AN1954 APPLICATION NOTE

Transcription

1 AN1954 APPLICATION NOTE How to Extend the Operating Range of the CRX14 Contactless Coupler Chip This Application Note describes how to extend the operating range of the CRX14 Contactless Coupler Chip, which is used to read RFID Tags. In the method described, the operating range is extended by increasing the power transmitted and amplifying the signal received. Introduction CRX14 is a short range Contactless Coupler Chip, compliant with the ISO14443 type B proximity standard. It generates a 13.56MHz signal, which is modulated according to the ISO14443 type B proximity standard. The CRX14 features the ST anti-collision mechanism, which allows the reader to detect and identify all the tags that are present in the operating range, and to access them individually. It also features the France Telecom-proprietary anti-clone function, so the reader can also authentication tags that are equipped with the France Telecom anti-clone capability. The CRX14 coupler provides the interface between: n the memory tags, through input/output buffers and the ISO14443 type B radio frequency protocol n the system master processor, through a 400kHz I 2 C bus. Operating from a 5V power supply, and delivered in a SO16N package, the CRX14 Contactless Coupler Chip is an excellent solution for building contactless readers, which are embedded in the final equipment, as they offer a good compromise between operating range and cost. Figure 1. Block Diagram Pre-demodulation Amplifier, Filter I2C Bus CRX14 Class E Power Amplifier Match Circuit Tags Ai08697 February 2006 Rev 2 1/10

2 Contents Contents 1 Application design Class E amplifier Class E Amplifier Components Class E amplifier used to boost RF power Antenna and matching circuit Receiving circuit Conclusion Appendix A Bill of material Revision history /25

3 List of tables List of tables Table 1. Bill of Material Table 2. Document Revision History /25

4 List of figures List of figures Figure 1. Block Diagram Figure 2. CRX14 Main Circuit Figure 3. CRX14 RFOUT Signal Figure 4. Typical Class E Amplifier Circuit Figure 5. Transistor Voltage and Current Waveforms Figure 6. Class E Amplifier Used to Boost RF Power from CRX Figure 7. Antenna and Matching Circuit Figure 8. Series-to-Parallel Circuit Equivalence Figure 9. Filter and Amplifier Circuit Figure 10. Negative Signal Cut Off Figure 11. Carrier Frequency Attenuated Figure 12. Signal Centered to V CC / Figure 13. Signal Cut-Off below V CC Figure 14. Signal Centered by C6 and R Figure 15. Signal Amplified together with 13.56MHz Carrier Figure 16. Q-spoiling Resistor to Reduce the Sharpness of the Resonance Peak Figure 17. Negative Signal Cut Off Figure 18. Signal Centered to V CC / Figure 19. PCB Board Figure 20. Top Layer Figure 21. Bottom Layer /25

5 Application design 1 Application design Figure 2 shows a basic design of an application using a CRX14. The RFOUT output signal is boosted by a Class E Amplifier. The RFIN input signal that comes from the antenna, is filtered and amplified. Figure 3 shows the signal on RFOUT. Figure 2. CRX14 Main Circuit C15 22µF V CC C13 100nF L4 10µH C37 100nF C33 22nF 50V FL7 RFIN V CC U1 Vref RFIN E0 E1 E2 GND_RF GND GND CRX14 V CC RF OUT GND_RF OSC1 OSC2 GND SCL SDA C14 22µF V CC To Class E Amplifier C34 7pF C35 7pF X MHz + C38 22µF 10V J1 AI /25

6 Application design Figure 3. CRX14 RFOUT Signal 1. X-axis = t, 50µs per division, Y-axis = V, 5V per division. 6/25

7 Class E amplifier 2 Class E amplifier The Class E Amplifier is a very efficient, high performance power amplifier used for narrowband or fixed-tuned operation. Figure 4 shows the schematic of a typical Class E Amplifier. The transistor acts as a switch. In the "ON" state, a high collector current flows, while the collector voltage is nearly zero. In the "OFF" state, the current is zero, while there's a high voltage. Thus, the voltage and current cannot be zero simultaneously, which avoids power loss form the transistor. The ideal waveforms of transistor voltage and current is shown in Figure 5. Figure 4. Typical Class E Amplifier Circuit Active Device Switch Load Network Load C2 L2 From Driver Q L1 C1 R + + V CC AI /25

8 Class E amplifier Figure 5. Transistor Voltage and Current Waveforms Voltage Across Switch Current Through Switch 0 Time Switch Switch "Off" State "On" State 0 Time AI Class E Amplifier Components In Figure 4 Inductor L1 is used to force a D.C. current in the circuit. Capacitor C1 holds the collector voltage at zero during the ON-OFF switch transition. The C2-L2-R circuit is designed so that the collector voltage falls back to zero just before the OFF-ON transition. The following equations can be used to calculate the values of of the components used in the circuit. Equation 1: Load Resistor R L R L = ( U cc U cesat ) P S Equation 2: Capacitor C C 1 = ω R L Equation 3: Inductor L 1 10 L ω 2 C 1 Equation 4: Inductor L 2 Q L L R L 2 = ω 8/25

9 Class E amplifier Equation 5: Capacitor C 2 ω L = R ω C L 2 Where: P S is the output power U cc is the supply voltage U cesat is the saturation voltage of the transistor ω is the signal frequency For example, if the amplifier s output power is 0.5W and the voltage supply is 5V, U cesat is 0.4V and the signal frequency is 13.56MHz, then from: Equation 1, R L = 24.41Ω Equation 2, C 1 = 96.16pF Equation 3, L µH Equation 4, L 2 = 1.146µH Equation 5, C 2 = 169pF 9/25

10 Class E amplifier used to boost RF power 3 Class E amplifier used to boost RF power Figure 6 shows how the Class E Amplifier is used to boost the RF power from the CRX14. The CRX14 RFOUT signal switches the PZT2222 transistor. The operating frequency is 13.56MHz according to the ISO14443 standard. R L is replaced by a tuning circuit and its antenna. Figure 6. Class E Amplifier Used to Boost RF Power from CRX14 V CC C17 100nF C19 22µF L5 22µH C23 100pF L6 22µH L8 1µH L7 0.15µH C18 22µF R11 10K C22 100pF C25 option C24 10pF To Anntena From CRX14 C16 330pF Q1 PZT2222 R12 47 R12 68 R13 0 GND AI /25

11 Antenna and matching circuit 4 Antenna and matching circuit In Figure 6 the antenna together with the Matching circuit, replaces the load resistor of the Class E Amplifier (shown in Figure 4). So the input impedance of the Matching circuit and antenna must be tuned to match the value of R L. It can be done by adjusting Capacitors C26+C27 and C28+C31 and Resistors R14+R15 (see Figure 7). Figure 7. Antenna and Matching Circuit From Class E C26 R20 option C27 27pF R14 option 0 C28 5pF C31 120pF A1 Antenna GND R15 0 AI09544 According to Application Note AN1806 Antenna (and Associated Components) Matching- Circuit Calculation for the CRX14 Coupler, the serial capacitor C26//C27 can be calculated by the following formula: Equation 6: Serial Capacitor C S C 1 S = Z ω R P and the parallel capacitor C28//C31 can be calculated as follows: Equation 7: Parallel Capacitor C P 1 C P = C Lω 2 S where: ω is 13.56MHz L is the inductance of the antenna loop Z equals R L of the Class E Amplifier R P is the equivalent parallel resistor as shown in Figure 8. 11/25

12 Antenna and matching circuit Figure 8. Series-to-Parallel Circuit Equivalence R S R P X P X S ai08464 R P can be calculated using the following equation: Equation 8 2 X S R P = R S where: X S = ω.l R S = total resistance of antenna loop In Figure 8, R S is the sum of the natural resistance of the antenna loop and the resistance of the additional resistor required to have the correct value of the antenna parameters. For example, if Z is 25Ω, R S is 0.5Ω, and the inductance of the antenna loop is about 0.9µH, then from: Equation 8, R P =11760Ω Equation 6, C S = 22pF Equation 7, C P = 131pF 12/25

13 Receiving circuit 5 Receiving circuit The signal from the antenna is sent to CRX14 after being pre-demodulated, amplified and filtered as shown in Figure 9. Figure 9. Filter and Amplifier Circuit V CC C4 22µF L1 C1 100nF C2 22µF Power R18 39K From AntennaT C32 15pF D2 IN4148 D1 IN µΗ C8 18pF C9 180pF R4 8.2K R1 33K D3 R5 33K IN4148 R7 1K D5 IN4148 C6 180pF R2 10K R17 56K R6 2.2K U1 OUT A IN A + IN A V AD8062 C nf V+ OUT B IN B + IN B R19 10K RFIN POWER R3 15K C12 C5 C3 68pF R9 22K 180pF D4 IN pF R8 1 L2 0.47µΗ C10 180pF C3 22pF R8' 1 AI /25

14 Receiving circuit C32, R18 and D1 rectify the signal from the antenna. They cut off the negative part of the signal as shown in Figure 10. Figure 10. Negative Signal Cut Off D2, C8, R4 pre-demodulate the signal. As a result, the 13.56MHz carrier frequency is greatly attenuated as shown in Figure 11. Figure 11. Carrier Frequency Attenuated C9, R1 and R5 are then used to center the signal to V CC /2 as shown in Figure /25

15 Receiving circuit Figure 12. Signal Centered to V CC /2 The negative part of the signal and the part between 0V and V CC are removed by D3, D5 and R7, as shown in Figure 13. Figure 13. Signal Cut-Off below V CC After that, the signal is centred at 0 volts with C6 and R2, as shown in Figure /25

16 Receiving circuit Figure 14. Signal Centered by C6 and R2 An AD8062 rail to rail OP amplifier is used to make a non-inverting amplifier, where the gain is calculated using the following equation: Equation 9: Gain Gain = 1 + R R16 Thus the sub-carrier signal is amplified together with the 13.56MHz carrier, as shown in Figure 15. Figure 15. Signal Amplified together with 13.56MHz Carrier After being amplified, the signal is filtered by a passive band filter tuned at 14.4MHz. The filter is composed by C3 and C3', C10, L2, R8 and R8'. The resonant frequency can be calculated as follows: 16/25

17 Receiving circuit Equation 10: Resonant Frequency ω ω = L 2 C 3 + C 3 + C 10 R8 and R8' are used as Q-spoiling resistor to reduce the sharpness of the resonant peak, as shown in Figure 16. Figure 16. Q-spoiling Resistor to Reduce the Sharpness of the Resonance Peak Again, the negative part of the signal is cut by C5 and D4, as shown in Figure 17. Figure 17. Negative Signal Cut Off Finally the signal should be centered to V CC /2 by C12, R3 and R9, as shown in Figure 18. The CRX14 will demodulate this signal and get data from the tag. 17/25

18 Receiving circuit Figure 18. Signal Centered to V CC /2 18/25

19 Receiving circuit Figure 19. PCB Board VCC L1 10µH C4 22µF C2 22µF VCC C34 7pF X MHz C35 7pF C33 FL7 22nF50V C32 RFIN VCC + C38 22µF 10V C1 100nF 5V C19 C23 22µF l5 22µH D2 150pF L7 L6 22µH 0.15µH L8 1µH C17 C15 C24 100nF C18 L4 10µH 22µF 10pF C25 option C22 100pF R11 10K 22µF C13 100nF C37 C26 option C27 27pF DEMOD C28 5pF R15 0 R20 option R14 0 C31 120pF A1 ANTENNA Q1 PZT2222 C16 100nF C14 22µF 330pF R13 0 R12 68 R12 47 VCC J1 15pF R18 1N K D1 1N4148 C5 C3 22pF C12 D5 1N4148 R17 C36 C10 L2 22pF R7 56K U1 1K 1 OUTA V+ 8 C3 68pF 180pF 0.47µH C6 2 -INA OUTB 7 100nF 3 +INA -INB 6 R19 1N D4 180pF R2 V- +INB 5 10K R8 R8 1N K AD R1 33K R3 15K 180pF R9 22K RFIN D3 C9 R6 2.2K 180pF R5 33K R4 8.2K C8 18pF DEMOD Vcc 16 RF OUT 15 GND_RF 14 OSC1 13 OSC2 12 GND 11 SCL 10 SDA 9 U2 Vref RF IN E0 E1 E2 GND_RF GND GND CRX14 ai /25

20 Receiving circuit Figure 20. Top Layer Figure 21. Bottom Layer 20/25

21 Conclusion 6 Conclusion CRX14 is a short range Contactless Coupler Chip, compliant with the ISO14443 type B proximity standard. The operating range of the CRX14 can be extended by increasing the power transmitted by the CRX14 to the tag using a high performance Class E power amplifier, and by pre-demodulating, amplifying and filtering the signal received from the antenna. This can extend the operating range to up to 10cm. The solution proposed is ideal to improve the read/write distance of a type B reader, or to read/write tags which have a higher power consumption, such as contactless MCU cards. 21/25

22 Bill of material Appendix A Bill of material Table 1. Bill of Material Component Value Footprint Comment R Ω ±1% R Ω ±1% R Ω ±1% L7 0.15µH 1210 ±10% L2 0.47µH 1210 ±10% R8' 1Ω Ω ±1% R8 1Ω Ω ±1% R7 1kΩ Ω ±1% L8 1µH 1210 ±10% R6 2.2kΩ Ω ±1% C28 5pF V ±5% C3' 7pF V ±5% C34 7pF V ±5% C35 7pF V ±5% R4 8.2kΩ Ω ±1% R19 10kΩ Ω ±1% R2 10kΩ Ω ±1% R11 10kΩ Ω ±1% C24 10pF V ±5% L1 10µH 1210 ±10% L4 10µH 1210 ±10% X MHz XTAL2 - C8 18pF V ±5% R3 22kΩ Ω ±1% R9 22kΩ Ω ±1% C33 22nF V ±5% C5 22pF V ±5% C32 22pF V ±5% C18 22µF 1210 TANTALUM ±10% 6.3V C38 22µF 1210 TANTALUM ±10% 6.3V C2 22µF 1210 TANTALUM ±10% 6.3V C15 22µF 1210 TANTALUM ±10% 6.3V C14 22µF 1210 TANTALUM ±10% 6.3V C19 22µF 1210 TANTALUM ±10% 6.3V 22/25

23 Bill of material Table 1. Bill of Material (continued) Component Value Footprint Comment C4 22µF 1210 TANTALUM ±10% 6.3V L5 22µH 1210 ±10% L6 22µH 1210 ±10% C27 27pF R5 33kΩ Ω ±1% R1 33kΩ Ω ±1% R18 39kΩ Ω ±1% R12' 47Ω Ω ±1% R17 56kΩ Ω ±1% R12 68Ω Ω ±1% C3 68pF V ±5% C36 100nF V ±5% C13 100nF V ±5% C17 100nF V ±5% C37 100nF V ±5% C1 100nF V ±5% C22 100pF V ±5% C31 120pF V ±5% C23 150pF V ±5% C12 180pF V ±5% C10 180pF V ±5% C9 180pF V ±5% C6 180pF V ±5% C16 330pF V ±5% D5 1N D4 1N D3 1N D1 1N D2 1N FL WURTH U1 AD8062 SO-8 - J1 CON4 SIP4 - U2 CRX14 SO-16 CRX14 STMicroelectronics Q1 PZT2222 SOT C26 option C25 option R20 option /25

24 Revision history 7 Revision history Table 2. Document Revision History Date Version Changes 25-Jan First Issue 14-Feb Figure 19: PCB Board modified. 24/25

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