Design and Implementation of a Novel Directional Coupler for UHF RFID Reader

Transcription

1 22 ELETRONIS, VOL. 20, NO. 1, JUNE 2016 Design and Implementatin f a Nvel Directinal upler fr UHF RFID Reader Jianxing Li, Shanlin Sng, Xiayu hen, Hua Nian and Weiguang Shi Abstract The directinal cupler is applied t islating RX frm TX because f lw cst and simplicity cmpared t the circulatr in the radi-frequency identificatin (RFID) reader. Because f unequal phase velcity between dd and even mde, the drawback f the traditinal micrstrip directinal cupler is pr islatin. In this paper, t btain a gd islatin between RX and TX, a nvel directinal cupler is prpsed t be applied t the UHF RFID system with a single antenna. Measurement result shws that the prpsed directinal cupler pssesses a gd islatin f -35dB in perating frequency band. Index Terms Islatin, Directinal upler, Radifrequency identificatin (RFID) Original Research Paper DOI: /ELS L R I. INTRODUTION adi-frequency identificatin (RFID) technlgy is becming mre and mre ppular frm the 1990s [1], its applicatins are extending rapidly and the cmmercial ptential is getting huger and huger [2]. It uses radi-frequency signal t realize autmatic identificatin f bject thrugh space cupling. The biggest advantage f the UHF RFID is that it can cmplete identificatin withut physical cntact [3]. A set f the UHF RFID system is cmpsed f the reader, the electrnic tag and the applicatin sftware. In the cmmn passive UHF RFID system, the reader transmits cntinuus waves t the tag, part f the cntinuus waves will be cnverted int energy by the tag, and the tag sends its back-scattered data t the reader. Because cntinuus wave frm the reader and back-scattered wave frm the tags simultaneusly ccupy the same frequency band, TX age t RX leads t many technical difficulties. Fr example, the strng TX age makes the lw nise amplifier (LNA) f the Manuscript received 4 September Accepted fr publicatin 25 December Jianxing Li is with the Schl f Electrnics and Infrmatin Engineering, Tianjin Plytechnic University, Tianjin, hina ( lijianxing@tjpu.edu.cn). Shanlin Sng is with the Schl f Electrnics and Infrmatin Engineering, Xiayu hen is with the Schl f Electrnics and Infrmatin Engineering, Hua Nian is with the Schl f Electrnics and Infrmatin Engineering, Weiguang Shi is with the Schl f Electrnics and Infrmatin Engineering, receiver saturated and the dynamic range f the LNA decreased [1]. In rder t imprve the perfrmance f the RFID reader, a nvel directinal cupler is prpsed t btain high islatin between RX and TX. An attenuatr, a phase shifter and a pwer synthesizer are added at the cupling prt f the transmit chain at the same time. The signal f the anti-phase and equal magnitude f the TX age is designed by using the attenuatr and the phase shifter at the cupling prt, and cmbined with the TX age at the islatin prt. nsequently, these tw signals are ffset by using the pwer synthesizer. Accrdingly, RX is islated frm the TX age effectively and thus the prpsed directinal cupler can be applied t the UHF RFID reader. II. INTRODUTION OF THE PRINIPLE A. Intrductin f the directinal cupler The directinal cupler is a kind f passive micrwave cmpnent and pwer distributin cmpnent, which is widely used in pwer distributin and synthesis [4]. The fur prts f directinal cupler are the input prt, the utput prt, the cupling prt and the islatin prt, respectively. The transmitted signals frm the input prt are mved t the utput prt and the cupling prt accrding t certain prprtin, n signals t the islatin prt ideally. As reciprcal netwrk, the functin f each prt is interchangeable. Each prt can be the input prt, the functin f ther prts changes in turn. Because f its prt reciprcity, the prpsed directinal cupler is applied t the RFID reader which shares a single antenna, as shwn in Fig. 1. In TX, transmitter cnnects prt 1 f the directinal cupler. At this pint, prt 1 serves as the input prt, prt 2 as the islatin prt, prt 3 as the cupling prt, prt 4 as the utput prt. The transmitted signals transferred t the utput prt are radiated thrugh the antenna. In RX, the antenna cnnects prt 4 f the directinal cupler. At this pint, prt 4 serves as the input prt, prt 1 as the utput prt, prt 2 as the cupling prt, prt 3 as the islatin prt. The received signals are transferred thrugh the cupling prt. In this way, the directinal cupler prvides the maximum islatin between RX and TX. The islatin between the transmitter and the receiver is prvided by the directinal cupler cmpletely. Nevertheless, the islatin f the directinal cupler is limited, and the

2 ELETRONIS, VOL. 20, NO. 1, JUNE transmitted signal will t RX inevitably, which makes the perfrmance f the receiver wrse. B. Analysis f the carrier age The passive UHF RFID system cmmunicates by back-scattered cmmunicatin. Althugh the reader and the tag send infrmatin in half-duplex cmmunicatin which agreement stipulated, back-scattered cmmunicatin requires that the tag sends signal when energy signal is transmitted by the reader, s it als can be regarded as full-duplex cmmunicatin. Other full-duplex cmmunicatin devices cannt be interfered with each ther by using different frequency band. Hwever, cntinuus wave frm the reader and back-scattered wave frm the tags simultaneusly ccupy the same frequency band. Therefre, they cannt be filtered by the filter if they enter int RX at the same time. At present, the directinal cuplers are extensively applied t insulate RX frm TX in the UHF RFID reader, as shwn in Fig. 2.The transmitted signal f the transmitter will int RX inevitably due t the fact that the directinal cupler has limited islatin. And the age signal is strnger than the back-scattered signal f tags, which makes the LNA f the receiver saturatin easily. As a result, the received signal cannt be amplified effectively. Since the age signal will lwer the signal t nise rati, it has an adverse effect n the detectin rang f the tag [5]. If the transmitted pwer f the transmitter is 30dBm, accrding t the frmula f Friss: 2 t t r 2 PG G Pr (1) 4 R where G t is the gain f the reader antenna, G r the gain f the P the tag antenna, P t the transmitted pwer by the reader, r received pwer by the reader, the wavelength. When the tag is 10 meters away frm the reader, G 8dBi and G 1.2dBi, the back-scattered pwer f the tag is abve r -60dBm fr prper cmmunicatin. If the islatin f the directinal cupler is -20dB, there will be 10dBm transmitted signal s int RX. In this way, the 10dBm nise added t the -60dBm effective signal, which will lead t the signal t nise rati very pr inevitably. In additin, the linearity f the LNA will nt very high, if the 1dB cmpressin pint f the LNA less than 10dBm, the 10dBm nise will cause the LNA saturatin which is unable t enlarge effective signal. Therefre, we must try t increase the islatin f the Receiver Transmitter Prt 2 Prt 3 Prt 1 Prt 4 t Antenna directinal cupler.. Imprving methd f the directinal cupler This paper uses directinal cupler chip RP890A05 as the islatin cmpnent f the transceiver. The prpsed technique t increase the islatin is that the canceling lp is designed and ffsets TX age. The canceling lp includes the attenuatr, the phase shifter and the pwer synthesizer and the principle circuit is created in Agilent ADS 2009 as shwn in Fig. 3. The mdel f the directinal cupler is set by the parameters f the RP890A05 chip in the simulatin. Prt 1 cnnects the transmitter as the input prt, prt 2 cnnects the antenna as the utput prt, prt 3 is the cupling prt, prt 4 is the islatin prt. The cupling signal travels thrugh the attenuatr and the phase shifter, and cmbines with the age f the islatin prt by the pwer synthesizer which cnnects t the receiver. The phase difference between the cupling signal and the age changes frm 90 t 180 thrugh the phase shifter. Then the cupling signal becmes the same magnitude with the age thrugh the attenuatr. At last, tw signals are cancelled ut in the pwer synthesizer. nsequently, the prpsed directinal cupler increases the islatin. Assuming the age f the islatin prt is S t A cs t (2) The back-scattered signal which the antenna receives is S t A t cs t (3) V TX RX V PA LNA The cupling signal f the transmitter is S0 t A cs t 90 (4) The cupling signal thrugh the attenuatr and the phase shifter is cs 90 S t ka t (5) where k is the cefficient f the attenuatin and is the amunt f the phase-shift. Three signals are cmbined in the pwer synthesizer, that is S t S t S t S t (6) R V Leakage Fig.2 ircuit diagram f the transceiver islatin Antenna Directinal cupler Fig.1 Structure f single antenna transceiver

3 24 ELETRONIS, VOL. 20, NO. 1, JUNE 2016 TABLE I OMPARISON OF THE PRINIPLE SIMULATION RESULTS RP890A05 directinal cupler Islatin. -26dB dB 2 Attenuatr 4 Pwer synthesizer 1 RP890A05 Fig.3 Principle circuit f the prpsed directinal cupler If ne wants t increase the islatin, the age S shuld be reduced as much as pssible. Ideally, S S 0. As a result, tw signals will be cancelled ut fr the same magnitude and the cntrary phase, as shwn in (7) and (8): A ka (7) (8) In practice, hwever, it is difficult t btain abslutely equal value, and the frequency change will cause phase change. S it can nly minimize the age, but cannt cancell ut the age cmpletely. The simulatin results using Agilent ADS 2009 in Table I shw that the prpsed directinal cupler imprves the islatin f the chip frm -26dB t -342dB, which all the cmpnents are ideal. Therefre, in thery, the prpsed methd is feasible. Fig.4 Layut f the prpsed directinal cupler 3 Phase shifter III. DESIGN OF THE DIRETIONAL OUPLER Actual mdel is added in Agilent ADS 2009, as shwn in Fig. 4. The attenuatr uses the π attenuatin netwrk instead, the phase shifter uses the micrstrip line instead, and the pwer synthesizer is the Wilkinsn pwer synthesizer which is made up f micrstrip line. Adjusting the attenuatin f the attenuatin netwrk makes the age f the islatin prt and the cupling signal the same magnitude, then nly adjusting the size f the micrstrip line f the phase shifter makes the phase difference between the age and the phase-shifted signal equal t 180. Layut is made t d electrmagnetic simulatin and frequency band f the simulatin is frm 860MHz t 960MHz. The simulatin results f the physical layut at frequency f 915MHz are shwn in Fig. 5. The insertin lss (m4) is abut -1.7dB in TX, the islatin (m6) between RX and TX is -53dB which shws that the prpsed directinal cupler has higher islatin, and the cupling (m5) between the antenna and the receiver is -8dB in RX. Because the actual bard and the layut are nt ideal, we als need t adjust the size f the phase shifter and the attenuatr accrding t measured results finally. Fig.5 Simulatin results f the layut IV. TEST AND ANALYSIS OF THE PROPOSED DIRETIONAL OUPLER The main functin f the directinal cupler is t islate RX frm TX, s the imprtant index f testing is the islatin. There will be energy lss when the signal is transmitted frm the transmitter t the antenna thrugh the directinal cupler, s ne needs t test the insertin lss between the input and the utput. One als needs t test the cupling f RX due t the fact that the received wave is transferred thrugh the cupling prt [6]. Fig. 6 shws the physical picture f the prpsed directinal cupler. T demnstrate the crrectness f the prpsed directinal cupler, the experimental set-up uses a netwrk analyzer fr measuring the S-parameters f the directinal cupler. The prpsed directinal cupler has the islatin f -35dB at 915MHz as shwn in Fig. 7 which is higher islatin than the directinal cupler chip as expected. The actual circuit has many unideal factrs s that the measured result is nt -53dB btained by the simulatin. As shwn in Fig. 8, the insertin lss is almst similar with that f the directinal cupler chip.

4 ELETRONIS, VOL. 20, NO. 1, JUNE Fig.6 Physical picture f the prpsed directinal cupler Fig.8 Insertin lss f the prpsed directinal cupler Fig.7 Islatin f the prpsed directinal cupler Fig.9 upling f the prpsed directinal cupler In Fig. 9, the cupling f the prpsed directinal cupler exhibits -9.1dB. The return lss f each prt is less than -10dB which is a gd match t the 50 lad. TABLE II is the perfrmance cntrast f the chip RP890A05 and the prpsed directinal cupler. The prpsed directinal cupler imprves the islatin f abut 10dB cmpared with the chip, which decreases the design difficulty f the LNA t a great extent, achieving the purpse f imprvement. Hwever, the cupling lwers a little and it is still in the acceptable range. In general, the prpsed directinal cupler has reached the purpse. V. ONLUSION New methd fr increasing the islatin f the directinal cupler f the UHF RFID reader has been presented. The prpsed directinal cupler includes the canceling lp t ffset TX age. The prpsed directinal cupler pssesses a gd islatin f -35dB at 915MHz, abut 10dB higher than befre, and is able t remarkably enhance the sensitivity f the receiver. TABLE II PERFORMANE ONTRAST OF THE DIRETIONAL OUPLER RP890A05 Islatin -26dB -35dB upling -6.2dB -9.1dB Insertin lss -3.2dB -3.2dB REFERENES directinal cupler [1] Kim. Wan-Kyu, Lee. Mn-Que, Kim. Jin-Hyun, Lim. Hyung-Sun, Yu. Jng-Wn, Jang. Byung-Jun, Park. Jun-Sek, A passive circulatr with high islatin using a directinal cupler fr RFID, IEEE MTT-S Internatinal Micrwave Sympsium Digest, p , [2] Desng Wan, Bin Yu, Lingling Sun. Radi link analysis and design f directinal cupler with high islatin fr UHF RFID reader, mmunicatin Technlgy, IT th IEEE Internatinal nference n. [3] K. Finkenzeller, RFID Handbk, Radi-Frequency Identificatin Fundamentals and Applicatins, New Yrk, Wiley, [4] Jin Wk Jung, Kwan Kyu Nae, Jay Prakash Thakur, Hng Gu h, Jun Sek Park. Directinal cupler fr UHF mbile RFID reader, Micrw. Opt. Technl. Lett.,2007,497.

5 26 ELETRONIS, VOL. 20, NO. 1, JUNE 2016 [5] Quan Xu, Kang Xie, Jun Tang, Jiang Ping. Directinal cupler design based n cupled cavity waveguides in phtnic crystals, Optik - Internatinal Jurnal fr Light and Electrn Optics,2010, [6] Zhngba Wang, Shajun Fang, Shiqiang Fu, Xiaming Li. A Bradband 3 db planar Waveguides Directinal upler with nductr-backed Asymmetric planar Waveguides mpensatin, IETE Jurnal f Research,2012,581.