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Radio Frequency Identification (RFID) Antenna and System Design

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  • 1. Radio Frequency Identification (RFID) Antenna and System Design Markus Laudien ( ANSOFT Corp) Application Engineer
  • 2. RFID antenna and system design Agenda Requirements of UHF-RFID systems and limitations of analytical treatment Reader antenna Transponder unit ( chip, package, antenna ) Field-Simulation of a whole transmission setup. Integration of nonlinear components into the transmission setup. Conclusions
  • 3. What is RFID (Radio Frequency IDentification) Wireless identifications of piece goods Reader: Communication with Tags, tracking of piece goods Frequencies of operation : status : LF: 125 kHz ; 13.56 MHz ( near field coupling ) established, mass production, short range (typ <1 m ) UHF: 433 MHz, 868 MHz, 915 MHz , 2.45 GHz market introduction, ( far field operation ) larger range ( <8m)
  • 4. Block diagram of an RFID transmission system antenna ASK Transponder IC RECEIVER DEMODULATOR READER LOGIC SYSTEM MEMORY PSK or TRANSMITTER MODULATOR (READ/WRITE) ASK •constant near field conditions at the Variable near field The transponder circuit is reader (stationary) conditions. powered by the Incident field at •Sometimes variable •Change of material the antenna near field conditions properties (mobile reader) •Neighboring transponders. The operating distance and system reliability in most cases is given by the forward link which determines the power supply of the transponder. The backward link is mostly not critical due to the high receiver sensitivity
  • 5. Link Budget Due to the indirect power supply it is essential to make a careful calculation of the power budget. Safety margins have to be included Example: Transmitter 33dBm Contributions from the transmitter and the reader 33dBm output antenna are relatively easy to evaluate, however, in the operational environment the transmission Antenna (circ) Antenna (Omni) +5dB setup and antenna are subject to strong variations 1.4dB gain 5dBi gain -3dB due to the strongly variable environment. 3dB Polasation-loss 0.5 dB mismatch 35dBm Free Space Modern systems are targeted for communication Transmission -40dB up to a few hundred tags thus requiring good -40 dB loss -5dBm reliability Antenna (Omni) Strongly varying region: Dependence on the 0dBi distance, objects in the vicinity of the 1 dB Gain 1 dB matching loss -5dBm transponder antenna, material variation Receiver The use of RF simulation tools in all -14 dBm sensitivity -5dBm parts of the system helps to predict the i.e. safety margin range of reliable operation of approx 9 dB
  • 6. Design of transponders for RFID Systems System Specification Simulation potential ( system ) Selection of Hardware ( Reader, Chips ) Decision for manufacturing technique Simulation sensitivity towards process (printing, etching … ) and assembly process; possible size of label, … Packaging parasitics, statistical Antenna design Parametric antenna studies & opti- misation, increase of bandwidth Prototype assembly of transponder sensitivity analysis ( chip & antenna ) Simulation of simplified transmission Test under „laboratory conditions“ between reader and tag (only if reader ant model available ) Customer Samples . . Modeling of a „real“ situation with Test under „real“ conditions ( customer) reader , many tags and other objects in between RELIABILITY ???
  • 7. Theoretical estimation of the transmission distance ( Friis equation) DISTANCE = R TRANSMIT ANTENNA WAVELENGTH OF SIGNAL = λ RECEIVE ANTENNA GAIN* = GT GAIN* = GR Polarization = ρT Polarization = ρR PR ⎛ λ ⎞ ( )( ) 2 REFL. COEFF = ΓR REFL. COEFF = ΓT =⎜ GT GR ρT • ρ R 1 − ΓT2 1 − ΓR 2 ⎟ 2 ˆ ˆ PT ⎝ 4πR ⎠ Analytical approaches like the Friis equation assume •Non-disturbed near field conditions (no proximity of dielectric and metal objects ) •Known antenna characteristics •No diffraction and reflection effects … this is not the case in most real RFID situations which means that this is a very rough estimation. A full system simulation including reader , tags and the environment is needed.
  • 8. Reader-Antenna A reader antenna in most cases uses circular polarization in order to avoid potential strong polarization losses due to a linear polarization of the transponder antenna With HFSS & Optimetrics different quantities (e.g. S11, axial ratio, field quantities ) can be improved in one single optimization setup: Goal quantity calculation range Goal Value Weighting
  • 9. Reader-antenna Optimization Process ( at 915 MHz ) Model similar to: L. Boccia, G. Amendola, G. Di Massa: Design a high-precision Antenna for GPS ; Microwaves&RF online January 2003
  • 10. Reader-antenna: Results Return Loss Gain Axial Ratio vs. Realized gain ( Phi vs Theta polarisation ) Theta
  • 11. Reader-antenna: E-Field and mesh Mag E-Field ( log scale, vertical excitation )
  • 12. Transponder: Antenna Matching of the tag Mainly three elements are influencing the matching characteristic of an UHF RFID-Tag: Influence of the tag designer on performance 1) Chip Impedance: given by input circuit, technology, power level of incident signal - 2) Package: Depending on the carrier technology different package techniques may apply like wire bonds, Flip Chip and SOC housing. Due to parasitic behaviour this part ís not neglectable. Here the user 0 has a certain amount of influence 3) Antenna: The antenna impedance is given by the shape of the antenna, the used materials and ++ changes in the environment. Here the user has the largest influence to adapt it to a specific application
  • 13. Transponder: Antenna Matching of the tag Antenna matching for conjugate complex impedance of chip AND package The power efficiency of the transponder is strongly influenced by the antenna losses and the package losses P Chip P Chip = η Chip = + P Chip + P Loss relative P Antenna
  • 14. Transponder: input impedance of the chip UHF transponder, which uses the energy from the incident field, exhibits a cascaded rectifier switch on behalf to the antenna. There are many different topologies that have been extensively examined *,**: These circuit parts (if necessary an additional overvoltage protection) affect considerably the chip impedance, which is seen by the antenna. This impedance depends on the input power. *Z. Zhu, B. Jamali, P.Cole: Brief Comparison of Different Rectifier structures for HF and UHF RFID ; (Phase II, Draft version 0.0 ) University of Adalaide 24.04.2004 **Qiang Li, Yfeng Han, Hao Min, feng Zhou : Fabrication and Modeling of SChottky Diode Integrated in CMOS Process; State Key Lab of ASIC& System; Fudan University, Shang Hai 200433
  • 15. Transponder: input impedance of the chip Example of a cascaded input circuit of a transponder IC . Today's common CMOS processes allow the integration of Schottky diodes. The input power at 915 MHz is swept within the relevant input power range of -40 dBm... +10 dBm. The most critical range is, depending upon the technology, at the minimum power levels which still allow an operation of the transponder circuit (approx. -23 dBm... -10 dBm). Input circuit, antenna, and receiver should be optimized for the range of the lowest permitted power levels. Depending on the circuit topology and input power level Re (Z_chip) and Im ( Z_chip ) may change 20 % and more within the range of operation condition.
  • 16. Transponder: resulting chip impedance Impedance match: CMOS technology of the UHF transponders provides low real part of impedance to the antenna (approx.. 5... 30 ohms) and a high reactive part around -200... -500 ohms. The Smith plot above shows that typical output impedance of the transponder chips lie in the marked range. Due to the low Re(Z) the matching to the antenna requires some inductive series element(s) in combination with shunt capacitance(s). Matched circuits are sensitive towards •power dependent changes of the chip impedance •Parasitic contributions of the package
  • 17. Transponder: contribution of the package Different package technology have a strong influence on impedance matching and have to be taken into account by simulation or measurement: Bondwires Flip Chip TSSOP Simplified equivalent circuit of different packages
  • 18. Transponder: contribution of the package Example*: mounting on a label antenna in flip chip technique Metallized bumps on the chip surface are pressed into the metallisation on the tag carrier. It can happen that overlaps between bumps and chip metallisation are forming a significant capacititance due to an overlap. *: pictures with permission of Philips AG / Gratkorn ( Austria )
  • 19. Transponder: contribution of the package Example: mounting in flip chip technique, Simulation of parasitic effects with HFSS Extraction of parasitic elements with 3D Simulation in HFSS, Extraction shows a sensitivity towards Change of position, angle of mounting, distance between chip metallisation and laben substrate.
  • 20. Transponder: Antenna Matching As the UHF labels use mostly printed or etched metallisation almost no design uses discrete components. This means the matching will be done within the antenna. Several antenna topologies are common which match to the complex chip impedance. Usual antenna topologies are e.g.: These antenna topologies mostly contain an inductive sections close to the chip and one or more capacitive sections Position of the chip
  • 21. Transponder: Antenna Matching This subject is very suitable for the use of 2.5D or 3D field simulation tools, which make the adjustment by parametric variation of the antenna geometry. It is important to merge the complex chip impedance directly into the model. Example of a parametric variation in HFSS: mperiod The material of the smart width card (thickness: thick_card) can also be a variable selected based on the material of the carrier Ysize_carr which is under it msize (xsize_carr, ysize_carr, thick_carr) distbar Xsize_carr
  • 22. Transponder: Antenna Matching Tuning the antenna on an initial configuration (e.g. a certain carrier material with well- known permittivity and loss tangent): A variation of one of the sizes is connected with a simultaneous change of the resonant frequency and antenna impedance Variation of m_period: Effect on the resonant frequency Effect on re ( Z_ant )
  • 23. Transponder: Antenna Matching Variation of load_bar distance: The variation “load_bar" of the distance can be used for the variation of the resonant frequency and by Re(Z_ant). By variation of two (not completely independent ) parameters it is quite simple to achieve the desired resonance frequency and a good matching: Parameter Enlargement Reduction msize, mperiod f_res down f_res up dist_bar f_res up f_res down w f_res down f_res up
  • 24. Transponder antenna: sensitivity towards changes in the proximity of the antenna For a particular configuration (e.g. a 5mm thick mother board from PA) the transponder antenna shows a very strong sensitivity of the resonance behavior in relation to variations (e.g. the thickness or the material properties): It is interesting to ask, from a system point of view, how such a change affects the receiver behavior, especially considering the nonlinear elements within the transmission circuit.
  • 25. Sensitivity towards changes of the antenna and in the proximity of the antenna Many more scenarios in the near field of a tag antenna can be simulated Effect of humidity absorption of the carrier Metal objects in the proximity Near Field interaction with other resonant tags Material changes Variation of conductor of carrier (tolerances ) thickness and width
  • 26. Simulation of a realistic UHF RFID transmission setup in HFSS (64 bit) Assumed is a close-to-reality model of a wall-mounted 915 MHz reader antenna and at a distance of approx. 2.20 m a pallet with 12 transponder antennas. This setup is simulated with HFSS 10. Reader antenna with •Total of 14 Ports two polarisation setups •Frequency range from 800 MHz..1GHz Distance from Pallet with 12 reader to the center boxes ( PS-foam) of the pallet approx.. and RFID 2.20 m transponders Detail image of two neighboring transponder antennas
  • 27. Simulation of a realistic UHF RFID transmission setup Computation on 2 GHz dual Opteron PC: used memory: 4.5 GB RAM Computing time approx. 3.5h ( including 8 adaptive passes to a convergence of <2 % ) Computing time may be further reduced by: •Increased Multiprocessing •Performing the solution at a very small bandwidth ( e.g. 914 MHz… 916 MHz )
  • 28. Simulation of a realistic UHF RFID transmission setup: Typical transmission losses between reader antenna and transponders tags (S-parameters). Variation of transmission-loss at 915 MHz between -35dB and -52 dB
  • 29. Simulation of a realistic UHF RFID system : Integration of the transmission circuit into an overall simulation with nonlinear input circuits at the transponders TX P_tag1 Reader 3D Simulation of the distance between reader P_tag2 vertical antenna and polarisation transponder . Φ . mag . horizontal polarisation P_tagn The direct link of the simulated transmission setup with nonlinear input circuits of the tags allows a very realistic estimations of the receiving conditions ( input DC power at the different transponders: P_tagn )
  • 30. Simulation of a realistic UHF RFID system : An equivalent circuit of an RFID input section ( under non-disclosure agreement ) was provided by Philips AG / (Gratkorn , Austria) to study the effects of the power dependent receiving conditions of an ensemble of RFID tags. The model has been transferred to an encrypted format. ed pt ry nc a e t da Equivalent circuit of the flip chip mounting was accomplished by measurements and simulations with HFSS. The modulation input for FSK transmission is not used here.
  • 31. Simulation of a realistic UHF RFID system EM - cosimulation: Link between circuit simulator ( NEXXIM ) and 3D field simulator (HFSS) Circular polarisation accomplished by two splitted signals with 90° phase offset "dynamic link": i.e. use of the parameters of the 3D model and the according results within the circuit simulator
  • 32. Simulation of a realistic UHF RFID system- polarization effects Feed of the antenna in horizontal polarization ( equivalent with orientation of the tag antennas ). As no over-voltage protection is included and the distance is quite short the voltages are somehow higher than in real cases. Reader operating frequency
  • 33. Simulation of a realistic UHF RFID system- polarization effects Circular polarisation vertical polarization This scenario clearly shows the advantage of a circular polarized reader antenna: While linear polarized reader antennas may provide a higher power level in case of polarisation- alignment is may also cause low power level if polarisation of reader and tag are perpendicular. The effects of polarisazion studies can be accomplished within the circuit simulator by a combining the two linear polarisation contents of the reader
  • 34. Simulation of a realistic UHF RFID system- reader-power sweep & frequency sweep : DC input power at three transponder chips at varying transmitting power of the reader ( a) and varying frequency ( b) :
  • 35. Simulation of a realistic UHF RFID system variance of geometry: Parametric change of geometry in the vicinity of the tag-antennas:
  • 36. Simulation of a realistic UHF RFID system time domain analysis : Time domain simulation of ASK modulated reader signal ( Cosimulation System & NEXXIM ) 868MHz RFID ASK test ( one arbitrary tag ) tag_in SP SP output U2 tag_2p2 tag_out 1 8 AWGN Input_PAD RChip 2 9 SP 3 10 4 11 power splitter 5 12 90° shift 6 7 13 14 NEXXIM subcircuit ref of transponder input 0 circuit CCONST ASK modulator 90% modulation, 50 % duty cycle
  • 37. Simulation of a realistic UHF RFID system– time domain analysis: Time domain simulation of ASK modulated reader signal ( Cosimulation System & NEXXIM ) Charging of the buffer C of the passive transponder
  • 38. Simulation of a realistic UHF RFID system proposed approach: Most recent approaches for UHF-RFID systems are based on trial and error or on the simulation of some single parts within the system. Supposed the reader antenna geometry and equivalent transponder circuit are available the simulation of a whole scenario is feasible with Ansoft simulation tools: 3D Simulation of reader Antenna (HFSS ) Simulation of complete 3D system setup including piece goods in the environ- Optimisation of ment Parametric studies transponder ( HFSS, parametrized com- of environmental Antenna (HFSS + ponents transferred by copy changes ( HFSS, Optimetrics ) and paste ) Optimetrics ) Extraction of Cosimulation EM Packaging para- sitics ( Q3D, HFSS) Equivalent circuit and circuit ( HFSS, of transponder input NEXXIM, DESIGNER) ( chip vendor)
  • 39. Simulation of a realistic UHF RFID transmission setup: Conclusions: • Todays solver efficiency of HFSS and circuit cosimulation with DESIGNER allow the simulation of a realistic UHF-RFID system within a few hours. • Semi-analytical statements based on “ideal” antenna parameters ( like directivity, polarisation ) are not always reliable if many other parts are in the vicinity of the antennas. • Even transient simulations of such scenarios and the integration of a system simulation for the characterisation of ASK/PSK modulation, BER etc. can be made. • The availability of 64 bit solver technology on LINUX and WINDOWS significantly shifts the complexity limit for such simulation scenarios.

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