THE DESIGN OF A LOW POWER FLOATING GATE BASED PHASE FREQUENCY DETECTOR AND CHARGE PUMP IMPLEMENTATION

International Journal of VLSI design & Communication Systems (VLSICS) Vol.4, No.2, April 2013
DOI : 10.5121/vlsic.2013.4206 63
THE DESIGN OF A LOW POWER FLOATING
GATE BASED PHASE FREQUENCY DETECTOR
AND CHARGE PUMP IMPLEMENTATION
Md Monirul Islam1
and Ankit Shivhare2
1
Department of Electronics Engineering, KIIT University, Bhubaneswar, India
imoniofficial@gmail.com
2
Department of Electronics Engineering, KIIT University, Bhubaneswar, India
ankitshivharenitdgr@gmail.com
ABSTRACT
A simple new architecture of phase frequency detector with low power and low phase noise is presented in
this paper. The proposed phase frequency detector is based on floating gate, consist of 4 transistors
including one floating gate pMOS and one floating gate nMOS constructed with two GDI (gate diffusion
input) cells and maintain main characteristics of conventional phase frequency detector in 180 nm
technology. Floating gate based methodology reduced the power of phase frequency detector about 51%.
Introduction of floating gate based phased frequency detector also reduces the number of transistor as
compared with conventional phase frequency detector.
KEYWORDS
Phase frequency detector, floating gate MOSFET, Voltage controlled oscillator, Gate diffusion cell,
Charge pump.
1. INTRODUCTION
The recent development of communication system has brought an increase demand for low power
and low noise system in phase-locked loop (PLL) design. Phase frequency detector is the most
key element in PLL design. The two non linear components present in PLL are phase frequency
detector (PFD) and voltage controlled oscillator (VCO). The main concept of phase frequency
detector or PFD is comparing two input frequencies in terms of both phase and frequency. In a
PLL the two frequencies are reference frequency (clk_ref) and voltage controlled oscillator
(VCO) output after division by N (clk_vco). The output is a pulse proportional to the phase
difference between the inputs and it drives the charge pump to either increase the control voltage
of the VCO or decrease it or keep it without changing. A PFD is usually built using by state
machine with memory element such as D flip-flop. There are many methodologies to simplify the
circuit. This paper presents a novel architecture of PFD design that floating gate based PFD
improves the power dissipation as compared with conventional PFD. As Floating gate based
circuit can operate at power supply voltage levels which are well below the intended operational
limits for a particular technology and consume less power than the minimum required power of a
circuit designed with only MOS devices in the same technology with the same performance.

64
2. PHASE FREQUENCY DETECTORS
Figure1 illustrates a common linear PFD architecture using two D flip flop implemented using
CMOS logic and a AND gate in the reset path. The PFD generates an UP and a DOWN signal
that passed through the charge pump followed by a loop filter and VCO. When clk_ref goes high
it will charge the upper D flip flop and results in UP signal to high and when clk_vco signal goes
high it will result DOWN signal to high. This will result both the inputs of AND gate high and
make the reset signal high to make both outputs low. It is known as the lock condition for the
PFD when both the outputs of PFD – UP signal and DOWN signal are low. When clk_ref is
leading it will result UP signal high and when lagging result DOWN signal high. The width of the
UP signal or DOWN signal is the measure of error. With the use of this error the control voltage
is generated and then that will correct out the frequency difference between input CLK and the
clock coming from the VCO from feedback path of the PFD [2].
Figure 1.Linear PFD detector
2.1. GDI BASIC CELL METHODLOGY
GATE DIFFUSION INPUT (GDI) technique has been used for the simultaneous generation of
digital logic functions.The GDI method is based on the use of a simple cell as shown in Figure 2.
At a first glance the basic cell resembles the standard CMOS inverter, but there are some
important differences: GDI cell contains three inputs–G (the common gate input of the nMOS and
pMOS transistors), P (input to the outer diffusion node of the pMOS transistor) and N (input to
the outer diffusion node of the nMOS transistor).The Out node (the common diffusion of both
transistors) maybe used as input or output port, depending on the circuit structure[3].the most
advantages of GDI cell over CMOS technology are low power circuit design, allows reducing
power consumption, reducing propagation delay and area, it also maintains low complexity of
logic design.

65
Figure 2.GDI basic cell
N P G Out Function
‘0’ B A ĀB F1
B 1 A Ā+B F2
1 B A A+B OR
B ‘0’ A AB AND
C B A ĀB+AC MUX
‘0’ 1 A Ā NOT
Table 1. Logic function of GDI cell
2.2. GDI BASED PHASE FREQUENCY DETECTOR
Figure 3.PFD state diagram
Figure 3 illustrates the finite state machine (FSM) of phase frequency detector. Initially both UP
and DOWN both signal are low. When one of the PFD input rises then corresponding output
becomes high. This state remains same until second input goes high which resets the circuit and
returns the FSM to the initial state [2].

66
Figure 4.GDI based phase frequency detector.
From basic GDI cell view table 1 when the supply voltage B is applied to PMOS and nMOS is
grounded and given input voltage in gate terminal is A then it gives output in the common drain
is ĀB.In figure 4, DOWN signal comes from the common drain terminal of pMOS PM1 and
nMOS NM3 and UP signal comes from the common drain of pMOS PM0 and nMOS NM4. The
UP signal of this circuit is clk_ref¯ ×clk_vco and the DOWN signal is clk_vco¯ ×clk_ref. So we
take output from common drain of this inverter configured on GDI cell.In this congiguration there
are only four transistors are use [1].the body bias substrate of pMOS is connected with VDD and
nMOS is connected with VSS ground.
Figure 5. Simulation of GDI based phase frequency detector
2.3. CHARGE PUMP
A charge pump is a three position electronic switch which is controlled by the three states of
PFD. When switch is set in UP or DOWN position, it delivers a pump voltage ±VP or a pump
current ±IP to the loop filter. When both UP and DOWN of PFD are off, the switch is open, thus
isolating the loop filter from the charge pump and PFD.
The charge pump coupled is a vital building block in the design topology, it helps provide an
effective controlling mechanism for the charging and discharging of the low pass filter.Also it is

67
used to manipulate the amount of charge on the loop filter capacitor depending on the UP and
DOWN signals from phase frequency detector [4]
Figure 6. Concept illustrating charge pump circuit
3.1. FLOATING GATE MOSFET
An FGMOS can be fabricated by electrically isolating the gate of a standard MOS transistor, so
that there are no resistive connections to its gate. A number of secondary gates or inputs are then
deposited above the floating gate (FG) and are electrically isolated from it. These inputs are
capacitive connected to the FG, since the FG is completely surrounded by highly resistive
material. So, in terms of its DC operating point, the FG is a floating node. [5]
Figure 7. Cross Sectional view of n channel FGMOS

68
Figure 8.Equilvalent Schematic of n channel N input FGMOS transistor
The distinctive feature of the FGMOS device is the set of input capacitors, denoted Ci where i =
[1, N] in Figure 8, between the effective inputs and the FG. The parasitic capacitances, CGD and
CGS, represented using dotted lines, are the same parasitic capacitances that would be present in
an MOS transistor fabricated using the same technology with the same active area. The
relationship between the DC drain to source current and the FG voltage, VFG, of an FGMOS is not
affected by parasitic capacitances. However, CGD, CGS and CGB do affect the relationship between
VFG and the effective input voltages Vi.
The value of VFG can be obtained by applying the charge conservation law to the floating node
(FG) shown in Fig.8. If there is an infinite resistance between the FG and all the surrounding
layers, there will be no leakage current between them, and so, the FG will be perfectly isolated.
So the voltage at the FG is given by [5]
ܸிீ = ෍
‫ܥ‬௜
‫ܥ‬்
ே
௜ୀଵ
ܸ௜ +
‫ܥ‬ீௌ
‫ܥ‬்
ܸௌ +
‫ܥ‬ீ஽
‫ܥ‬்
ܸ஽ +
ܳிீ
‫ܥ‬்
− − − − − − − (1)‫ݐ݅ݓ‬ℎ‫ݐݑ݋‬ ܾ‫ݕ݀݋‬ ܾ݅ܽ‫ݏ‬
ܸிீ = ෍
‫ܥ‬௜
‫ܥ‬்
ே
௜ୀଵ
ܸ௜ௌ +
‫ܥ‬ீ஻
‫ܥ‬்
ܸ஻ௌ +
‫ܥ‬ீ஽
‫ܥ‬்
ܸ஽ௌ +
ܳிீ
‫ܥ‬்
+ ܸௌ − − − (2)‫ݐ݅ݓ‬ℎ ܾ‫ݕ݀݋‬ ܾ݅ܽ‫ݏ‬
where N is the number of effective inputs.
The term QFG refers to a certain amount of charge that has been trapped in the FG during
fabrication. As this term is constant, it can be interpreted as a voltage offset at the FG, or
alternatively, as an offset in the threshold voltage of the device. The term CT refers to the total
capacitance seen by the FG and is given by [5]
‫ܥ‬் = ‫ܥ‬ீ஽ + ‫ܥ‬ீௌ + ‫ܥ‬ீ஻ + ෍ ‫ܥ‬௜
ே
௜ୀଵ
− − − (3)
The equations modelling the large signal behaviour of the FGMOS can now be obtained by
replacing VGS in the equations describing the large signal behaviour of the MOS transistor, with
the expression describing the voltage between the FG and source which can be obtained by
referring VFG to the source terminal rather than the bulk.

69
4. PROPOSED PHASE FREQUENCY DETECTOR USING FGMOS
.
Figure 9.Circuit diagram of the proposed PFD
The proposed phase frequency detector consist of 4 transistor including both FGMOS of n type
and p type.V1 of the FGPMOS is connected with bias voltage Vbias1 and V2 of FGNMOS of is
connected with Vbias2 voltage. Common gate of both FGMOS are given clk_vco input. The
source of the FGPMOS is given clk_ref as reference signal. The output DOWN signal is drawn
from common drain of two FGMOS. UP signal is drawn from common drain of PM1 and
NM1.When REF signal leads the feedback VCO signal the PFD detects a rising edge on the REF
signal and it will produce the UP signal and when feedback clk_vco signal leads reference signal
clk_ref it will produce the DOWN signal.
5. SIMULATION RESULT
The proposed phase frequency detector is given two pulse sequences clk_ref and clk_vco.
For simulation we apply clk_ref at 50MHz and clk_vco at 33.33MHz.

70
Figure 10.Input and Output waveform of proposed PFD
The output of the proposed PFD is given to the VCO after passing through charge pump and loop
filter. Charge pump provide an effective controlling mechanism for the charging and discharging
of the low pass loop filter. It is used to manipulate the amount of charge on the loop filter’s
capacitors depending on the UP and DOWN signal generated from phase frequency detector.
When UP signal goes up, the charge pump drives current into the loop filter which change the
VCO control voltage in such a way that frequency is increased and when DOWN signal goes up,
charge pump draw current from the loop filter to change the control voltage and slowdown the
VCO output frequency. Loop filter also removes jitter from charge pump to smoothen the control
voltage.

71
Figure11. Proposed PFD and Charge pump
Figure 12.Input output waveform of PFD and charge pump
5.1. PHASE NOISE ANALYSIS
The phase noise in Up and Down translates to random modulation of the time during which noise
is injected into the loop filter. There are three possible cases. The phase noise may modulate the
widths of Up and Down by the same amount, in which case the CP produces no net output. The
phase noise modulates only the position of Up with respect to Down. This effect is negligible.
Lastly, the phase noise may modulate the widths of Up and Down pulses differently and it is this

72
case that matters most [6]. The power supply 1.8V.the phase analysis is simulated cadence
virtuoso 180nm technology where the clk_vco in 25MHz and clk_ref 50MHz.
Figure 13.GDI based PFD phase noise
Figure 14. Proposed PFD phase noise
SI no PFDS Number of
transistor
Power
consumption
Types of transistor
Conventional 48 33µ nMOS, pMOS
Proposed PFD 4 16.1µ FGPMOS, FGNMOS, nMOS, pMOS
Table 2: Performance comparison between different PFD topologies
SI no PFDS Phase noise Offset frequency at
Conventional -112dbc/Hz
1 MHZGDI based PFD -123.9dbc/Hz
Proposed PFD -127.2dbc/Hz
Table 3: Performance comparison between different PFD topologies

73
6. CONCLUSIONS
This paper presents a new phase frequency detector or PFD architecture and charge pump
implementation using cadence 180nm 1.8V CMOS process technology. Most important
advantage of using floating gate reduce the power consumption. Use of floating gate reduces the
power 51% in proposed circuit, which is dominant factor in low power application This paper
also presents the minimum number of transistor for implementing PFD which becomes a crucial
factor in deep submicron as space is very important factor for complex circuit. The proposed
phase frequency detector maintain all functionality of a conventional PFD. Implementation of
charge pump circuit in the PFD circuit for phase lock loop application is also verified to present
the accuracy of proposed circuit. The presented circuit also very much noise free as its
investigation result shows -127dbc/Hz phase noise at 1 MHz frequency offset which is less than
the both conventional and simple GDI based phase frequency detector circuit. So the presented
circuit confirms its applicable identity for charge pump operation within a high performance PLL
application.
REFERENCES
[1] S.Siva, S.Yellampalli, Design of phase frequency detector and charge pump for high frequency
PLL,international journal of soft computing and engineering.vol-2,issue-2,May 2012.
[2] M.Mansuri, DeanLie, Ken yang Fast frequency acquisition phase-frequency detector for
Gsample/sphase locked loops.IEEE journal of solid state circuits.vol.37,no 10,oct 2002.
.[3] A.Morgenshtein, M.Moreinis, R.Ginosar,Asynchronous gate-diffusion input(GDI) circuits,Electrical
engg.dept,Isreal Institute of technology,Isreal.
[4] A.Mann,A.Karalkar,Lili He,M.Jones, department of electrical engineering,San Jose State
Univrsity,CA,USA
[5] Esther Rodriguez-Villegas ,Low Power and Low Voltage Circuit Design with the FGMOS Transistor,
Published by The Institution of Engineering and Technology, London, UK.
[6] A.Homayoun,B.Razavi, Analysis of Phase Noise in Phase/Frequency Detectors, IEEE transactions on
circuit and system regular paper,vol .60,no-3,March 2013
Authors
Md Monirul Islam : He received his B.E degree in electronics and communication
from The University of Burdwan, West Bengal, India in 2011.He is currently perusing
his M.tech course in VLSI and Embedded System from KIIT University,
Bhubaneswar, India. His area of research topic in digital VLSI design, logic design
verification, Low power VLSI.
Ankit Shivhare : He is currently working as Assistant Professor in School Of
Electronics Engineering, Kalinga Institute of Industrial Technology (KIIT
University), Bhubaneswar, Odisha. He has completed his BE in 2010 field of
Electronics and Communication from Government Engineering College, Jabalpur,
Madhya Pradesh and MTech in 2012 specialization in Microelectronics and VLSI
from National Institute Of Technology Durgapur, West Bengal. He has lecture
subjects including Electromagnetic Theory, Semiconductor Device, Control System,
Analog Electronic Circuits, Digital Electronic Circuits, MOSFET Device Modelling,
VLSI Design, Communication System- Analog and Digital, Basic Electrical Theory etc.
His current interest includes Analog, Digital, Mixed Signal IC Design.

THE DESIGN OF A LOW POWER FLOATING GATE BASED PHASE FREQUENCY DETECTOR AND CHARGE PUMP IMPLEMENTATION

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