Analysis of UHF-band Passive RFID Antennas Loaded byDickson Charge Pump
Yuta Watanabe, and Hajime Igarashi
11Graduate School of Information Science and Technology, Hokkaido University, JapanIntroduction

The UHF-band passive radio frequency identification(RFID) systems consist of a reader-writer and several RFID tags.
They attain considerable attentions due to their two practicaladvantages that the RFID tag does not need battery andcommunicate distance between reader-writer and RFID tag is afew meters [1]. The RFID tag is composed of a tag antenna
receiving electromagnetic waves from reader-writer and IC chip
which rectifies and amplifies the received voltage. Previous
studies of UHF-band passive RFID have presumed a linear
equivalent circuit of IC chip [2, 3]. However, the IC chip is nonlinear circuit including often FET and diode in real situations. In[4], the authors have described non-linear analysis of themeander line antennas loaded by the Cockcroft-Walton (CW)
circuit, where the performance of RFID tag is computed byhybridization of finite-difference time-domain (FDTD) andmodified nodal analysis (MNA). However, it would be difficult
to realize the CW circuit in integrated circuits. This study
assumes that the antenna is loaded by the Dickson charge pump(DCP) which can be realized in integrated circuits. To consider
the non-linearity of DCP, the V-I characteristics of FET isrepresented by the Akima interpolation [5]. The planer latticeantenna (PLA) composed of line antenna is assumed to be thetag antenna.
2Hybridization of FDTD method and MNA

The coupling of the FDTD method and MNA will briefly bedescribed in this section. The Maxwell equations are given by∂H∂t∂E∇× H = J +ε.
∂t∇ × E = −µ(1)(2)

where J=σE is conductive current. In the FDTD method, (1) and(2) are approximated by spatial and time central difference andcalculated in explicit time domain as follows:Line Antenna

CircuitXSZzYyx

Fig. 1 Line antenna parallel to z-axis loaded by nonlinear circuit.

ILIC0

VL
Non-linear
circuitFig. 2 Equivalent circuit

En =1 − σ∆t / 2ε n−1∆t / εE+∇ × H n −1/ 2 (3)1 + σΛt / 2ε
1 + σΛt / 2ε∆t(3)H n+1/ 2 = H n−1 / 2 + ∇ × E n .

µLet us consider the line antenna loaded by non-linear circuit,shown in Fig. 1, parallel to z-axis. It is assumed that the spatial
size of non-linear circuit is sufficiently smaller than that of theantenna. ∆X, ∆Y and ∆Z are cell size of the FDTD method. By
integrating (2) on the surface S of FDTD cell, we obtain

C0∂VL+ I L (VL ) = I∂t(4)where VL = Ez∆Z is voltage imposed to the non-linear circuit, C0= ∆X∆Y / ∆Z is spatial capacitance of the FDTD cell, IL iscurrent flowing into the non-linear circuit and I is the total
current given byI = ∫∂S H ⋅ ds .
(5)
The equivalent circuit corresponding to (4) is composed ofparallel circuit of current source I obtained by (5), capacitanceC0 and non-linear circuit shown in Fig. 2. The node voltages of
equivalent circuit shown in Fig. 2 are analyzed by MNA.
The followings are procedures in coupling analysis of theFDTD method and MNA.
1) The magnetic field Hn-1/2 is computed in the FDTD process,where n denotes the time step.
2) The total current I is calculated from Hn-1/2 by (5) in theMNA process.
3) The electric field in non-linear circuit is calculated by
Ez=VL/∆Z, and other electric field En is computed in theFDTD process.
4) Steps 1) to 7) are repeated until the steady state solutions
are obtained.
33.1

RFID tagTag antenna
In this work, the PLA shown in Figure 3 is considered,
which contains the floating lines and loops. The structure will be
optimized to maximize the output voltage of DCP.

21xz

99zyFig. 3 Planer lattice antennaEz

3.2Dickson charge pump
In this study, the DCP circuit shown in Fig. 4 is considered.
The DCP circuit rectifies and amplifies the input voltage
received by the tag antenna. Although the DCP circuit iscomposed of diodes and capacitances in general, the diode isrepresented by using FET. The V-I characteristics of FET isshown in Fig. 5, where VGS is voltage between gate and source
terminal, VDS is voltage between drain and source terminal. Thedrain current ID is interpolated by Akima interpolation. TheAkima interpolation is carried out twice because ID depends on
VGS and VDS.
4NZyHxNXxNYFig. 6 Analysis model

1.0

Numerical Results

The UHF-band passive RFID model shown in Fig. 6 isanalyzed by hybridization analysis of the FDTD method andMNA. The incident electromagnetic wave from reader-writer isassumed to be plane wave. In FDTD process, the size of FDTD
cell, where ∆X=∆Y=∆Z, is set to 3 mm. The number of FDTD
cell, where NX=NY=NZ, is set to 100. The frequency of incident
wave is set to 1 GHz. The amplitude of incoming electric field isassumed to be 20 V/m. The perfect match layer is employed toenforce the free space conditions on the domain boundary. The
time evolution of resultant output voltage Vout of PLA shown inFig. 3 loaded by the DCP circuit is shown in Fig. 7. It can beobserved in Fig. 7 that Vout tends to increase almost
monotonously and reach at the steady value although it includes
small ripples. If the ripple amplitude becomes too large, thedigital circuit, to which Vout is supplied, would have
VinCCCCC

V out [V]

0.8

0.4

0.2

0.00CR

VoutRg

GND

Fig. 4 Dickson charge pump circuit

[2]

4.0
VGS = 0.25 V

3.5

VGS = 0.5 V

3.0

VGS = 0.75 V] 2.5
Am[ID2.0[3]VGS = 1.0 V
VGS = 1.25 V
VGS = 1.5 V

1.5

[4]

VGS = 1.75 V

1.0

VGS = 2.0 V
VGS = 2.25 V

0.5

VGS = 2.5 V
0.0
2.0

VDS [V]

Fig. 5 Drain current of FET

80

2.5Conclusions

References
[1]

1.560

The characteristics of RFID tags composed of PLA andDCP circuit is computed by hybridization of the FDTD methodand MNA. The sinusoidal input voltage received by PLA is
successfully rectified and amplified by the DCP.
In extended paper, we will discuss optimization of PLA
shape in order to maximize the output voltage of DCP circuit.

4.5

1.040
Time [ns]Fig. 7 Output voltage of DCP circuit

50.5

20malfunctions in its operation. It is found from computations thatthe ripple can be reduced by increasing C in Fig. 4, whereas therise time becomes longer with C.

Cout

0.0

0.6

[5]

R. Want, An introduction to RFID technology, IEEE Pervasive
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H. Makimura, Y. Watanabe, K. Watanabe, H. Igarashi, Evolutional
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5. Pp. 1510-1513, 2011
Y. Watanabe, H. Igarashi, Accelerated FDTD analysis of antennas
loaded by electric circuits, vol. 60. No. 2, pp. 958-963, 2012.
H. Akima, A new method of interpolation and smooth curve fitting
based on local procedures, J. ACM, vol. 17, no. 4, 1970.