Numerical Determination of Frequency Guard Band Resonances for Chipless RFID Tags

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Numerical Determination of Frequency Guard Band 
Resonances for Chipless RFID Tags 
Gilberto T. Santos-Souza, Andreia Ap. de C. Alves, 
Leonardo L. Bravo-Roger, Member, IEEE 
Telecommunication Division School of Technology 
University of Campinas - UNICAMP 
Limeira - SP, Brazil 
g081491@dac.unicamp.br, a116142@dac.unicamp.br, 
leobravo@ft.unicamp.br 
Abstract-This paper presents a numerical study to show the 
frequency guard band effect in the design of spiral resonators 
coupled in son microstrip line for encoding Chipless RFID Tags 
in S-band. Simulations in the software HFSS version 15.0 were 
performed in this study. 
Keywords-Chipless RFID Tag, Resonator, Frequency Guard 
Band 
I. INTRODUCTION 
In the Chipless RFID Tags systems, the resonators are 
responsible for encoding data, where each resonator represents 
one bit. The function of these resonators is creating a low 
impedance path through 50n microstrip line, the resonator and 
the ground, when it works on its resonance frequency, creating 
a short-band effect on this frequency due to its internal 
resistance. The inductances [1] and capacitances [2] of the 
resonator are the parameters that define its resonance 
frequency by (1): 
1 
fr 
= 
2rr.JIT 
(1) 
Basically, these resonant frequencies are tuned by 
changing the resonator's length. The increase of the 
resonator's length will be also increasing the inductance and 
capacitance, lowering the resonant frequency. 
In the literature is possible to find several Chip less RFID 
Tags technologies [3-6], but there are not studies of frequency 
guard band for cascaded resonators. To avoid reading errors, 
each resonator should resonate or not in the frequency for 
which it is designed. For reasons of limited spectrum, it is 
desirable that the resonant frequencies of the resonators are 
close, this is, larger number of bits in the same band. 
However, adjacent resonators can resonate in the same 
frequency resulting in read errors from the ID Tag. At S-band, 
typically each resonator occupies 20MHz of bandwidth, 
approximately. This paper shows how to design a six bits 
Chipless RFID Tag in the substrate Taconic TLX-O 
(Er = 2.45, tan 0 = 0.0019 and h = 0.87mm) with a 
frequency guard band that avoids interference in the 
resonances. This study can be also used for other binary 
classifications of Chipless RFID Tags in the S-band. 
Department of Microwaves and Optics (DMO), School of Electrical and 
Computer Enginnering (FEEC), University of Camp in as (UNICAMP). 
Hugo E. Hernandez-Figueroa, Senior Member, IEEE 
Departament of Microwaves and Optics (DMO) 
13083-970, Campinas-SP, Brazil 
hugo@dmo.fee.unicamp.br 
II. DESIGN OF CHIPLESS RIFD TAG 
Fig. 1 illustrates the parameters construction of the spiral 
resonator. The design of the Chipless RFID Tag is shown on 
Fig. 2, where Ll, L2, L3, L4, Ls and L6 are the different lengths 
of the resonators to tune its resonance at different frequencies, 
respectively. This tag is classic in the literature and your in­
depth study can be found in [3]. 
Jw 
.... 1 
I' • ... 
Fig. 1. Spiral resonator: Dgap spiral = O.3mm, Wspiral = O.8mm, 
W = 2.26mm and Dgap reed = O.4mm, [3]. 
Fig. 2. HFSS model for six bits Chipless RFTD Tag. 
III. RESULTS 
To simplify the results analysis, the Tag's antennas are 
removed from the simulation model and replaced by ports that 
feed the RF transmission line. Table I shows the resonators's 
lengths values in millimeters (mm) for two different designs. 
TABLE I. LENGTH PARAMETERS OF THE RESONATORS TN 
MILIMETERS 
PROJECTS Ll L2 L3 L4 L5 
1 10,1 9,7 9,1 8,7 8,4 
2 11 10,2 9,4 8,7 8 
L6 
8,1 
7,5 
In Project 1 [3], there are resonant frequencies very close to 
adjacent resonators. The Project 2 shows a case where the 
resonances are separated by appropriates frequency guard 
bands. The six resonances obtained in the Project 1 when 
multiresonator is excited with a frequencies sweep in S-band 
are shown in Fig. 3. Analyzing the second and third 
resonances (correspond ing to the resonator length L2 and L3), 
it is observed a separation of only 78MHz between their 
central resonance frequencies (worse case). 
Figs. 4 to 9 show the magnetic distribution field of the 
multiresonator when it is energized with the resonances 
frequencies of the resonators length Ll (1.996GHz), L2 
(2.076GHz), L3 (2.154GHz), L4 (2.294GHz), Ls (2.408GHz) 
and L6 (2.502GHz), respectively. It is clearly observed that the 
resonance occurs in expected resonator and also in the 
adjacent resonator (except L3 because it has a frequency guard 
band relatively large with respect to L4) at each frequency and 
absorbing energy from the line, which could cause a reading 
error in the Tag code. 
-2.00 
'" 
�3.25 
� 
-4.50 
-5.75 
Freq[GHz] 
Fig. 3. Project 1: Six bits Chipless RFlD Tag resonances. 
Fig. 4. Resonance error of Project 1 at 1. 996GHz. 
Fig. 5. Resonance error of Project I at 2. 076GHz. 
Fig. 6. Resonance of Project I at 2. I 54GHz. 
Fig. 7. Resonance error of Project I at 2.294GHz. 
Fig. 8. Resonance error of Project 1 at 2.408GHz. 
Fig. 9. Resonance of Project I at 2.502GHz. 
Obviously a solution for this problem is to increase the 
frequency guard band between adjacent resonances. However, 
excessive separation is unacceptable due to restrictions on 
spectrwn use. This paper proposes an attainment of 
appropriate values for frequency guard bands in the design of 
this type of Tags in S-band using nwnerical simulations. 
The simulations corresponding to multiresonator in the 
Project 2 are shown in Fig. 10. 
XYPIot 1 
0.00 -::r-�-==-'-��---r��������������---rc""", 
-1.00 
-2.00 
�3.oo 
� !g-4.oo 
-5.00 
-6.00 
Fig. 10. Project 2: Six bits Chipless RFTD Tag resonances. 
The Fig. 10 shows a better distribution of the resonances 
within the S-band, wherein the lowest frequency guard band 
resonances now occurs between the lengths of the resonators 
L1 and L2, with a value of l62MHz. 
The Fig. 11 to 16 show the magnetic distribution field 
when exciting the Tag with the resonant frequency of the 
resonators length L1 (1.8lGHz), L2 (1.972GHz), L3 
(2.l48GHz), L4 (2.334GHz), L5 (2.S02GHz) and L6 
(2.702GHz), respectively. Using this frequency guard band of 
at least l62MHz there is no resonance in the adjacent 
resonators. 
Fig. II. Resonance of Project 2 at 1.810Hz. 
Fig. 12. Resonance of Project 2 at 1. 9720Hz. 
Fig. 13. Resonance of Project 2 at 2. I 480Hz. 
Fig. 14. Resonance of Project 2 at 2.3340Hz. 
Fig. 15. Resonance of Project 2 at 2.5020Hz. 
Fig. 16. Resonance of Project 2 at 2.7020Hz. 
IV. CONCLUSION 
In the Chip less RFID systems, adjacent resonances are 
intolerable, because the coding is set through the tag's 
hardware, as well, since the tag is constructed, its ID may not 
be modified. To avoiding spurious resonances that could cause 
errors when reading the tag's ID, is necessary to establish a 
minimum frequency guard band value between the resonances 
for each type of Chipless RFID Tag.enabling to accommodate 
the largest possible number of bits in the tag's band operation. 
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