2006 INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS, SEPTEMBER 12 - 14, 2006, PUTRAJAYA, MALAYSIA

RFID Transponder Using Bow Tie Antenna for Wireless Application

Allen Wong Seng Hung1 and Widad Ismail2

'Electrical and Electronic Department, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan,14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia

Tel: +6 012-57797342Tel: +6 04 599 5999 ext. 6050

Email: allenwshggmail.com, Email: eewidadgeng.usm.my

Abstract - A dual port Bow Tie patch antenna isdesigned to integrate with the RFID syste. A Bow Tiepatch antenna is integrated together withSuperheterodyne receiver and transmitter to boost theperformance of the RFID system. Hence, the usage ofLNA, mixers, LO and I & Q discrete chip are integratedto a dual port microstrip Bow Tie patch antenna toproduce a full product of wireless communication. Inthis RFID design, ADS (Advanced Design System) toolis used to analyze and simulate the entire architecture.The whole complete system is then fabricated on lowdielectric FR4 PCB for testing and measurement.

Keywords - RFID Transponder, superheterodyne, GilbertMixer

1. Introduction

Radio frequency identification, or RFID, is ageneric term for technologies that use radio waves toautomatically identify people or objects. The RFIDsystem consists of readers/interrogators and tags/transponders. RFID operates in different frequencybands (e.g.,125 kHz, 13.56 MHz, 868/915 MHz, 2.45GHz, and 5.8 GHz).RFID has several importantadvantages over the traditional barcode [1]. There areseveral methods of identification, but the most commonis to store a serial number that identifies a person orobject, and perhaps other information, on a microchipthat is attached to an antenna (the chip and the antennatogether are called an RFID transponder or an RFIDtag) [2]. The antenna enables the chip to transmit theidentification information to a reader. The readerconverts the radio waves reflected back from the RFIDtag into digital information that can then be passed onto computers that can make use of it.

In most RFID systems, the tags have a uniquenumerical code associated with it. In some systems, thecode is preprogrammed at the factory by themanufacturer and in other systems; the code can beprogrammed by the user and can be changed as often asneeded. When a tag senses, the RF signal from theinterrogator, it modulates its code onto a sine wave andcontinuously transmits it until the interrogator, itmodulates its code onto a sine wave and continuouslytransmits it until the interrogator receives it. Theinterrogator demodulates the transponder's signal and

0-7803-9745-2/06/$20.00 (©)2006 IEEE.

then knows exactly which one it is by its unique code[3].

Antennas are key components of microwavetagging systems as the antenna dimensions mainly limitminiaturization of the transponder or tag [4]. Noveltagging systems based on spread spectrum technique tocombat against channel fading make great demands onthe impedance and gain bandwidth of the tag antenna[5]. As most applications require small tag size, patchantennas are the preferable antenna type because theyallow separating the radiating patch from the activecircuitry resulting in a high front-to-back ratio. Besides,the electrical antenna specifications can be met byoptimizing the substrate parameters of the radiatingpatch without having to increase the antenna sizesignificantly. The microstrip patch antenna has becomean important area of research because it can giveexcellent results in term of efficiency, compactness,lightweight and low cost compared to the conventionalsystems. The main disadvantage of microstrip antennais an intrinsic limitation in bandwidth, which is due tothe resonant nature of the patch structure [6]. Toachieve a compact and high efficiency RFIDtransponder, microstrip Bow Tie antenna is designed.Under this scope, measurements result of antenna,mixer, filter and the whole Superheterodyne system arealso presented.

2. Proposed RFID Transponder

Figure 1 shows the block diagram of the proposedRFID Transponder for wireless applications. In thereceiver portion, microstrip patch antenna will receivethe RF signal. A low noise amplifier (LNA) isemployed to improve the noise performance of theradio frequency (RF) signal. A local oscillator (LO)signal is applied to the gate of the mixer to downconvert the signal to intermediate frequency (IF) signal.The IF signal will once again been amplified beforepassing through the I & Q demodulation discrete chip.The local oscillator (LO) is applied at the gates of eachmixer (Gilbert Mixer) with a 90° phase shift. The baseband signals are extracted from the drain of each mixer.The extracted base band signals are then filtered using alow pass filter (LPF) [7].

For the transmitter portion, the desired signalswith the base band frequency of 100 MHz is applied

21

U-55 1

10-

-15-

-20

-2530

30 l11.4 1.6 1.8 2.0 2.2 2.4

m2freq=2 .075G HzdB(S(1 ,1))=-29.382

10

-2

-30-

-40-l1.4

2.61.6 1.8 2.0 2.2 2.4 2.6

m4freq=2.225G HzdB(Allen4..S (1, 1))=-35.784

Figure 2: Measurement results ofpatch antenna.

into the I & Q modulator discrete chip for themodulation process. Then, the IF modulated signal isup-converted to RF signal before going through theband pass filter. The filtered RF signal is then beenamplified using a low noise amplifier (LNA) beforetransmission through microstrip patch antenna.

Figure 1: RFID Transponder Block Diagram.

3. Design Procedures

3.1 Microstrip Bow Tie Patch Antenna.In this RFID Transponder, a dual port microstrip

antenna is designed using FR4 with dielectric constant,Er of 4.5, height of 1.6 mm and tangent loss (0.018)and copper thickness (35 gm). Here the usage offormula below (1) to calculate the length of the BowTie antenna where f is the resonant frequency, c is thespeed of light and a is the antenna length. A fineoptimization is needed too.f 2c (1)

3a 6r

Thus, a= 0.04578 m 45.78 mm

Figure 2 shows the measurement results of theproposed patch antenna as shown in Figure 3 where L

is length and W is the width. The two feed microstriplines are matched at 50Q load impedance.

The Bow Tie patch antenna is simulated usingAdvanced Design System (ADS) momentum analysissoftware. The hardware measurement result shows thatthe designed antenna has good return loss, Si, (2.075GHz) and S22 (2.225 GHz).

Figure 3: Proposed patch antenna.

3.2 FET Resistive MixerThe mixers in the RFID transponder design are

implemented using a FET resistive mixer. Theadvantageous of a FET mixer are low level ofintermodulation distortion and spurious responses.

The PHEMT mixer of Agilent's ATF 34143 ischosen because it offers better conversion loss thanMOSFETs. In addition extra biasing is needed to makesure that the FET mixer is not overloaded withexcessive current which will damaged the FET mixer.

Figure 4 below shows the INV relationship ofATF34143 mixer. It clearly shows that high linearity, totalchannel resistance of 12.5 Q and an open circuit forgate-source voltages from -1 V to 0.6 V [8].

22

200J

150

LW

50-

0-

ml-

Oi ii liUi i I I 0ii2 iL2 ii3

Figure 4: IV characteristics ofATF 34143 mixer.

3.3 LNA AmplifierThe LNA amplifier used is MGA-83563 Low

Noise Amplifier from Agilent Technology is used toboost the received signal because of the high gainprovided which is around 22 dB (from 0.5 GHz to 6GHz) [9]. In the RFID, LNA is used to amplify RFsignal and also the IF (down-convert signal) andfurthermore to increase the signal to noise ratio level

3.4 I & Q Discrete Chip (RF2713)The RF2713 used is a monolithic integrated

quadrature modulator/demodulator discrete chip. Thedemodulator is used to recover the I and Q basebandsignals from the amplified and filtered IF signal. The IFsignal is down-converted from the mixer in thesuperheterodyne receiver. The inputs and outputs canbe reconfigured to modulate the I/Q signals onto an RFcarrier. The RF2713 is intended for IF systems wherethe IF frequency ranges from lOOkHz to 250MHz, andthe LO frequency is two times the IF. The IC containsall of the required components to implement themodulation/demodulation function and contains adigital divider type 90° phase shifter, two doublebalanced mixers, and baseband amplifiers designed tointerface with Analog to Digital Converters.

The prototype of RFID system is fabricated ontwo layers of FR4. Figure 5 and 6 show the layout ofthe whole RFID system. The bottom layer is the dualfrequency patch antenna whereas the superheterodynesystem is on the upper layer. The two layers areconnected via through holes.

4. Measurement Results

From the measurement in figure 7, it is clear thatthe highest power received at 90° when the bow tieantenna is facing another antenna with a line of sightwhich mean surface to surface. This will enable the

maximum signal power to be transmitted from anotherantenna to the receiving Bow Tie antenna.

mlVDS=0.100IDS.i=0,008VGS=-0ooo0o0

Figure 5: Bottom layer ofRFID transponder.

Figure 6: Top layer ofRFID Transponder.

Figure 7: Radiation pattern (polar field) ofBow TiePatch antenna at 2.075 GHz.

23

- Power (normalised)

-N

Signal generator

Signal generator

Figure 8: Equipment Setup for RFID Measurement.

The measurement setup for RFID transpondersystem is shown in figure 8 above. At the transmitterside, a signal generator is used to transmit a RF signalat 2.075 GHz to the RFID transponder. Here, two signalgenerators are connected to the RFID transponder. Oneis used for the down conversion FET mixer to down-convert the signal from 2.075 GHz to 100 MHz. Thenanother signal generator is to generate LO signal for theI & Q IF baseband signal demodulation process. Hence,a spectrum analyzer is also used to measure the powerlevel of the base band signal. Oscilloscope is alsoneeded to check the phase difference for both I & Qdemodulated signal to make sure that they are 900 phasedifference.

Table 1: RFID Transponder hardware measurementresults.

.;-- Agilent 12:07:33 Apr 6, 2006

Ref CPeakLog10dB/

S3 F,

Mkrl 100.3 MHzRtten 10 dB -35.74 dBm

Marlfort_ )~ystem, Rlignnments, f lign Now RlI re quired

Dcmwtt J<t4T-n T 1

l.Start -10.83 MHzRes BW 3 MHz

Stop 300 MHzVBH 3 MHz Sweep 4 ms (401 pts)

Figure 9: Spectrum of I demodulated signal.

Agilent 12:08:15 Rpr 6, 2006

Ref 0 dBmPeakLog10dB/

W1 SS3 F

Mkrl 100.3 MHz-23.32 dBm

st l Alignments, Rlign Now, All required

100.3 000 lHz-

292 }ty===

_-_____Start -10.83 MHzRes BW 3 MHz VBW 3 MHz

Stop 300 MHzSweep 4 ms (401 pts)

Figure 10: Spectrum ofQ demodulated signal.

24

Rtten 10 dBItem measured Frequency Measurement

I & Q signal 100.3 MHz 900 phase different

Demodulated I &QIsinl 357dBsignal in terms of 100.3 MHz I signal -35.74 dBm

power level Q signal -23.32 dBm

I & Q signal to noise 100.3 MHz I Signal 22.04ratio Q signal 34.68

RF Signaltransmitted from 2.225 GHz -42.24 dBmRFID Transponder

I I I I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Oscilloscope

0 dBm

Conclusion

Figure 11: Measurement of I & Q signal fromoscilloscope.

It is clear that the complete system of RFIDtransponder can be used effectively in wirelesscommunication. The integrated patch antenna and thesuperheterodyne architecture yield a compact front-endreceiver and transmitter. The whole design of RFIDtransponder has been designed and simulated usingADS. The down-converted baseband signals of I & Qsignals have been successfully demonstrated in bothsimulations and hardware measurements.

References

The measurement result from figure 9, 10 and 11shows that the there are two side band which weredemodulated from the I & Q signal. But the power levelof the lower sideband is much higher than the uppersideband because at the output of the I and Qdemodulator chip, there is a T network low pass filter tofilter all the spectrum signal that are above 250 MHz.

From the two measurement results, there have bean offset of 0.1MHz of frequency that are differentfrom the simulation result. From observation, it is clearthat the power level of I signal is slightly higher thanthe power level ofQ signal. The I demodulated signal isaround -33.63 dBm while the Q demodulated signal isaround -41.33 dBm. The percentage difference, %offsetis around

ofset 33.63 - (-41.33) 1000 (2)= 22.896%

Distance (m) 0.5 0.75

-Spectrum power at 100MHz (dBm)

Figure 12: Demodulated signal at 100 MHz versusdistance.

From figure 12, it is clear that the signal powerreceived by the RFID transponder decreased when thedistance between the transmitting antenna and RFIDtransponder increases.

[1] Bing Jiang, Kenneth P. Fishkin, Sumit Roy,Matthai Philipose, IEEE Transactions OnInstrumentation and Measurement, Vol. 55, No. 1,February 2006, pp. 187-196

[2] Klaus Finkenzeller, "RFID Handbook", 2ndEdition, John Wiley & Sons, Inc., Publication2004.

[3] Carl. J. Weisman, "The Essential Guide to RF andWireless", 2nd. Edition, Prentice Hall, 2002, pg:261-263

[4] Ryan Y. Miyamoto, Yongxi Qian, and Tatsuo Itoh,"An Active Integrated Retrodirective Transponderfor Remote Information Retrieval-on-Demand",IEEE Transactions On Microwave Theory AndTechniques, Vol. 49, No. 9, September 2001

[5] Marcel Kossel, Hansruedi Benedickter and WernerBaechtold, "Circular Polarized Aperture CoupledPatch Antennas for an RFID System in the 2.4GHz ISM Band", IEEE Radio and WirelessConference, August 1999, RAWCON 99, pg: 235-238

[6] S. Maci, B. Gentili, "Dual-Frequnecy PatchAntenna, IEEE Antennas and PropagationMagazine, IEEE Antennas and PropagationMagazine, Vol. 39, No.6, December 1997.

[7] W. Ismail, P. Gardner, "Active Integrated Antenna(AIA) With Image Rejection", IEEE 6th. EuropeanConference on Wireless Technology, 2003 pp.427-430.

[8] J.J. Tiang and W. Ismail, "Direct ConversionReceiver with Integrated Antenna" InternationalWireless and Telecommunications Symposium2006 (IWTS '06), 15 - 17 May 2006, Berjaya TimeSquare Convention Center, Kuala Lumpur,Malaysia, pp 13-15.

[9] W. Ismail and Sohiful A. Z. Murad, "DualFrequency Integrated Antenna (DFIA) With ImageRejection", 2005 7th IEEE Malaysia InternationalConference on Communications, Kuala Lumpur,Malaysia, pp 005-007.

25

1.25 1.5 1.75

-10

-20

-30

-40

-50

-60

-70