Automatic Impedance Matching for 13.56 MHz NFC Antennas Michael Roland 1 , Harald Witschnig 2 , Erich Merlin 2 , Christian Saminger 1 1 Upper Austria University of Applied Sciences, Hagenberg, Austria 2 NXP Semiconductors, Gratkorn, Austria 6 th Symposium on Communication Systems, Networks and Digital Signal Processing, 2008 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 1 / 24
Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 2 / 24
Overview of the NFC Research Lab NFC Research Lab Part of the R&D department of the Upper Austria University of Applied Sciences Funded by the BMVIT (Federal Ministry for Transport, Innovation and Technology) and FFG (Austrian Research Promotion Agency) We are focusing on NFC hardware development interoperability and performance testing for NFC systems implementation of NFC applications secure aspects for NFC systems and applications 1st Austrian NFC Trial Annual NFC Congress in Hagenberg Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 3 / 24
Motivation NFC Equipment and its Antennas Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 4 / 24
Motivation NFC Equipment and its Antennas NFC Equipment and its Antennas NFC transmission module IC has external antenna Mobile devices have though space-saving requirements Non-standard antennas necessary to fit casings antennas have varying characteristics (shape, impedance) each antenna has a certain impedance measured at its clamps Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 5 / 24
Motivation Impedance Matching Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 6 / 24
Motivation Impedance Matching What is Impedance Matching? Antenna’s impedance is the NFC IC’s load resistance 2 parameters depend on the NFC IC’s load resistance the transmitter supply current and the emitted RF power Both parameters decrease with increasing load resistance 140 140 transmitter supply current 120 120 emitted RF power 100 100 80 [mW] 80 [mA] 60 60 40 40 20 20 0 0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 load resistance [Ohm] load resistance [Ohm] Good trade-off between both parameters around 40 to 50 ohms Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 7 / 24
Motivation Impedance Matching NFC Antenna Topology to RX TX1 L0 C1 RQ C0 C2 NFC IC GND C0 C2 C1 L0 RQ TX2 NFC antenna topology with EMC filter, matching circuit, quality factor damping resistors and antenna coil C 1 and C 2 are adjusted until the whole topology circuit matches 50 ohms at its clamps (TX1, TX2) Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 8 / 24
Motivation Impedance Matching Disadvantages of Manual Impedance Matching Expensive equipment (network analyzer, impedance analyzer) Time-consuming procedure Good sense of choosing the right capacitor values necessary Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 9 / 24
Automatic Impedance Matching Automatic Tuning System Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 10 / 24
Automatic Impedance Matching Automatic Tuning System Requirements Most NFC devices are mobile handsets Automatic tuning system must be small in shape and size Single chip solution preferred ⇒ ideal solution has only integrable parts Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 11 / 24
Automatic Impedance Matching Automatic Tuning System System Design Automatic tuning system design split into its three components: measuring circuit, matching circuit and controller component. waveform generator voltage follower voltage follower amplitude antenna measuring bridge phase Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 12 / 24
Automatic Impedance Matching Measuring Circuit Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 13 / 24
Automatic Impedance Matching Measuring Circuit Measuring Bridge Wheatstone bridge with 13 . 56 MHz sinusoidal supply Bridge is balanced ( V diff = I 2 · R 2 − I Z · Z = 0 V) R1 R3 50 50 amplitude & phase measurement R2 50 tuning circuit ⇒ Tuning circuit is matched to 50 ohms (at the operating frequency) Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 14 / 24
Automatic Impedance Matching Measuring Circuit Balancing the Amplitude of V diff Measuring rectifiers are used to rectify V SINE 1 = I 2 · R 2 and V SINE 2 = I Z · Z Resulting signals are low pass filtered V diff is calculated in software after analog-digital conversion R3 D1 V- R1 4 VSINE 2 R4 VRECT 1 GND D2 3 C1 U1A R2 8 V+ GND Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 15 / 24
Automatic Impedance Matching Measuring Circuit Balancing the Phase of V diff V SINE 1 and V SINE 2 shaped into rectangular signals Phase detector: phase-offset is converted into a pulse-width modulated signal PWM signal is low pass filtered and fed into an analog-digital converter R1 U1A VSINE1 2 4 GND 1 R2 /Q SET RESET GND U1D 15 PHASE Q 13 VSINE2 16 R4 R3 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 16 / 24
Automatic Impedance Matching Measuring Circuit Verification of the Measuring Circuit Verified with a manually tunable antenna circuit The antenna is connected to the measuring circuit and tuned Tuning is verified with a network analyzer Result is approximately the same with several tested antennas: Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 17 / 24
Automatic Impedance Matching Matching Circuit Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 18 / 24
Automatic Impedance Matching Matching Circuit Matching Circuit C 1 and C 2 in the antenna topology are replaced with capacitance arrays Capacitance array: network of parallel switched capacitors Capacitors switched with low-capacitance DMOS switches S0 S1 S2 S3 Sn 1 * C 2 * C 4 * C 8 * C (2^n) * C Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 19 / 24
Automatic Impedance Matching Controller Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 20 / 24
Automatic Impedance Matching Controller Controller Analyses the measurands by reading the values from analog-digital converter Controls the capacitance arrays Tuning algorithm to minimizes amplitude and phase of V diff by optimal adjustment of C 1 and C 2 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 21 / 24
Automatic Impedance Matching Results Outline Overview of the NFC Research Lab 1 Motivation 2 NFC Equipment and its Antennas Impedance Matching Automatic Impedance Matching 3 Automatic Tuning System Measuring Circuit Matching Circuit Controller Results Summary and Outlook 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 22 / 24
Automatic Impedance Matching Results Automatic Tuning Results Antenna 1 Antenna 2 Antenna 3 Antenna 4 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 23 / 24
Recommend
More recommend
