Characterization of RFID Strap Using Single-Ended Probe
Characterization of RFID Strap
Using Single-Ended Probe
Sung-Lin Chen,Student Member,IEEE,and Ken-Huang Lin,Member,IEEE
Abstract—Radio frequency identi?cation(RFID)strap attach-ment modality is more reliable,low cost,and easy to assemble, and therefore,it becomes increasingly more popular in RFID tag designs.This paper presents a single-ended probe method with power transmission coef?cient compensation for the char-acterization of RFID straps.Approximate identi?cation of the Read/Write threshold power and impedance is based on the charge status of voltage multipliers and charge pumps instead of on the Read/Write-modulated commands.For comparison pur-poses,the conventional source-pull system is also brie?y reviewed and applied to verify the measurement results of absorbing power and impedance of RFID straps using the presented method.An enhanced source-pull system,named RFID source-pull system,for an RFID strap that can accurately measure the threshold power and impedance for Read/Write-modulated commands is also con-structed for verifying the presented method.Alien and Texas Instruments(TI)straps are used for measurement examples in this paper.It is found that the measurement results of both RFID straps obtained by the presented method agree well with those by the conventional source-pull system and the RFID source-pull sys-tem.The single-ended probe method can measure the approximate Read/Write threshold power and impedance of the RFID strap with minimum operating procedures;furthermore,the compli-cated radio frequency(RF)facilities are not required.Obtaining the Read/Write threshold power and impedance of RFID straps allows designers to estimate the maximum read range of the designed RFID tag in advance.Therefore,the implemented cost and design cycle times can substantially be reduced.
Index Terms—Impedance measurement,power transmission coef?cient,radio frequency identi?cation(RFID),RFID strap, RFID tags,single-ended probe.
I.I NTRODUCTION
R ADIO frequency identi?cation(RFID)is a technology used for object identi?cation and has become very pop-ular in the retail,transportation,manufacturing,and supply chains[1].The tag antenna acts as a power receiver that trans-forms the emitted electromagnetic wave into electric energy for the chip,as well as a radiating source that sends out digital in-formation embedded inside the chip.For the purpose of energy conversion,the chip includes a charge capacitor that causes the chip to have additional capacitive impedance,making the tag antenna more dif?cult to match with the chip than with a general radio frequency(RF)system of purely real impedance of50Ωload.
Manuscript received April29,2008;revised August9,2008.First published June16,2009;current version published September16,2009. The Associate Editor coordinating the review process for this paper was Dr.Sergey Kharkovsky.
The authors are with Department of Electrical Engineering,National Sun Yat-sen University,Kaohsiung80424,Taiwan(e-mail:xview@https://www.360docs.net/doc/3515553765.html,). Digital Object Identi?er10.1109/TIM.2009.2018697
The electromagnetic power from the tag antenna is maxi-mally delivered to the chip when the antenna has a conjugate impedance of the chip[2].Since the energy interaction between the chip and the antenna is the most important issue,a suc-cessful antenna design is determined by conjugate impedance matching between both components[3].
During an RFID tag design,the read range is the most im-portant performance index.The read range R can be calculated using the Friis free-space transmission formula as follows[4]:
R=
λ
4π
P r G r G tτ
P th
1/2
(1)
whereλis the wavelength,P r is the power transmitted by the reader,G r is the gain of the reader antenna,G t is the gain of the tag antenna,P th is the minimum threshold power of powering on the RFID chip,andτis the power transmission coef?cient that will be discussed in detail later.To estimate the maximum read range of the designed tag using the Friis formula,the threshold power and chip impedance are the es-sential parameters for the RFID tag designers.Unfortunately, RFID chip or strap manufacturers treat these parameters as con?dential information in the business.They provide very few speci?cations about their devices for general customers or research laboratories.Therefore,the RFID tag designers encounter great dif?culties in obtaining these two parameters before designing RFID tags.In addition to threshold power and impedance measurement issues,the strap attachment modality becomes increasingly more popular in RFID tag designs.Under this circumstance,the related measurement method should be carried out.
In the past few years,several papers have been published on RFID tag design.Rao et al.give an overview and a process of antenna design for RFID tags[5].Fang et al.introduce a design process for broadband impedance matching for RFID tags[6].In the design of tags based on power re?ection coef-?cient analysis,Nikitin et al.apply the Smith chart based on Kurokawa’s power re?ection coef?cient method with measure-ment impedance normalized to the real part of the impedance of the source to the RFID tag design[7].These designs require that the RFID chip impedance be known in advance.However, very few papers provide measurement methods for threshold power and input impedance of the RFID chip or strap.
This paper presents a simple and fast measurement method, namely,the single-ended probe method,to measure the thresh-old power and impedance of RFID straps.In addition,an enhanced source-pull system for RFID straps is also presented.
0018-9456/$26.00?2009IEEE
It can be used to measure more accurate threshold power and impedance of RFID straps with different RF-modulated commands.Section II explains the conventional source-pull system,single-ended probe method,and RFID source-pull system for characterization of RFID straps.The measurement and comparison results are demonstrated in Section III.Finally, conclusions are drawn in Section IV.
II.M EASUREMENT M ETHODS FOR RFID S TRAPS This section brie?y reviews the conventional source-pull sys-tem for impedance measurement of power-dependent devices. The presented single-ended probe method and the RFID source-pull system are also explained in this section.
A.Source-Pull System
For comparison purposes,the conventional load pull system is reviewed[8]–[10].The load pull system is composed of source-pull and load-pull measurements for the input and out-put properties of a device under test,respectively[11].The sys-tem is used to measure the input impedance or characteristics of an RF device or component in which the impedance is different from the ideal50Ωor has a complex form.For the impedance measurement of an RFID strap,only input impedance has to be measured.Therefore,only the source-pull measurement is re-quired for the input impedance measurement of the RFID strap. The block diagram of measurement processes and con?guration of the conventional source-pull measurement system are shown in Fig.1.In a source-pull measurement,the source impedance with a minimum return loss or re?ection coef?cient is typically regarded as the conjugate impedance of the device under test. To provide more freedom of tuner adjusting,we use a triple-stub tuner as the source tuner.
First,we probe the RFID strap with a source tuner output and adjust the triple-stub tuner for the perfect match with 50Ωat the source tuner input reference plane.Second,we remove the RFID strap and connect the second port probe to the source tuner output.Fig.1(a)and(b)shows block diagrams that illustrate the?rst and second measurement processes, respectively.Finally,the impedance Z so of the source tuner output port is measured,which,ideally,is equal to the complex conjugate of the RFID strap input impedance Z strap=Z?so.The main function of the source-pull measurement asserts that the absorbed power of the RFID strap is equal to the RF output power of the network analyzer,assuming that the source tuner is lossless.To?nd the relation curve between the absorbed power and impedance of RFID straps,the measurement process should be taken repeatedly for each sweeping power.Hence, this is a highly time-consuming process.
B.Single-Ended Probe Method for an RFID Strap
Source-pull measurement is conventionally performed using stub tuners that can be dif?cult to use and consume measure-ment time considerably.Hence,a simple and rapid measure-ment method for an RFID strap will be more convenient to the RFID tag designers.The presented method,namely,the
single-Fig.1.Measurement block diagrams and con?guration of the conventional source-pull system for an RFID
strap.
Fig.2.Equivalent circuit model of an RFID tag.
ended probe measurement with power transmission coef?cient compensation,can achieve this requirement.
Fig.2shows the equivalent circuit model of the RFID tag. The circuit is with two complex impedances,namely,Z c and Z a,which are the chip impedance and antenna impedance, respectively.In this case,the voltage-or current-basis re?ection coef?cient based on traveling waves is not suitable for use as the performance index of an RFID tag design.Kurokawa[12]pre-sented the physical meaning of power waves and the properties of the scattering matrix.The power re?ection coef?cient was de?ned as
|s|2=
Z a?Z?c
Z a+Z c
2,0≤|s|2≤1(2)
where Z c=R c+X c is the RFID chip impedance,and Z a= R a+X a is the RFID tag antenna impedance.To apply this
CHEN AND LIN:CHARACTERIZATION OF RFID STRAP USING SINGLE-ENDED PROBE
3621 Fig.3.RFID die,chip,strap,inlay,and label(or
tag).
Fig.4.Simpli?ed equivalent circuit model of an RFID strap.
concept to measure the threshold power and complex im-
pedance of an RFID chip with a single-ended probe,we
represent the power re?ection coef?cient asγand de?ne its
complementary coef?cient,i.e.,power transmission coef?cient,
τ(γ=1?τ)as
τ=
4R a R c
[(R a+R c)2+(X a+X c)2]
.(3)
For complex impedance measurement applications,the power transmission coef?cient is more useful than the power re?ection coef?cient.
For manufacturing considerations,increasingly more RFID chip manufacturers supply the customers with RFID straps rather than RFID chips.Fig.3shows RFID die,chip,strap, inlay,and label(or tag).Each RFID strap is packaged with an RFID chip and two terminal pads on a thin substrate.The equivalent circuit of the RFID chip can be represented as either a parallel or a series model with a resistor and a capacitor[13]. Due to the terminal pads and binding interface,straps can be represented as a series resistor and capacitor.In this paper,we simplify the equivalent circuit of the RFID strap as series model of a resistor and a capacitor.The simpli?ed equivalent circuit model of the RFID strap is shown in Fig.4.
For microwave design,the engineers are familiar with many measurement skills for50-Ωdevices.The majority of the measured equipment are designed for measuring50-Ωdevices; they are not directly applicable to measuring RFID straps.Not only do RFID straps have complex impedance characteristic, but they also directly bind with the chip without any connector. For RFID antenna impedance measurement,Camp et al.[14] present a novel impedance measurement procedure for
RFID Fig.5.Con?guration of the single-ended probe method for an RFID strap. tag antennas using a modi?ed on-wafer probe,which usually probes on chip for characteristic measurement.It provides a good concept of measurement platform setup.Fig.5shows the con?guration of the measurement platform that we use in this paper.
For the measurement of the threshold power and impedance of an RFID strap,a simple measurement procedure is imple-mented.A single-ended probe is combined with a network analyzer(Agilent ENA5071B)to perform the measurement. Fig.5shows the single-ended probe measurement setup for the RFID strap impedance measurement.The network analyzer can take an inverse transformation from the re?ection coef?cient to the impedance of device under test.Therefore,the RFID strap impedance Z strap can be measured from the network analyzer directly.
To measure the threshold power of an RFID strap,we sweep the RF output power of the network analyzer from?20to 10dBm and record the measurement data.In reality,the RF output power of a network analyzer is not equal to the actual absorbed power of an RFID strap.Because the RFID strap is not matched with the50-Ωcoaxial cable of the network analyzer, the sweeping RF output power P RF should be corrected with the power transmission coef?cient.The absorbed power P strap can be compensated by the following:
P strap=P RF+10×log(τ)(in decibel milliwatts).(4) Now P strap is the actual absorbed power of the RFID strap. Therefore,the resistance and reactance curves related to the actual absorbed power of RFID strap are obtained.
RFID chips are not purely passive components but active devices whose input impedances vary with absorbed power and operating frequency[15].The architecture of an RFID chip is shown in Fig.6.A voltage multiplier converts the incident RF signal power into a dc voltage to provide the regulated voltage required for the operation of RFID chip.When the voltage multiplier is charged,the demodulator,control unit,
3622IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT,VOL.58,NO.10,OCTOBER
2009
Fig.6.Architecture of an RFID
chip.
Fig.7.Variation of chip impedance with absorbed power.
charge pump,and modulator start running.In other words,the RFID chip is powered on and can reply to Read commands but not to Write commands.The resistance of RFID chips is strongly dependent on the dc current taken from the voltage multiplier output V DD [16].Therefore,when the RFID chip powers on,the input resistance shows an unusual increased status.Fig.7shows an illustration of the resistance curve.After taking the difference of the resistance curve varied with the absorbed power,the reading threshold power P rth and reading impedance Z rth =R rth +jX rth of the RFID chip are obtained.The difference of resistance is de?ned as
R di?=R (n +1)?R (n )
(5)
where R (n )is the n th measurement resistance data measured under different absorbed power conditions,and n is the index of the measurement data varied with increased absorbed power.For the writing function,the dc output voltage of the voltage multiplier is not enough to supply the control unit writing information into electrically erasable programmable read-only memory (EEPROM),except when the charge pump is charged.After both voltage multiplier and charge pump are charged,more incident power leads to more forward current on the Schottky diodes.Due to the fact that the reactance of an RFID chip is mainly determined by the junction and substrate capacitances of the Schottky diodes,the increased forward current changes the diode capacitance [15].Therefore,the reactance curve varies with increasing absorbed power and has a monotonically decreasing capacitive reactance when the voltage multiplier and charge pump are charged.Fig.7shows an illustration of reactance curve varied with absorbed power [17].The writing function can be done only if the
charge
Fig.8.Enhanced source-pull system for an RFID strap.
pump is charged;hence,we can determine the writing threshold power P wth and writing impedance Z wth =R wth +jX wth of the RFID chip at this turning point.
Therefore,the single-ended probe method can approximately measure the threshold power and impedance for reading-and writing-modulated commands of the RFID strap according to the status of voltage multiplier and charge pump,respectively.The detail measurement and comparison results are presented in Section III.It will be demonstrated that the measurement results of the presented method are in good agreement with the other two measurement methods.With these parameters,designers can estimate the maximum Read/Write range of the tags with the Friis equation before implementing their RFID tag designs.C.RFID Source-Pull System
The conventional source-pull system and single-ended probe method can be used to measure the impedance behavior with the absorbed power of the RFID strap,in which the threshold powers P rth and P wth are de?ned according to the charge status of the voltage multiplier and charge pump.However,the RFID chip is not a purely passive component but an active device when it powers on.It includes an RF-to-dc voltage multiplier,a demodulator,a modulator,an EEPROM,a charge pump,and a control unit [16].The measurement results of the RFID chip using the conventional source-pull system or single-ended probe method are the behaviors of the standby mode,and not the turn-on state,unless the RFID chip receives the speci?ed modulated commands,such as the Read or Write command.For this reason,we enhance the conventional source-pull system with the RFID reader,spectrum analyzer,circulator,and atten-uators.Fig.8shows the con?guration of the enhanced source-pull system for RFID straps.The enhanced system can measure the Read/Write threshold power and impedance of RFID straps through responding to the modulated commands from RFID reader.Therefore,we can use it to verify the measurement results of the single-ended probe method.
In the RFID source-pull system,RFID reader system,which includes an RFID reader and a control laptop,sends the com-mand to and receives the response signal from an RFID strap with certain RF output power.Due to the fact that the RFID reader transmits and receives the RF signals in the same coaxial cable,a circulator is used to separate the response signal of the RFID strap and transmits it to the spectrum analyzer for
CHEN AND LIN:CHARACTERIZATION OF RFID STRAP USING SINGLE-ENDED PROBE
3623
Fig.9.Con?guration of attenuator,circulator,triple-stub tuner,and
probe.
Fig.10.Spectrum of modulated signal from an RFID strap.
observing the modulated signal from the RFID strap.The minimum controllable RF output power of the RFID reader is 15dBm,which is much higher than the threshold power of the RFID strap.Therefore,an attenuator is connected between the RFID reader and the circulator.The con?guration of the triple-stub tuner and probe is the same as that in the conventional source-pull system.Fig.9shows the con?guration of the atten-uator,circulator,triple-stub tuner,and probe and an illustrated picture of probed strap.Take note that the probe is directly connected to the RFID strap and that the RFID tag antenna is not required.
The ?rst step is the same as in the source-pull system,except for sending out a Read-modulated command from the RFID reader with a certain RF power level from low to high.Tuning the triple-stub tuner until the RFID reader detects the modulated signal from the RFID strap or the spectrum analyzer displays a modulated signal,as shown in Fig.10.Second,remove the RFID strap and connect the probe of network analyzer to the source tuner output probe.Finally,the impedance Z so of the source tuner output port is measured,which is ideally equal to the complex conjugate of the RFID strap input impedance,
i.e.,Z strap =Z ?
so
.Then,remove the triple-stub tuner from the circulator and measure the RF output power at the circulator output.Then,the reading threshold power P rth of the
RFID
Fig.11.Measurement results of an RFID strap with different input power levels using the single-ended method.
strap is measured.Similarly,by repeating the measurement procedures above and sending out a Write-modulated command instead of a Read one,we can measure the writing threshold power P wth when the RFID strap is written successfully.
The RFID source-pull system can measure practical Read/Write threshold power and impedance of the RFID strap,which respond to modulated commands from the RFID reader with minimum turn-on power.However,the measurement process cycle should be taken when the modulated command or RF output power is changed.In other words,to perform the RFID source-pull system requires a considerable amount of time and effort.Again,after obtaining the threshold power and impedance information of the RFID strap,the designers can estimate the maximum Read/Write range of the designed RFID tags before implementing prototypes.
III.M EASUREMENT R ESULTS
First,we demonstrate the measurement results of the single-ended probe method with power transmission coef?cient com-pensation and show that the compensated results agree well with the measurement results of using the conventional source-pull system.Then,to demonstrate the feasibility of using the presented methods to measure the Read/Write threshold power and impedance of the RFID strap,the conventional source-pull system,single-ended probe,and the RFID source-pull system all have been used.Measurement results of the RFID straps from two companies are presented in this section.The ?rst RFID strap is produced by Alien Technology (Alien),while the second one is produced by Texas Instruments (TI).A.Measurement Results of the Single-Ended Probe Method For the purpose of demonstration,the raw measurement results of the conventional source-pull and single-ended probe are shown in Fig.11.It shows the measurement results with dif-ferent input power levels,and both of them are entirely different to each other.However,we apply (4)to recalculate the mea-surement results of the single-ended method.After applying the
3624IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT,VOL.58,NO.10,OCTOBER2009
https://www.360docs.net/doc/3515553765.html,pensation results of an RFID strap with different input power levels using the single-ended method.Fig.14.Impedance curves of an Alien strap with different absorbed power for the Read command.
Fig.15.Impedance curves of an Alien strap with different absorbed power for the Write command.
impedance curves and the threshold powers and impedances of Read commands for three different measurement methods. The measurement results of the Read commands for the Alien RFID strap agree with each other very well.Fig.15shows the threshold powers of Write commands for three different measurement methods.The measurement results of the three measurement methods also agree well.The detailed measure-ment data,the threshold power for Read/Write commands,and the corresponding impedances are listed in Table I.
C.Measurement Results of the TI RFID Strap
For TI RFID strap measurement,the difference curve of re-sistance measured by a single-ended probe is shown in Fig.16. It shows that the threshold power of the Read command is about ?9.8dBm using the presented method.Whereas,?10.8and ?10.0dBm are the threshold power of the Read command using
RFID source-pull and conventional source-pull measurement methods,respectively.Fig.17shows the threshold powers and impedances of Read commands for three different measurement
CHEN AND LIN:CHARACTERIZATION OF RFID STRAP USING SINGLE-ENDED PROBE3625
TABLE I
M EASUREMENT I MPEDANCE OF A LIEN S TRAP S AMPLE1(f c=925MHz
) Fig.17.Impedance curves of a TI strap with different absorbed power for the Read
command.Fig.18.Impedance curves of a TI strap with different absorbed power for the Write command.
TABLE II
M EASUREMENT I MPEDANCE OF TI S TRAP S AMPLE1(f c=925MHz
) methods.The measurement results of the Read commands for the TI RFID strap also agree with each other well.Fig.18 shows the impedance curves and the threshold powers of Write commands for three different measurement methods.The measurement results of the three measurement methods also agree with each other very well.The detailed measurement data,the threshold power for Read/Write commands,and the corresponding impedances are listed in Table II.
IV.C ONCLUSION
In this paper,a single-ended probe measurement method with power transmission coef?cient compensation for the character-ization of an RFID strap has been presented.The simpli?ed method can be used more easily and can be used to measure the Read/Write threshold power and corresponding impedance of an RFID strap without complicated operating procedures and additional facilities.An RFID source-pull measurement system is also constructed;the system provides more accurate
3626IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT,VOL.58,NO.10,OCTOBER2009
measurement results for different RFID-modulated commands. The measurement results demonstrate that the threshold power and impedance for the Read/Write commands of an RFID strap are in good agreement for both Alien straps and TI straps measured by three measurement methods.Furthermore,the measured impedances of Alien and TI straps are the actual straps input impedance in which the effects of the RFID chip or die as well as terminal conductive pads impedance are all included.This is more useful for designing RFID tag antennas to achieve the actual conjugate match condition.Obtaining the Read/Write threshold powers and impedances of the RFID strap allows designers to estimate the maximum read range of a designed RFID tag and to select an appropriate tag antenna in advance.In this paper,we have shown that our method can be successfully applied to Alien and TI straps.While we expect that the method can still be applied to other RFID chips or straps,further work should be conducted to verify this.
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Sung-Lin Chen(S’06)was born in Kaohsiung,
Taiwan,in1974.He received the B.S.degree
in electronic engineering from I-Shou University,
Kaohsiung,in1997,and the M.S.degree in elec-
trical engineering from the National Sun Yat-Sen
University,Kaohsiung,in2002,where he is currently
working toward the Ph.D.degree.
He concentrates on novel RFID tag antenna design
for products tracking applications,particularly on
RFID applications in the steel industry.In addition
to RFID tags design,his research interests include electromagnetic band-gap structures and high-gain and high-directive antenna
design.
Ken-Huang Lin(M’93)was born in Kaohsiung,
Taiwan,in1961.He received the B.S.degree
from the National Sun Yat-sen University(NSYSU),
Kaohsiung,in1984,the M.S.degree from the
National Taiwan University,Taipei,Taiwan,in
1986,and the Ph.D.degree from the University of
Illinois,Urbana-Champaign,in1993,all in electrical
engineering.
He joined the Department of Electrical Engineer-
ing,NSYSU,in1993,where he is currently a Profes-
sor.Since August2005,he has served as the Director of the NSYSU Incubation Center.His research interests include radiowave propagation,antennas,EMC,and wireless communications.
Dr.Lin received the Young Scientist Award at the XXVth General Assembly of the International Union of Radio Science in1996and was listed in Who’s Who in the World in1998.