Circuits and Systems, 2011, 2, 190-195
doi:10.4236/cs.2011.23027 Published Online July 2011 (
Copyright © 2011 SciRes. CS
Voltage Controlled Ring Oscillator Design with Novel
3 Transistors XNOR/XOR Gates
Manoj Kumar1, Sandeep Kumar Arya1, Sujata Pandey2
1Department of Electronics & Communication Engineering Guru Jambheshwar,
University of Science & Technology, Hisar , India
2Department of Electronics & Communication Engineering, Amity University, Noida, India
Received April 14, 2011; revised May 6, 2011; accepted May 13, 2011
In present work, improved designs for voltage controlled ring oscillators (VCO) using three transistors
XNOR/XOR gates have been presented. Supply voltage has been varied from [1.8 - 1.2] V in proposed de-
signs. In first method, the VCO design using three XNOR delay cells shows frequency variation of [1.900 -
0.964] GHz with [279.429 - 16.515] µW power consumption variation. VCO designed with five XNOR de-
lay cells shows frequency variation of [1.152 - 0.575] GHz with varying power consumption of [465.715 -
27.526] µW. In the second method VCO having three XOR stages shows frequency variation [1.9176 - 1.029] GHz
with power consumption variation from [296.393 - 19.051] µW. A five stage XOR based VCO design shows
frequency variation [1.049 - 0.565] GHz with power consumption variation from [493.989 - 31.753] µW.
Simulations have been performed by using SPICE based on TSMC 0.18µm CMOS technology. Power con-
sumption and output frequency range of proposed VCOs have been compared with earlier reported circuits
and proposed circuit’s shows improved performance.
Keywords: CMOS, Delay Cell, Low Power, VCO, XOR and XNOR Gates
1. Introduction
The Phase locked loops (PLL) are widely used circuit
component in data transmission systems and have exten-
sive applications in data modulation, demodulation and
mobile communication. Voltage control oscillators (VCO)
are the critical and necessary building blocks of these
PLL systems. Two widely used VCOs types are LC tank
based and CMOS ring circuits. Combination of inductor
and capacitor consumes large layout area in LC tank
based oscillators [1-3]. CMOS ring based oscillators
have advantages due to ease of controlling the output
frequency and no requirement for on chip inductors [4,5].
CMOS based ring oscillators are easier to integrate and
also gives wide tuning range. Due to flexibility of on
chip integration, CMOS based ring oscillators have be-
come essential building blocks in various battery oper-
ated mobile communication systems. Rising requirement
of portable devices like cellular phones, notebooks, per-
sonal communication devices have aggressively en-
hanced attention for power saving in these devices. Pow-
er consumption in very large scale integration (VLSI)
systems includes dynamic, static power and leakage
power consumption. Dynamic power consumption re-
sults from switching of load capacitance between two
different voltages and dependent on frequency of opera-
tion. Static power is contributed by direct path short cir-
cuits currents between supply (Vdd) and ground (Vss) and
it is dependent on leakage currents components [6,7].
VCOs being the major components in PLL system and is
responsible for most of the power consumption. Some
draw back of ring based oscillators includes large power
consumption, phase noise and the limit of highest
achievable frequency. In modern VCOs design power
consumption and output frequency range are significant
performance metrics [8-13]. A ring oscillator consist of
delay stages, with output of last stage fed back to input
of first stage. A VCO block diagram with single ended
N-delay stages is shown in Figure 1.
The ring must provide a phase shift of 2π and unity
voltage gain for oscillation occurrence. Each delay cell
also gives a phase shift of π/N, where N is number of
delay stages. The remaining π phase shift is provided by
dc inversion using the inverter delay cells. For single
ended oscillator design the odd numbers of delay stage
are required for dc inversion. Frequency of oscillation
with N-single ended delay stages is given by o
12 d
Nt , where N is the number of delay stages and td
is delay of each stage [9,14]. Delay stages are the basic
building blocks in any VCO design and improved design
of these delay cells will improve the overall perform-
ances of VCO. Various types of delay cells have been
reported for VCO design including multiple-feed- back
loops, dual-delay paths and single ended delays. These
delay cells have been implemented by various ap-
proaches like simple inverter stage, latches, cross cou-
pled cells etc. [15-18].
In present work modified VCOs circuits with three
transistor XNOR/XOR delay cells have been presented
with reduced the power consumption and wide output
frequency range. The paper is organized as follows: In
Section 2 three & five stages XNOR/XOR based ring
VCOs have been presented. In Section 3 results for the
three proposed VCOs have been obtained and compari-
sons with earlier reported structures have been made.
Finally, in Section 4 conclusions have been drawn.
2. System Description
The frequency of single ended ring VCO is dependent on
the delay provided by the each delay cell. In the pro-
posed designs new delay cells based on three transistor
XNOR/XOR gates have been used. Inverter operation
has been implemented by XNOR/XOR gates. Direct path
between Vdd and ground has been eliminated in proposed
delay cells, due to which leakage power is reduced and
the designs are power efficient. The circuits have been
designed in 0.18 μm CMOS technology with supply vol-
tage of 1.8 V. Supply voltage/control voltage has been
varied from 1.8 to 1.2 V for obtaining the output fre-
quency at different supply voltages.
First proposed delay cell is shown in Figure 2. XNOR
delay stage is made up of two NMOS transistors and one
PMOS transistor. Out of two input terminal of XNOR
gate, one is connected to ground and signal is applied to
other terminal. This circuits works as inverter without
having direct path between Vdd and ground with saving in
power consumption. A small capacitance of 0.01 pf at
output of each delay cell has been included. The gate
Figure 1. Block diagram of single ended VCO.
lengths of all three transistors have been taken as 0.18 μm.
Widths (Wn) of NMOS transistors (N1 & N2) have been
taken 2.5 µm and 0.5 µm respectively. Width (Wp) for
transistor P1 has been taken as 1.0 µm. Output frequency
is controlled by varying the supply voltage of XNOR
delay stage. Three and five stages ring VCOs have been
designed using proposed XNOR delay cell as shown in
Figures 3(a) and (b).
Figure 4 shows proposed XOR delay cell, which con-
sist of two PMOS transistors (P1 & P2) and one NMOS
transistor (N1). One input terminal of XOR gate is con-
nected to control voltage (Vc) and signal is applied to
other terminal so that circuit works as an inverter. The
gate length of all three transistors has been taken as 0.18
μm in XNOR delay cell. Width (Wn) of NMOS transistor
N1 has been taken 0.25 µm. Width (Wp) for P1 & P2
transistors has been taken as 2.0 µm. Output frequency is
controlled by varying the control voltage (Vc) of second
input terminal of XOR delay stage. Three and five stages
ring VCOs have been designed using proposed XOR
delay cell as shown in Figures 5(a) and (b).
Figure 2. Proposed delay cell based on XNOR gate.
Figure 3. (a) 3 stages, (b) 5 stages ring VCO based on
XNOR gate delay cell.
Copyright © 2011 SciRes. CS
Figure 4. Proposed delay cell based on XOR gate.
Figure 5. (a) 3 stages, (b) 5 es ring VCO with on XOR
. Results and Discussions
imulations have been performed using SPICE based on
ows output waveform for three & five
ges ring
Table 1. Results for XNOR delay based VCO.
gate delay cell.
TSMC 0.18 μm technology with supply voltage varia-
tions from [1.8 - 1.2] V. Table 1 shows the results for three
and five stages VCOs designed with XNOR delay cells.
Supply /control voltage (Vc) has been varied from [1.8 -
1.2] V. Output frequency of three stage VCO shows vari-
ation from [1.900 - 0.964] GHz with power consumption
variation of [279.429 - 16.515] µW. In five stages ring
VCO frequency shows variation from [1.152 - 0.575] GHz
with varying power consumption [465.715 - 27.526] µW.
Figures 6(a) and (b) shows frequency and power con-
sumption variation for three and five stages XNOR based
ring VCOs.
Figure 7 sh
ages XNOR VCOs at supply voltage of 1.8 V.
Table 2 shows results for three and five sta
COs designed with XOR delay cells. Control voltage at
the second input terminal of delay cells has been varied
from [1.8 - 1.2] V. In three stage VCO, output frequency
shows variation [1.917 - 1.029] GHz with varying power
consumption of [296.393 - 19.051] µW. For five stage
XOR VCO frequency varies from [1.049 - 0.565] GHz
with varying power consumption of [493.989 - 31.753] µW.
Figures 8(a) and (b) shows frequency and power
consumption variation for three and five stages XOR
based ring VCOs. Figure 9 shows output waveform for
three & five stages XOR based VCO at supply voltage of
1.8 V.
Three stages XNOR VCO Five stages XNOR VCO
fr con fr con
Output Power
Output Power
1.8 2 4 1.900 79.429 1.152 65.715
1.7 1.773 210.349 1.071 350.582
1.6 1.632 151.432 0.978 252.388
1.5 1.469 102.570 0.884 170.950
1.4 1.312 63.786 0.789 106.311
1.3 1.144 35.179 0.686 58.632
1.2 0.964 16.515 0.575 27.526
Figure 6. (a) Frequency, (b) power consumption variations
of 3 and 5 stages XNOR based VCO.
Copyright © 2011 SciRes. CS
Figure 7. Wave forms at 1.8r (a) 3 stages XNOR VCO,
(b) 5 stages XNOR VCO.
r XOR delay based VCO.
V fo
Table 2. Results fo
Three stages XOR VCO Five stages XOR
Voltage (Vc)
fr fr
(GHz) (µW)
(GHz) (µW)
1.8 1.917 296.393 1.049 493.989
1.7 2 3 1.794 25.6140.981 76.023
1.6 1.660 164.559 0.908 274.266
1.5 1.507 113.140 0.827 188.568
1.4 1.363 71.500 0.747 119.167
1.3 1.202 40.054 0.656 66.757
1.2 1.029 19.051 0.565 31.753
Figure 8. (a) Frequency, (b) power consumption variations
of 3 and 5 stages XOR VCOs.
Figure 9. Wave forms at 1.8 V for (a) 3 stages, (b) 5 stages
In reported circuits, power consumption is increasing
with increase in number of delay stages whereas output
frequency is showing downward trend. Number of stages
may be decreased or increased depending upon the ap-
plication, requirement for frequency range and power
consumption. A comparison with earlier reported circuits
in terms of power consumption and output frequency
range is given in Table 3. Proposed circuits’ shows better
Copyright © 2011 SciRes. CS
Copyright © 2011 SciRes. CS
Table 3. Comparison of
VCO performance.
Vdd (V) Technology (µm) Power consumption VCO designs Operating frequency (GHz)
[1] 2.17 - 2.73 0.9 0.18 2.7 mW
[5] 0.39 - 1.41 1.8 0.18 12.5 mW
[10] 0.12 - 1.3 0.5 0.18 0.085 mW
[13] 16.8 mW
[16] 0.65 - 1.6 1.8 0.18 39 mW
Present work [3 stages XNOR] [
ent work [5 stages XNOR] 1 [465.7
ent work [3 stages XOR] [296.3 µW
nt work [5 stages XOR] 1. [493. µW
1.57 - 3.57 1.8 0.090
1.900 - 0.964 1.8 0.18 279.429 - 16.515] µW
Pres.152 - 0.575 1.8 0.18 15 - 27.526] µW
Pres1.917 - 1.029 1.8 0.18 93 - 19.051]
Prese049 - 0.5651.8 0.18 989 - 31.753]
perform in terms of power consumptiotput
4. Con
reported work improved power efficient designs for
CO with XNOR
iation [1.900 - 0.964] GHz with devia-
ption from [279.429 - 16.515] µW.
o. 7, July 2008, pp.
ancen and ou
cy range than compared c
three and five stages CMOS ring VCOs have been pre-
sented. In first methodology design with XNOR delay
ages have been presented. Three stages Vst
shows frequency var
on in power consumti
Five stages XNOR delay based VCO gives output fre-
quency range [1.152 - 0.575] GHz with power consump-
tion variation [465.715 - 27.526] µW. In the second me-
thodology VCO designed with three stages XOR based
delay cell shows frequency variation [1.917 - 1.029] GHz
with power consumption variation [296.393 - 19.051] µW.
Finally the VCO designed with five stages XOR delay
cells shows frequency variation [1.049 - 0.565] GHz with
power consumption variation [493.989 - 31.753] µW.
Proposed designs have been compared with previously
reported design and present approach shows significant
power saving with wide tuning range.
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