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MIXED
DESIGN
MIXDES 2013, 20th International Conference "Mixed Design of Integrated Circuits and Systems", June 20-22, 2013, Gdynia, Poland
Wide-Frequency-Range Low-Power Variable-Length
Ring Oscillator in UMC CMOS 0.18 Pm
Maciej Frankiewicz, Andrzej Kos
AGH University of Science and Technology
Department of Electronics
Kraków, Poland
[email protected], [email protected]
Abstract—The paper describes structure and simulation
results of the novel ring oscillator designed in UMC CMOS
0.18 Pm (1.8 V). Frequency generated by the oscillator is tuned
by scaling the supply voltage, additionally ring length is digitally
controlled. Presented ring oscillator has very wide tuning range
(250 MHz-2.1 GHz) with small current consumption (34-689 PA).
Index Terms—VCO, ring oscillator, low-power, CMOS
I. INTRODUCTION
Voltage-Controlled Oscillators (VCOs) are commonly used
circuits in modern electronics and have many different
applications, for example in PLLs (Phase-Locked Loops) or
DPM (Dynamic Power Management) systems such as DFS
(Dynamic Frequency Scaling). Growing requirements of these
applications result with need of power-efficient oscillators with
very wide tuning range and possibly small phase noise [1-3].
The paper describes structure and presents some simulation
results of voltage-controlled ring oscillator with variable ring
length. The circuit was designed in UMC CMOS 0.18 Pm
technology using full-custom technique and has very wide
tuning range and small current consumption.
II. RELATED WORKS
In modern microelectronics the oscillator circuits are
present in almost every project. There is plenty of applications
for such circuits. Microprocessors and other digital circuits
which are designed nowadays contain built-in PLLs. Radio
transmitters and receivers for portable devices, which are
becoming important part of the market of electronics, are also
based on generators. As a result a lot of researcher’s attention
has been put to design low-power and wide-frequency-range
oscillators in late years. The most popular choices of circuit
structures with examples of applications will be shortly
described in this section.
Very frequently used structure of generator is resonant
cross-coupled circuit [1][2]. Popularity of this type of circuit
comes from the fact, that it is very well-known and it is quite
easy to obtain high frequency of oscillations. What is
additional advantage – LC circuit should produce accurate
sinusoidal wave which means very low phase noise [3]. Usage
of varactors in resonant circuit results with extremely easy
control of the generated frequency by scaling voltage
connected between the diodes. Unfortunately, this favourable
at first glance structure has some features which disqualify for
some specified applications. In CMOS technologies resonant
elements cover quite big area which cannot be used by other
elements. Capacitors are usually realized as poly-poly or metalmetal structures while inductors are metal spiral or round
geometries. Consequently capacitances and inductances have
very low values. That results with very high frequencies in
range of few GHz (which sometimes is advantage) and
necessity to use large elements. Other very important
disadvantages are low accuracy of resonant elements
fabrication (which leads to uncertainty of frequency of
generated sine) and quite big power consumption (which can
be a serious problem for portable devices and other
applications which require low power dissipation).
Another popular structure of generator is a ring-oscillator
[4][5]. The basic idea is to connect an odd number of delay
stages with a feedback loop to obtain constant change of their
outputs. Propagation time of single delay stage is very short so
total generated frequency is quite high, which is feature similar
to previously described resonant circuits. Great advantage of
ring oscillator structure is much smaller area used by the
elements and much smaller power consumption – in
comparison to resonant circuits. The problem in designing
VCOs with ring oscillator is how to control the frequency.
Several different approaches can be mentioned here: control of
the supply, including additional delay stages or usage CurrentControlled Oscillators (CCOs) and elements which transfer
control voltage into current (in that case the time of switching
single stage is proportional to the control current). Next issue is
quite big phase noise and malformed shape of wave as a result
of quite big rise/fall times in comparison to period of the
oscillator wave.
There are also some attempts to design other oscillator
structures, like multivibrator-based oscillators [6]. In this work
an example of ring-oscillator dedicated for a dynamic power
management system will presented.
III. RING OSCILLATOR STRUCTURE
Ring oscillator is a generator consisting of odd number
(2N-1) of inverters in series, as shown in Fig. 1. Such circuit
generates oscillations with frequency described by (1) where n
is number of inverters in ring and tP is a propagation time of
single inverter.
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decrease of current consumption. Additionally such circuit
propagation time can be controlled by change of supply voltage
value.
IV. RESULTS AND DISCUSSION
To verify assumptions described in previous section a ring
oscillator consisting of 3 to 7 inverters tuned by supply voltage
have been designed in UMC CMOS 0.18 Pm (1.8 V)
technology and tested in Spectre simulator.
f0
1
2nt P
(1)
Equation (1) means that generated frequency can be tuned
by controlling inverters propagation times. Such effect is often
used and can be achieved by including some additional delay
stages. More interesting approach is direct control of inverters
tP for example by controlling their supply voltage. When
supply voltage is lowered the internal capacitances of the
circuit are reloaded slower what affects the propagation time.
Such approach will be presented in this paper. Obviously,
another inverter with constant supply is needed as an output
buffer to ensure constant amplitude of the oscillations. Another
way to tune the oscillator frequency can be change of number
of inverters in series. For that reason some transmission gates
have been included in the ring oscillator structure. By enabling
one of the transmission gates ring can be shortened and
variable length of the ring can be achieved resulting with much
wider tuning range. Presented structure is ring oscillator with
length of 3, 5 or 7 inverters. For that three transmission gates
are necessary and 3-bit signal is needed to control the
transmission gates. Additionally, by disabling all the gates the
ring can be opened to stop the oscillations.
Tested circuit generated oscillations when was supplied
with 0.8 to 1.8 V. Tuning characteristic for different lengths of
the ring is shown in Fig. 3. Presented structure has very large
tuning range of 250 MHz to 2.1 GHz which is about 88.1%
compared to the maximum frequency. Fig. 4 shows maximum
current consumption of the ring oscillator in dependence of the
control voltage for different ring lengths. The circuit consumes
very small amount of current of 34 to 689 PA for different
cases, although the maximum value is consumed only at short
peaks and for most of the time current consumption is much
smaller.
2.2
2
n=3
n=5
n=7
1.8
Generated frequency [GHz]
Figure 1. Block diagram of the basic ring oscillator.
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0.8
1
1.2
1.4
Control voltage [V]
1.6
1.8
Figure 3. Tuning characteristic of the ring oscillator for different ring
lengths.
700
Figure 2. Schematic diagram of the CMOS XNOR gate as inverter.
CMOS inverters seem a good choice as a delay stage for
ring oscillator from power-efficiency point of view since they
consume current only when changing the output state.
Unfortunately, when circuit works as oscillator inverters are
continuously toggled and more power saving cell than simple
inverter is needed. The solution can be CMOS XNOR gate
shown in Fig. 2 [4]. After shortening one of the inputs to the
ground the XNOR gate would operate similar to the CMOS
inverter cell but in presented circuit structure there is no direct
path between supply voltage and ground resulting with
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Current consumption [uA]
600
n=3
n=5
n=7
500
400
300
200
100
0.8
1
1.2
1.4
Control voltage [V]
1.6
1.8
Figure 4. Maximum current consumption of the ring oscillator for different
ring lengths.
Another important issue for circuit work is change of the
ring length which should be made on the edge of the generated
signal. If controlled properly presented ring oscillator generates
square wave with digitally controlled frequency, as shown in
Fig. 5 from spectre simulator. In the figure the VCO is
controlled by constant supply voltage of 1.8 V. At first part of
the figure below there are 3 inverters in the ring (2.1 GHz), at
about 7 ns the ring is switched to 5 inverters (1.34 GHz) and
finally at about 12 ns there are 7 inverters in the ring (1.07
GHz). At the end the ring is opened and oscillations stop.
V. CONCLUSIONS
The paper presented structure and simulation results of
XNOR-based ring oscillator. Presented circuit produces a wide
range of frequencies from 250 MHz to 2.1 GHz and consumes
extremely small amount of current – respectively from 34 to
689 PA at peaks. Presented structure has also an option of
opening the ring to stop the oscillations.
Presented circuit is a part of the Dynamic Power
Management system designed by authors of the paper but can
be used to any other application which requires wide range of
generated frequency with small power consumption.
ACKNOWLEDGMENT
This work was supported by polish National Centre of
Science (Narodowe Centrum Nauki) (grant NCN N N515
500340).
REFERENCES
[1]
Figure 5. Transient response of the ring oscillator for different ring lengths
with 1.8 V control voltage.
Table 1 compares tuning ranges and current consumption
of presented circuit with some latest papers. Extremely wide
tuning range and very small current consumption of presented
work are clearly visible what makes described circuit very
useful for implementation in different applications.
TABLE I. COMPARISON OF PARAMETERS OF DIFFERENT VOLTAGECONTROLLED OSCILLATORS.
Paper
Tuning
range
[GHz]
Tuning
range
[%]
Current
consumption
Technology
[nm]
[7]
3.15-4.6
33.7
8.5 mA
130
[8]
0.68-1.65
58.8
2 mA
65
[9]
0.62-1.5
58.7
22.8 mA
180
[10]
0.96-1.9
0.58-1.15
49.5
49.6
9.2-279 PA
15.3-164 PA
180
This
work
0.25-2.1
88.1
34-689 PA
180
As can be clearly seen from the table above, the LC
structures [8][9] have much greater power consumption than
presented circuit. Power saving can be achieved by usage of
ring oscillator [10]. Power consumption of circuit presented in
this work is slightly greater than in case of [10] but here the
maximum (peak, not mean) values are given. What is more,
presented XNOR-based ring oscillator with variable ring length
has significantly wider tuning range than any other circuit from
the table.
J. Kim, J. Shin, S. Kim, H. Shin „A Wide-Band CMOS LC VCO with
Linearized Coarse Tuning Characteristics”, IEEE Transactions on
Circuits and Systems – II: Express Briefs, Vol. 55 No. 5, 2008
[2] B. Jung, R. Harjani „High-Frequency LC VCO Design Using Capacitive
Degeneration”, IEEE Journal of Solid-State Circuits, Vol. 39 No. 12,
2004
[3] P. Dudulwar, K. Shah, H. Le, J. Singh „Design and Analysis of Low
Power Low Phase Noise VCO”, Proceedings of the International
Conference Mixed Design of Integrated Circuits and Systems
MIXDES’2006, 2006
[4] W.H. Tu, J.Y. Yeh, H.C. Tsai, C.K.Wang „A 1.8 V 2.5-5.2 GHz CMOS
Dual-Input Two-Stage Ring VCO”, Proceedins of the IEEE Asia-Pacific
Conference on Advanced System Integrated Circuits AP-ASIC’2004,
2004
[5] P. Mroszczyk, A. Goáda, A. Kos „Niskomocowy generator
pierĞcieniowy CMOS sterowany napiĊciem”, Materiaáy IX Krajowej
Konferencji Elektroniki, 2010
[6] J. Jasielski, W. Koáodziejski „Low Voltage CMOS Voltage-Controlled
Multivibrator and Resonant VCO Circuits”, Proceedings of the
International Conference Mixed Design of Integrated Circuits and
Systems MIXDES’2009, 2009
[7] B.Saeidii, J. Cho, G. Taskov, A. Paff, “A wide-range VCO with
optimum temperature adaptive tuning,” Proceedings of the 2010 IEEE
Radio Frequency Integrated Circuits Symposium, Anaheim, USA, 2010,
pp. 337-340
[8] L. Lou, L. Sun, H. Gao, J. Wen, “A 0.68-1.65GHz CMOS LC voltagecontrolled oscillator with small VCO-gain and step variation,”
Proceedings of the 13th International symposium on Integrated Circuits,
Singapore, 2011, pp.79-82
[9] S.Y. Lee, S. Amakawa, N. Ishihara, K. Matsu, “Low-phase-noise widefrequency-range differential ring-VCO with non-integral subharmonic
locking in 0.18 um CMOS,” Proceedings of the 40th European
Microwave Conference, Paris, France, 2011, pp. 1611-1614
[10] M. Kumar, S.K. Arya, S. Padley, “Voltage controlled ring oscillator with
novel 3 transistors XNOR/XOR gates,” Circuits and Systems, Vol. 2,
2011, pp.190-195
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