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Powering up the RFID chip - Remotely

Powering up the RFID chip - Remotely. Basic Reader-Tag System. Rectifier. Logic & Memory. Reader. Tag. Z1’ and Z2’ can be used to represent resistors, capacitors etc. as required. I 1. I 2. Z 1 ’. Define self-impedance of each loop: Z 1 = Z 1’ +R1+ j w L1 Z 2 = Z 2’ +R2+ jwL2.

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Powering up the RFID chip - Remotely

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  1. Powering up the RFID chip - Remotely

  2. Basic Reader-Tag System Rectifier Logic & Memory Reader Tag

  3. Z1’ and Z2’ can be used to represent resistors, capacitors etc. as required I1 I2 . . Z1’ Define self-impedance of each loop: Z1 = Z1’ +R1+ jwL1 Z2 = Z2’ +R2+ jwL2 + Z2’ L1, R1 L2, R2 ~ Vi Vi = Z1.I1 - jwM.I2 0 = Z2.I2 - jwM.I1 Applying KVL in each loop Reflected impedance Simple Magnetically Coupled Circuit Input impedance General Expressions Transfer admittance

  4. Input impedance Current Transfer ratio General Expressions Transfer admittance

  5. I1 I2 . . jwM.I2 R1 + R2 L1 L2 ~ Vi Vi Voltage Current Source voltage I2 I1 Example: Inductively Coupled Resistive Circuit (Transformer) (R1 + jwL1).I1 (R2 + jwL2).I2 = jwM.I1 Vi = (R1 + jwL1).I1 - jwM.I2 0 = (R2 + jwL2).I2 - jwM.I1

  6. I1 I2 . . jwM.I2 R1 ~ 0 + R2 L1 L2 ~ Vi jwL1.I1 Vi Voltage Current Source voltage I2 I1 Ideal Transformer R1 << w.L1 R2 << w.L2 k ~ 1 (R2 + jwL2).I2 = jwM.I1 Vi = jwL1.I1 - jwM.I2 0 = (R2 + jwL2).I2 - jwM.I1

  7. Self Quiz • Inductively coupled circuit with R1= 1W, R2= 2W, L1=L2, w.L1=200W, k= 0.8 If I1= 1A, what is the approximate value of I2? (KVL) • If R2 = 1W, what is the approximate value of I2? • What is approximate input impedance in each case? • What is the approximate input impedance if k ~ 1?

  8. 0.8 A • 0.8 A (Same!) • (1+ j.72) W (Unchanged!) • 1 W

  9. I1 I2 . . Self impedances: Z1 = 1/ jwC1 +R1+ jwL1 Z2 = 1/ jwC2+R2+ jwL2 R1 R2 C1 + L1 Vi L2 C2 Let resonance occur at which is our excitation frequency ~ Transfer admittance Effectiveness to drive current through secondary – would like to maximize for effective power transfer Introduce resonance CAVEAT: Series resonance for illustration only!

  10. At w = w0, we have Z1 =R1, Z2 =R2 and Transfer admittance is Q1=30 Q2=40 Coupling Coefficient % Beyond this value of k, Transfer admittance falls! Peak occurs at

  11. Self Quiz Reader and Tag both has Q =25, and each has ESR (effective series resistance ) = 5W. The reader is excited by 1V. What is the current in the Tag for k = 1%, 4%, 10% if both primary and secondary tuned to same frequency?

  12. spacing Spacing ↑ => Coupling coefficient ↓ ~ Transfer admittance Tight coupling Small Separation Weak coupling Large Separation Coupling Coefficient % Diminishing return – does not help reducing the spacing beyond a certain point

  13. Weak Coupling Case then coupling is weak If In other words Then

  14. For non-resonant situation For resonant situation Resonant vs. Non-resonant Transfer admittance - general expression For weak coupling: => Current increases by Q1.Q2 (Product of loaded Q’s)

  15. Effects of Resonance • Resonance helps to increase current in coupled loop ~1000X  • But it causes strange behavior (reduction of secondary current at close range). Why ?

  16. Self Quiz The primary coil is tuned to a certain frequency and excited by a voltage source of the same frequency. A secondary coil, also tuned to the same frequency is gradually brought in from far distance. How does the current in the secondary coil behave with changing distance? (qualitative description) Two coils each of Q=50 is taken. Current is measured in second coil with and without tuning capacitor (tuned to frequency of excitation). What is the ratio of currents in the two scenarios?

  17. Self Quiz The primary coil is tuned to a certain frequency and excited by a voltage source of the same frequency. A secondary coil, also tuned to the same frequency is gradually brought in from far distance. How does the current in the secondary coil behave with changing distance? Increases till k.sqrt(Q1.Q2) = 1, then decreases Two coils each of Q=50 is taken. Current is measured in second coil with and without tuning capacitor (tuned to frequency of excitation). What is the ratio of currents in the two scenarios? 50*50 = 2500

  18. Self Quiz A Reader-tag system has a certain maximum read range determined by current needed to turn on the Tag chip. Q of the tag is halved. How much is the max read range compared to original? [Assume weak coupling] R2 is doubled  (wM/R1.R2) halved  range halved

  19. (R2+j.X2).I2 = jwM.I1 I2 + (R1+j.X1).I1 I1 I2 + . . R1 R2 C1 + L1 Vi L2 C2 -jwM.I2 I1 Voltage Current Source voltage ~ ~ Inductively Coupled Series Resonant Circuits Excitation at higher than resonant frequency Vi = [R1 + j(wL1-1/wC1)].I1 - jwM.I2 0 = [R2 + j(wL2-1/wC2)].I2 - jwM.I1 Phase angle between Vi and I1 may be > or < 0 depending on coupling

  20. R2.I2 = jwM.I1 I2 I1 I2 . . R1 R2 C1 R1.I1 + L1 Vi L2 C2 -jwM.I2 Vi Voltage Current Source voltage I1 ~ Inductively Coupled Series Resonant Circuits Excitation at resonant frequency Vi = [R1 + j(wL1-1/wC1)].I1 - jwM.I2 0 = [R2 + j(wL2-1/wC2)].I2 - jwM.I1

  21. (R1-j.X1).I1 I2 I1 I2 I1 . . R1 R2 C1 + L1 Vi L2 C2 (R2-j.X2).I2 = jwM.I1 -jwM.I2 Voltage Current Source voltage ~ Inductively Coupled Series Resonant Circuits Excitation at lower than resonant frequency Vi = [R1 + j(wL1-1/wC1)].I1 - jwM.I2 0 = [R2 + j(wL2-1/wC2)].I2 - jwM.I1 • Phase angle between Vi and I1 may be > or < 0 depending on coupling • I1 and I2 flowing in same direction for lossless case

  22. 2 2 2 1 1 1 + + + Below resonance (capacitive) Resonance (resistive) Above resonance (inductive) I2 I1 I1 I2 I1 I2

  23. Power Transmission Efficiency h Rectifier Logic & Memory Reader Equivalent Resistive Load Tag

  24. If Q>>1 then: Parallel to Series Transformation Cs At a certain frequency ≡ C RL RLs Example: f = 13.56 MHz C= 50.0 pF (XC = 235W) RL = 2000 W Cs pF (Exact): 50.7 pF Cs pF (Approx): 50.0 pF RLs (Exact): 27.2 W RLs (Approx): 27.6 W

  25. ~ I1 I2 C2 . . R1 R2 C1 + RLs L1 Vi L2 Zin Power dissipated at load = |I2|2.RLs Power available from source = |I1|2.Re(Zin) Assuming both Reader and Tag are resonant at excitation frequency

  26. wM = 15W wM = 5W For weak coupling, efficiency is maximum when R2 = RLs RL↑ => C2 ↓ for given R2 Low dissipation chips usually use less tank capacitance

  27. Special Case • Both Reader and Tag are resonant at excitation frequency L1.C1=L2.C2 = w02 • Weak coupling R1>> Reflected impedance • Tag is independently matched to load R2=RLs => Total resistance in Tag = 2R2 = 2RLs • Q of load (XC2/RLs) >> 1

  28. Self Quiz XC = 200 ohm (C~ 50 pF) RL = 10Kohm What is the value of Tag resistance for optimum power transfer at weak coupling? If XC is changed to 300 ohm, what is the value of Tag resistance for optimum power transfer at weak coupling?

  29. Self Quiz XC = 200 ohm (C~ 50 pF) RL = 10Kohm What is the value of Tag resistance for optimum power transfer at weak coupling? 200^2/10e3= 4 ohm [Traces could be too wide for a compact tag!] If XC is changed to 300 ohm (C~ 33 pF), what is the value of Tag resistance for optimum power transfer at weak coupling? 300^2/10e3= 9 ohm [Compact tag is realistic]

  30. Measurement of Resonance Parameters • Resonant frequency • Loaded Q • Caution: • Maintain weak coupling with probe loop Vector Network Analyzer Sensing Loop

  31. Measurement on a Tag attached to curved surface

  32. Principle of Measurement Z1 = R1 + j.wL1 Sensing Loop alone Z2 = R1 + j.wL1 + (wM)2. YDUT Sensing Loop + DUT Z2 - Z2 = (wM)2. YDUT YDUT If s-parameter is used Sensing Loop alone – stored in Memory Sensing Loop + DUT – ‘Data’ Data – Memory = s11_D - s11_M Approximation valid if Z0>> Z1, Z2. error for low values of YDUT Transmission method is more accurate

  33. Spectral Splitting

  34. Are these phenomena related? Weak coupling Large Separation Tight coupling Small Separation ~ secondary current Coupling Coefficient % spacing

  35. I1 I1 R1 L1-M L2-M R2 . . R1 R2 C1 ≡ + C1 L1 Vi L2 Vi C2 M C2 ~ ~ I1 I2 M I1 I2 . . R1 L1-M L2-M R2 R1 R2 + + + + L1 ≡ L2 V1 V2 M V1 V2 V1= (R1+jwL1).I1 + jwM.I2 V2= (R2+jwL2).I2 + jwM.I1

  36. I1 R1 ~1/w02.M L1-M C1 Vi M ~w02.M2/R2 +jw.M -jw.M (w0.M)/R2>>1 ~ ~ I1 Let:(L1, C1) => f0 (L2, C2) => f0 i.e. w0.L1=1/(w0.C1) w0.L2=1/(w0.C2) R1 L1-M L2-M R2 C1 Vi M C2 If M~0 (weak coupling), I1 exhibits series resonance behavior determined by L1, C1 If coupling is NOT weak: At f=f0: R2+j.[w0.(L2-M)-1/(w0.C2)] = R2- jw0.M Parallel resonance chokes current at f0 [+jw.M and –jw.M in shunt] Input is capacitive If R2 ↑ (Q2↓) => choking ↓

  37. Self Quiz • Lossless Resonators tuned at f1 and f2. When coupling is increased, at what frequency parallel resonance occurs?

  38. Self Quiz • Lossless Resonators tuned at f1 and f2. When coupling is increased, at what frequency parallel resonance occurs? • f2 when looking from resonator 1 and vice versa

  39. ~ ~ Series resonances I1 R1 L1-M Frequency↓=> Shunt arm more and more capacitive C1 f<f0 ‘Odd Mode’ Vi M I1 R1 L1-M L2-M R2 Frequency↑ => Shunt arm less and less capacitive and then more and moreinductive f>f0 ‘Even Mode’ Occurs when shunt arm is shorted C1 Vi M C2 Series and parallel resonances alternate

  40. R1=R2=6 ohm L1=L2=2700 nH C1=C2=50 pF Q1=Q2=38.7 f01=f02=13.7 MHz Critical coupling = 0.026 Excitation voltage = 1V

  41. Resonances for Lossless Identical resonators L1=L2=L C1=C2=C R1=R2=0 Series Parallel Series L-M C L-M C C L-M 2M

  42. Two NFC Tags ~ equally coupled with Sensing Loop

  43. Realistic Situation R1=R2=6 ohm L1=L2=2700 nH C1=50pF C2= 47pF Q1=38.7 (at f01)Q2=39.9 (at f02) f01=13.7 MHz f02= 14.1 MHz Critical coupling = 0.025 Excitation voltage = 1V

  44. Excitation Frequency as Parameter Significant degradation in weakly coupled region when frequency of excitation is outside the band between resonant frequencies with a little bit improvement in close range

  45. Review Quiz • For two magnetically coupled resonators tuned at same frequency, we observed that parallel resonance occurs above a certain M. To arrive at this we used an equivalent T network for magnetically coupled inductors. How this phenomenon is explained by reflected impedance?

  46. Review Quiz • For two magnetically coupled resonators tuned at same frequency, we observed that parallel resonance occurs above a certain M. To arrive at this we used an equivalent T network for magnetically coupled inductors. How this phenomenon is explained by reflected impedance? Primary current ~ is maximized when Z2 is minimum Series resonance in secondary => parallel resonance in primary

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