Download A 10-GHz Low Phase Noise Differential Colpitts CMOS VCO Using

A 10-GHz Low Phase Noise Differential Colpitts
CMOS VCO Using Transformer Coupling Technology
Chia-Shih Cheng , Yi-Tzu Yang , Chien-Cheng Wei , Hsien-Chin Chiu
Chang Gung University, Department of Electronics Engineering, Tao-Yuan, Taiwan,
333. R.O.C.
TEL:+886-3-2118800 FAX:+886-3-2118507 Email: [email protected]
Abstract- We demonstrated a 10GHz low phase noise differential Colpitts Voltage-Controlled
Oscillator (VCO) utilizing TSMC 0.18 μm 1P6M CMOS process with transformer coupling
technology. The transformer coupling approach not only reduces the phase noise but also
improves the power consumption owing to the number of active devices can be reduced. The
total power consumption of this VCO is 20 mW and there is only 8mW power consumption
in the oscillator core under lower supply voltages. By adding the buffers to the output
terminals, it avoids the influence from the parasitic effects and also achieves a good
performance in low phase noise.
Keywords : VCO, Colpitts , CMOS , low phase noise , Transformer Coupling
1.
INTRODUCTION
In recent years, CMOS technology progresses constantly makes it offer superior
efficiency, lower cost, higher frequency circuit design, leading to a progressive transceiver
and receiver systems of the wireless communication. As to DVB-S (Digital Video Broadcast
System) specification, CMOS technology shows the ability to combine it into a single chip [1].
In wireless communication systems, the voltage-controlled oscillator design is a key issue. It
consumes most of the area and power in a circuit. In order to get the best phase noise, we
must improve power consumption. And with the diminishing of the electron channel, the
bearing pressure degree of crystal will reduce. It makes the output signal amplitude be
restricted. Then how to design a good phase noise and low power consumption oscillator has
been a challenge of the high-frequency circuit designers. In this paper, we demonstrate a
CMOS differential Colpitts oscillator by utilizing the transformer coupling technology to
improve the phase noise and power consumption owing to active devices can be reduced.
2. OSCILLATOR
TOPOLOGY
In the basic oscillator circuits, one of the simplest structures is Colpitts oscillator. It only
needs to use an active component (electric crystal) and three passive components (an
inductance and two capacities). Fig.1 shows the simplest structure of a Colpitts oscillator,
I bias is the current source and Vbias is the voltage source [2]. They are both used for tuning
the quality-factor (Q) and the working area. After adding the proper passive components, it
can make the circuit start to oscillate.
Fig.1
A simplest structure of Colpitts oscillator
There are two shortcomings by using the Colpitts oscillator. On the one hand, in order to
avoid feedback circuit destroying the Q-factor of the passive resonance circuit, the values of
the resonance circuit components needs to be relatively great. But in this way, it may be
limited to some extent in practical application. On the other hand, resonance circuit only has
an active component, so the form of output is single-ended. But it’s inconvenient for the users
who need double-ended output. Therefore, in order to generate double-ended output signals,
we connect two Colpitts oscillator in parallel [2]. While considering the actual operation
states, this circuit is similar to the relaxing type oscillator and the mutual use of two pieces of
Colpitts circuits. Therefore, the bias current sources can be shared and using a converter to
switch. It simplifies the complexity of the original circuit.
The advantage of using a balanced Colpitts oscillator is to get a higher Open- Loop-Gain
and make the circuit easier to oscillate than other oscillators by simple oscillatory structures
when operating at a high frequency. As to the phase noise, the number of active devices
increases because of using a converter (switch).It not only increases the phase noise by active
devices itself but also make the converter not an ideal switch due to the non-linear
characteristic of the device. These results make the phase noise worse and the output
waveform undesirability.
Fig . 2
A Differential Colpitts CMOS VCO
Using Transformer Coupling
Fig. 3
Photograph of the VCO
Frequency (GHz)
In this paper, we utilize transformers replacing by converters in a differential Copitts
CMOS VCO. Because of transformers can make the output phase lack of 180 degree and it’s
also a passive device, so its phase noise is much smaller than active devices. Hence, this
approach not only improves the linearity of the circuit but also the phase noise due to the
active devices can be reduced. Additionally, the buffers are also added to the output terminal
to avoid the influence from the parasitic effects. The schematic of the CMOS VCO utilizing
transformer coupling technology is shown in Fig.2. The chip microphotograph is given in Fig.
3. The total chip size including pads is 1.024 x 0.570 mm2.All active devices and
center-tapped inductors(tranformer) were provided by TSMC 0.18 μm 1P6M CMOS
process.
10.75
10.70
10.65
10.60
10.55
10.50
10.45
10.40
10.35
10.30
10.25
10.20
10.15
10.10
10.05
10.00
9.95
9.90
measuerment
simulation
0.0
0.5
1.0
1.5
2.0
Control voltage(V)
Fig. 4
The oscillation frequency versus control voltage
0
measurement
simulation
Output power (dBm)
-2
-4
-6
-8
-10
-12
0.0
0.5
1.0
1.5
2.0
Control voltage(V)
Fig. 5
The output power versus control voltage
3. SIMULATION AND MEASURED RESULTS
The simulated and measured results of oscillation frequencies and output power versus
variable voltage are plotted in Fig.4 and Fig.5 respectively. The supply voltages of VDD and
Vb are 1.6 V and 0.8V.Under the controlled voltage was biased from 0V to 2V, the measured
oscillation frequency was ranging from 9.92GHz to 10.22 GHz and output power was ranging
from -10.7dBm to -7.66dBm. Additionally, the power consumption of this VCO is 20 mW
and there is only 8mW power consumption in the oscillator core. Fig.6 shows the measured
and simulated results of phase noise versus variable voltage. With a supply voltage of
VDD=1.6 V and Vb=0.8V, the measured phase noise was ranging from -110 dBc/Hz to -115
dBc/Hz under the controlled voltage was biased from 0-2V.
Phase noise (dBc/Hz@1MHz)
-106
measurement
simulation
-108
-110
-112
-114
-116
-118
-120
0.0
0.5
1.0
1.5
2.0
Control voltage (V)
Fig. 6 The phase noise versus control voltage
The measured tuning range of this VCO was 9.92-10.22GHz, i.e. 300MHz under the
controlled voltage was biased from 0V to 2V. The oscillator was measured with Agilent
E4407B spectrum analyzer.Fig.7 shows the phase noise versus offset frequency from 1kHz to
10MHz. The phase noise was measured as -111.16 dBc/Hz at 1-MHz frequency offset from a
center frequency of 9.88 GHz.
Fig. 7
The phase noise versus offset frequency from 1kHz to 10MHz
To evaluate the overall performance of the VCO, a common figure of merit (FOM) is
used, which is given by[3]
FOM = L{ f offset } − 20 log(
f0
) + 10 log(
f offset
PDC
)
1mW
(1)
where L{ f offset } is the phase noise at a certain frequency offset ( f offset ) , f 0 is the
oscillation frequency, and PDC is the power dissipation. The simulated and measured
results of FOM versus variable voltage is shown in Fig.8.With a supply voltage of VDD=1.6 V
and Vb=0.8V, the value of FOM is from -179.1 dBc/Hz to -184.1 dBc/Hz under the controlled
voltage was biased from 0V to 2V.
-170
-172
measurement
simulation
-174
-176
FOM
-178
-180
-182
-184
-186
-188
-190
0.0
0.5
1.0
1.5
2.0
Control voltage (V)
Fig. 8
The figure of merit (FOM) versus control voltage
4.
CONCLUSION
A 10-GHz differential Colpitts VCO using transformer coupling technology
in TSMC 0.18 μm 1P6M CMOS process has been presented. Due to replacing the converters
(switches) by transformers, the circuit achieves a good performance in low power
consumption and low phase noise. By adding the output buffers, the parasitic effects are
reducing and the performance of whole circuit is not influenced.
ACKNOWLEDGEMENT
The authors are grateful to the Construction Industry Council (CIC) for supporting the
TSMC 0.18 μm 1P6M CMOS process. This work is financially supported by the National
Science Council, ROC (NSC 94-2215-E-182 –005).
REFERENCES
1.
Santo A. Smerzi and Giovanni Girlando, “A Ku-Band Monolithic Receiver for DVB-S
Applications, “IEEE Communications Magazine , August 2004
2. Chun-Yi Lee , “ RF Quadrature Voltage-Controlled Oscillator , ” Thesis of Master of
Science , Department of Electrical Engineering , National Cheng-Kung university ,
Tainan , Taiwan , R.O.C , June , 2003
3. Tommy K. K. Tsang and Mourad N. El-Gama1, “A High Figure Of Merit And
Area-Efficient Low-Voltage (0.7-1V) 12 GHz CMOS VCO , “ IEEE Radio Frequency
Integrated Circuits Symposium , 2003