Fig. 4 Simulated results of: (a) QVCO output and coupling signals; (b) phase noise improvement of QVCO over single-phase VCO.
The selection of coupling-strength factor m is a trade-off between noise improvement and phase error of the QVCO outputs. The larger the coupling strength is, the smaller the phase error becomes, as shown in Fig. 5, yet less phase noise improvement can be achieved. As the coupling strength approaches to 1, where the proposed QVCO is similar to conventional one, the phase noise would be degraded compared to the single-phase VCO. While m is equal to 0, the two VCO becomes independent to each other without coupling and thus fail to produce quadrature outputs. Therefore, there is an optimum point of m to achieve the best phase noise improvement with acceptable phase error. Fig. 5 shows the
Fig. 6 Die photo of the implemented QVCO RFIC.
The phase noise is measured using an Agilent E4446A spectrum analyzer with phase noise option. The design provides the reconfigurability to form either a QVCO or a
single-phase VCO for comparison. Fig. 7 shows the phase noise for both QVCO and single-phase VCO measured under the same conditions. The QVCO and single-phase VCO achieved measured phase noise of -132.3dBc/Hz and 127.8dBc/Hz @ 3MHz offset while consuming 4.2mW and 2.1mW, respectively. The phase noise improvement of the QVCO over single-phase VCO is about 3 to 5dB from 100kHz to 10MHz offset frequency range. The FoMs at 3MHz are 190dB and 191.5dB for single-phase VCO and QVCO, respectively.
power consumption in mW, including both i-VCO and q-VCO cores. IV. CONCLUSIONS A CMOS enhance-swing Colpitts QVCO with noisecoupling reduction is proposed in this paper. The prototype CMOS QVCO was fabricated in 0.13µm CMOS technology with measured frequency tuning range about 4%. The QVCO achieves a FoM of 191.5dB while the FoM of a single-phase VCO of the same type is 190dB. The measurement results demonstrate the effectiveness of noise improvement using the proposed optimized capacitive coupling technique. The QVCO consumes only 4.2mW power with a 0.6 V supply and occupies a core area of 0.48 mm2. TABLE. I PERFORMANCE COMPARISON OF QVCOS. [3]
[4]
[5]
Frequency (GHz)
1.1
2.07
4.88
5.6
Power (mW)
5.4
4.5
22
4.2
Power Supply (V) Phase Noise (dBc/Hz)
1.8 -137 @3MHz 0.18µm CMOS
1.5 -124.4 @1MHz 0.25µm BiCMOS
-
0.625
-
0.48
181
186
185
191.5
Technology Core Area (mm2) FoM (dB)
Fig. 7 Measured phase noise of single-phase VCO (top) and QVCO (bottom).
The frequency tuning ranges from 5.4GHz to 5.62GHz for QVCO and from 5.46GHz to 5.68GHz for single-phase VCO, respectively, as shown in Fig. 8. The phase noise of QVCO varies from -129.5dBc/Hz to -132.3dBc/Hz @3MHz offset in the entire tuning frequency range. -124
VCO Frequency (GHz)
VCO Noise
fvco -126
5.6 -128
5.5
-130
fqvco -132
QVCO Noise 5.4 0.2
0.4
0.6
0.8 1 Control Voltage (V)
1.2
Phase Noise @ 3MHz Offset (dBc/Hz)
5.7
This work is supported by the U. S. Army under Contract No. W15P7T-09-C-S320. The authors would like to acknowledge Jonathan Corriveau, Andre Aklian, and Geoffrey Goldman for funding and managing this project. REFRENCES [1]
[2]
[3]
[4]
Fig. 8 Measured frequency tuning range and phase noise of QVCO and single-phase VCO.
f 2 1mW FoM = 10 log 0 − L ( ∆f ) ∆f P
[5]
[6] [7]
(5)
In the above definition, L(∆f) is the phase noise at the ∆f offset from the oscillator frequency f0, and P is the QVCO’s core
2.5 0.6 -125 -122@1MHz @1MHz -132.3@3MHz 0.25µm 0.13µm CMOS CMOS
ACKNOWLEDGEMENT
1.4
Table-I summarizes the performance of the proposed noise-coupling QVCO and comparison with previously published QVCO work. When compared with prior art, the proposed Colpitts QVCO achieves a FoM of 191.5dB, where the FoM is defined as [8]:
This work
[8]
A. Rofougaran, J. Rael, M. Rofougaran, and A. Abidi, “A 900MHz CMOS LC-oscillator with quadrature outputs,” in IEEE ISSCC, 1996, pp. 392–393. P. Andreani and X. Wang, “On the phase-noise and phase-error performance of multiphase LC CMOS VCOs,” IEEE J. Solid-State Circuits, vol. 39, No. 11, pp. 1883-1893, Nov. 2004. H. Kim, C. Cha, S. Oh, M. Yang, and S. Lee, “A very low-power quadrature VCO with back-gate coupling,” IEEE J. Solid-State Circuits, vol. 39, No. 6, pp. 952-955, Mar. 2004. B. Soltanian and P. Kinget, “A low phase noise quadrature LC VCO using capacitive common-source coupling,” Proceedings of the IEEE European Solid-State Circuits Conference, 2006, pp. 436-439. S. L. J. Gierkink,S. Levantino, R. C. Frye, C. Samori, and V. Boccuzzi, “A low-phase-noise 5-GHz CMOS quadrature VCO using superharmonic coupling,” IEEE J. Solid-State Circuits, vol. 38, no. 7, pp. 1148–1154, Jul. 2003. Thomas H. Lee and Ali Hajimiri, “Oscillator phase noise: A tutorial,” IEEE J. Solid-State Circuits, vol. 35, No. 3, pp. 326-336, Mar. 2000. F. Farhabakhshian, T. Brown, and T. Fiez, “A 475 mV, 4.9 GHz enhanced swing differential Colpitts VCO in 130 nm CMOS with an FoM of 196.2 dBc/Hz,” Proceedings of Custom Integrated Circuits Conference , 2010. P. Kinget, B. Soltanian, S. Xu, S. Yu, and F. Zhang, “Advanced design techniques for integrated voltage controlled LC oscillators,” Proceedings of Custom Integrated Circuits Conference, 2007, pp. 805811.
highly desirable to design a QVCO with excellent phase noise performance under very low supply voltage.
Index Terms — quadrature VCO (QVCO), phase noise, Colpitts oscillator, capacitive coupling, enhance-swing, ISF.
For a conventional QVCO shown in Fig. 1, the iVCO and qVCO couple with each other at the gate of the coupling transistor and the largest energy injection happens to be at the zero crossing of the VCO output swing. According to the impulse sensitivity function (ISF) theory [6], the VCO phase noise is most sensitive to disturbance near the zero crossings of the oscillation. Thus, the phase noise of the quadrature outputs is degraded due to the fact that the amplitude-to-phase noise conversion in this topology is maximum at their zerocrossings. It’s for this reason that a QVCO can only end up with worse phase noise than that of single-phase VCO.
I. INTRODUCTION Quadrature signals are widely used in radio-frequency (RF) transceiver to drive complex mixers in order to distinguish positive and negative frequency components. Several techniques can be employed to produce quadrature signals, i.e., (i) a frequency doubled voltage-controlled oscillator (VCO) followed by divide-by-two circuit, assuming the VCO can generate accurate 50% duty cycle output; (ii) poly-phase filter, which is narrow-band, followed by a power hungry amplifier to regain the signal strength and a limiting amplifier to reduce the amplitude mismatch; (iii) using two VCOs with variety of coupling mechanisms to inject-lock each other to generate quadrature outputs. An LC tuned quadrature VCO (QVCO) using parallel transistors for coupling is illustrated in Fig. 1 [1]. This type of QVCO has poor phase noise performance and large power consumption. Series transistor coupling scheme [2] can reduce the current consumption, yet it requires larger voltage headroom. QVCO using back-gate coupling is reported in [3], which dissipates low power, however fails to achieve high figure-of-merit (FoM). Other QVCO structures based on capacitive coupling [4] and super harmonic coupling [5] have been proposed with improved FOMs. Comparing to its single-phase counterpart, the source-coupled QVCO [4] improves its phase noise utilizing the tail-current shaping through its coupling components. The current trend of technology scaling presents challenges for circuit designs. Feature size shrinking forces the power supply drop below 1V. Lowered supply voltage limits the output swing that can be generated, which further limits the phase noise that an oscillator can achieve. Therefore, it’s
978-1-4577-0223-5/11/$26.00 ©2011 IEEE
Fig. 1 Conventional quadrature VCO.
Fig. 2 Proposed QVCO with optimized capacitive coupling.
This paper presents a novel QVCO as shown in Fig. 2. It utilizes an optimized capacitive coupling to improve the phase noise. Instead of using noisy transistors for coupling, capacitive coupling provides the quadrature signal coupling needed for QVCO. By using Cqc and Ccc to couple the two VCO cores, signals at the gates of the oscillation transistors are shaped such that the amplitude of the gate voltage is no longer the maximum during zero crossings of the VCO output swing. As a result, the amplitude-to-phase noise conversion between the two VCOs is reduced and the phase noise
Yin
Admittance for Different Colpitts VCO
Yin
Vb
25
Vb
Rb
20
C1
Real[Yin] (mS)
Rb C1
M1
M2 Ccc
Ccc Vc
15
ES VCO
10
C2
C2
5.2 5.4 5.6 5.8
6
5 ESEGm VCO
0
Tank 2 L2
-1 -1.5 -2 -2.5 -3 -3.5 -4 5
L2 -5 1
3 4 5 6 7 Frequency (GHz) (a) (b) Fig. 3 (a) VCO core with Gm enhancement; (b) admittance for ES and enhanced-Gm Colpitts differential VCO.
performance of the QVCO can be improved beyond what can be achieved by a single-phase VCO. Phase noise is further improved by placing diode junction varactors with reference to ground. To maintain large output swing required for good phase noise performance under a low supply voltage around 0.5V, an enhance-swing (ES) Colpitts VCO structure is adopted [7]. A combination of these techniques enables the proposed QVCO with low phase noise (-122dBc/Hz @ 1MHz offset) and low power consumption (4.2mW). Moreover, the proposed QVCO accomplishes 3~5dB noise improvement when compared with the phase noise of the single-phase VCO core. Therefore, the proposed QVCO outperforms the most of QVCOs published so far with good phase noise, low power consumption and small area. The proposed optimized capacitive coupling technique can also be employed for implementing multi-phase VCOs for phase array applications. II. QVCO WITH REDUCED NOISE-COUPLING A. Enhanced-Gm Colpitts VCO Conventional Colpitts VCO requires large transconductance to meet the start-up conditions in the presence of PVT variations. As a result, high power dissipation is necessary to ensure reliable start-up. The VCO core adopts a differential enhance-swing Colpitts VCO topology with crosscoupled positive feedback. The small-signal admittance looking into the drain of an ES enhanced-Gm (ESEGm) VCO as shown in Fig. 3(a) can be derived as Re[Yin ]ESEGm = −
2 g mω 2 L2C1 (ω 2 L2C2 − 1)
(1)
2
1 − ω 2 L2 ( C1 + C2 ) + 4 g m2 ω 2 L22
while that of an ES Colpitts VCO without cross-coupled feedback is given by Re[Yin ]ES = −
g mω 2 L2C1 (ω 2 L2C2 − 1) 2
(2)
1 − ω L2 ( C1 + C2 ) + g ω L 2
2 m
2 2 2
The difference between the two admittances is plotted in Fig. 3(b). As shown in the frequency range of 5~6GHz, the positive feedback almost doubles the admittance of ES Colpitts VCO and thus relaxes the start-up requirement. As a result, the power consumption can be reduced compared with that of an ES Colpitts VCO. The magnitude of negative Gm
2
decreases as frequency is lowered, i.e., it becomes more difficult for the VCO to meet the start-up condition. As frequency is further reduced, the Gm becomes positive and peaks at the resonant frequency of Tank 2 as shown in Fig. 3(a). The quality factor of Tank 2 has to be decreased in order to improve the start-up. However, the enhanced-Gm VCO doesn’t need to lower the Q of the tank since it inherently has smaller quality factor ES VCO and thus improved start-up. In addition, the resonant frequency of Tank 2 should be placed far below the VCO oscillation frequency to maintain a sufficient margin for start up. B. Noise-Coupling Reduction for Colpitts QVCO Theoretically, the phase noise of a QVCO can be reduced by 3dB compared to a single-phase VCO that draw half of the current of the QVCO. However, the strong coupling forces the QVCO to operate away from the resonant frequency in conventional QVCO topologies. The reason for the degraded phase noise of conventional QVCOs is due to the fact that the coupled signal is maximum when the QVCO is most susceptible to noise, i.e., the two VCO cores inject noise to each other during the zero-crossing point of their output swings. QVCOs using transistors for coupling will suffer additional noise from the coupling active devices. Nevertheless, regardless of the coupling schemes used, all the QVCOs reported so far have worse phase noise than their single-phase VCO counterparts. In order to lower the phase noise, it is beneficial to reduce the voltage swings of the coupled signals at their zero-crossing time. Furthermore, instead of coupling from QVCO outputs, the cross-coupling capacitor Ccc and quadrature cross-coupling capacitor Cqc are connected to source terminals of the oscillation transistors for the following reasons: (i) the voltage swing at the gate will be smaller than that at the QVCO output; (ii) the QVCO output frequency is less sensitive to the parasitics at the source terminals. For the QVCO circuit shown in Fig. 2, we define the coupling-strength factor m between the iVCO and qVCO as
m=
Cqc Cqc + Ccc
(3)
Vg , M 1 ( t ) = miVQsn ( t ) + (1 − m )iVIsn ( t )
(4)
By properly shaping the coupling signals with optimum coupling strength, the impulse sensitivity function (ISF) of the QVCO has been reduced as well, achieving better phase noise performance than a single-phase VCO. Moreover, the diode junction varactors are placed with reference to ground to reduce the noise picked up from substrate since the signal swing on n-type anode is isolated from substrate by connecting the p-type cathode to ac common control voltage.
Figure of Merit (dB)
As shown in Fig. 4(a), the transient voltages at the gates of oscillation transistors have been reduced during zerocrossing. Therefore, the noise contributed by amplitude-tophase conversion is decreased. The noise improvement for different coupling factor m is presented in Fig. 4(b). As the coupling strength increases, the phase noise improvement varies for different frequency offset. For instance, at 1MHz offset, the phase noise improvement is the best when the coupling factor falls from 0.3 to 0.4. Usually, the VCO design cares more about the out-of-band noise at large offset frequency since the close-in noise can be filtered by the phaselocked-loop (PLL).
noise performance and phase error of the proposed QVCO for different coupling-strength factor. After careful trade-off between the noise and phase error, coupling strength of 0.4 is chosen to implement the QVCO. The corresponding FoM and phase error in pre-layout simulation are 198.1dB and 0.04°, respectively.
199
1
198
0.8
197
0.6
FoM
196
0.4 Phase Error
195
194 0
Phase Error (Degree)
Assuming the transient voltages of the quadrature output signals as Isp=V0cos(ω0t) and Qsp= V0sin(ω0t), the voltage swing at the gate of M1 can be written as
0.2
0.2
0.4 0.6 Coupling Factor
0.8
0 1
Fig. 5 Simulated FoM and phase error with coupling strength factor.
III. IMPLEMENTATION AND MEASURED RESULTS
(a)
The QVCO was implemented in a 0.13µm CMOS technology and the die photo of the chip is shown in Fig. 6. The QVCO core including the pads and testing output buffers occupies an area of 1.2x1.2mm2, while the core of QVCO takes only 0.6x0.8mm2. The extracted parasitic capacitance at each source terminal is around 300fF mainly caused by the cross-coupled wiring and diode wiring. Therefore, the locations of the cross-coupled connections and the varactors have a large impact on the QVCO output frequency variation due to parasitic wiring capacitance.
Phase Noise Improvement
Noise Improvement (dB)
5
@ 1MHz Offset
0
@ 100kHz Offset -5 @ 10kHz Offset
-10 0
0.2
0.4 0.6 Coupling Factor m
0.8
1
(b) Fig. 4 Simulated results of: (a) QVCO output and coupling signals; (b) phase noise improvement of QVCO over single-phase VCO.
The selection of coupling-strength factor m is a trade-off between noise improvement and phase error of the QVCO outputs. The larger the coupling strength is, the smaller the phase error becomes, as shown in Fig. 5, yet less phase noise improvement can be achieved. As the coupling strength approaches to 1, where the proposed QVCO is similar to conventional one, the phase noise would be degraded compared to the single-phase VCO. While m is equal to 0, the two VCO becomes independent to each other without coupling and thus fail to produce quadrature outputs. Therefore, there is an optimum point of m to achieve the best phase noise improvement with acceptable phase error. Fig. 5 shows the
Fig. 6 Die photo of the implemented QVCO RFIC.
The phase noise is measured using an Agilent E4446A spectrum analyzer with phase noise option. The design provides the reconfigurability to form either a QVCO or a
single-phase VCO for comparison. Fig. 7 shows the phase noise for both QVCO and single-phase VCO measured under the same conditions. The QVCO and single-phase VCO achieved measured phase noise of -132.3dBc/Hz and 127.8dBc/Hz @ 3MHz offset while consuming 4.2mW and 2.1mW, respectively. The phase noise improvement of the QVCO over single-phase VCO is about 3 to 5dB from 100kHz to 10MHz offset frequency range. The FoMs at 3MHz are 190dB and 191.5dB for single-phase VCO and QVCO, respectively.
power consumption in mW, including both i-VCO and q-VCO cores. IV. CONCLUSIONS A CMOS enhance-swing Colpitts QVCO with noisecoupling reduction is proposed in this paper. The prototype CMOS QVCO was fabricated in 0.13µm CMOS technology with measured frequency tuning range about 4%. The QVCO achieves a FoM of 191.5dB while the FoM of a single-phase VCO of the same type is 190dB. The measurement results demonstrate the effectiveness of noise improvement using the proposed optimized capacitive coupling technique. The QVCO consumes only 4.2mW power with a 0.6 V supply and occupies a core area of 0.48 mm2. TABLE. I PERFORMANCE COMPARISON OF QVCOS. [3]
[4]
[5]
Frequency (GHz)
1.1
2.07
4.88
5.6
Power (mW)
5.4
4.5
22
4.2
Power Supply (V) Phase Noise (dBc/Hz)
1.8 -137 @3MHz 0.18µm CMOS
1.5 -124.4 @1MHz 0.25µm BiCMOS
-
0.625
-
0.48
181
186
185
191.5
Technology Core Area (mm2) FoM (dB)
Fig. 7 Measured phase noise of single-phase VCO (top) and QVCO (bottom).
The frequency tuning ranges from 5.4GHz to 5.62GHz for QVCO and from 5.46GHz to 5.68GHz for single-phase VCO, respectively, as shown in Fig. 8. The phase noise of QVCO varies from -129.5dBc/Hz to -132.3dBc/Hz @3MHz offset in the entire tuning frequency range. -124
VCO Frequency (GHz)
VCO Noise
fvco -126
5.6 -128
5.5
-130
fqvco -132
QVCO Noise 5.4 0.2
0.4
0.6
0.8 1 Control Voltage (V)
1.2
Phase Noise @ 3MHz Offset (dBc/Hz)
5.7
This work is supported by the U. S. Army under Contract No. W15P7T-09-C-S320. The authors would like to acknowledge Jonathan Corriveau, Andre Aklian, and Geoffrey Goldman for funding and managing this project. REFRENCES [1]
[2]
[3]
[4]
Fig. 8 Measured frequency tuning range and phase noise of QVCO and single-phase VCO.
f 2 1mW FoM = 10 log 0 − L ( ∆f ) ∆f P
[5]
[6] [7]
(5)
In the above definition, L(∆f) is the phase noise at the ∆f offset from the oscillator frequency f0, and P is the QVCO’s core
2.5 0.6 -125 -122@1MHz @1MHz -132.3@3MHz 0.25µm 0.13µm CMOS CMOS
ACKNOWLEDGEMENT
1.4
Table-I summarizes the performance of the proposed noise-coupling QVCO and comparison with previously published QVCO work. When compared with prior art, the proposed Colpitts QVCO achieves a FoM of 191.5dB, where the FoM is defined as [8]:
This work
[8]
A. Rofougaran, J. Rael, M. Rofougaran, and A. Abidi, “A 900MHz CMOS LC-oscillator with quadrature outputs,” in IEEE ISSCC, 1996, pp. 392–393. P. Andreani and X. Wang, “On the phase-noise and phase-error performance of multiphase LC CMOS VCOs,” IEEE J. Solid-State Circuits, vol. 39, No. 11, pp. 1883-1893, Nov. 2004. H. Kim, C. Cha, S. Oh, M. Yang, and S. Lee, “A very low-power quadrature VCO with back-gate coupling,” IEEE J. Solid-State Circuits, vol. 39, No. 6, pp. 952-955, Mar. 2004. B. Soltanian and P. Kinget, “A low phase noise quadrature LC VCO using capacitive common-source coupling,” Proceedings of the IEEE European Solid-State Circuits Conference, 2006, pp. 436-439. S. L. J. Gierkink,S. Levantino, R. C. Frye, C. Samori, and V. Boccuzzi, “A low-phase-noise 5-GHz CMOS quadrature VCO using superharmonic coupling,” IEEE J. Solid-State Circuits, vol. 38, no. 7, pp. 1148–1154, Jul. 2003. Thomas H. Lee and Ali Hajimiri, “Oscillator phase noise: A tutorial,” IEEE J. Solid-State Circuits, vol. 35, No. 3, pp. 326-336, Mar. 2000. F. Farhabakhshian, T. Brown, and T. Fiez, “A 475 mV, 4.9 GHz enhanced swing differential Colpitts VCO in 130 nm CMOS with an FoM of 196.2 dBc/Hz,” Proceedings of Custom Integrated Circuits Conference , 2010. P. Kinget, B. Soltanian, S. Xu, S. Yu, and F. Zhang, “Advanced design techniques for integrated voltage controlled LC oscillators,” Proceedings of Custom Integrated Circuits Conference, 2007, pp. 805811.
