Research on Resonance Frequency Measurement of Contactless Smart IC Card

In recent years, as large as finance, public transportation and social security, as well as libraries, campuses, and access control, smart card applications have become increasingly diversified, and there have been more and more smart card design and production companies. Because the smart card is completely sealed, it is difficult to measure the overall electrical parameters L, C, and R. The resonant frequency is an important indicator that can reflect the electrical parameters of the smart card antenna port, and is widely used by companies and R&D units for design or production. Reference has long been used extensively. But so far, there is no uniform standard in the industry for measuring resonant frequencies. At the same time, when referring to the resonant frequency value, each link often ignores its measurement method and clear error range. Therefore, in the field of smart card measurement, the reliability and reliability of the resonant frequency parameter have long been neglected.

Taking a smart card that conforms to the ISO/IEC 14443 standard as an example, the protocol specifies that the carrier frequency for communication is 13.56 MHz, but no standard value is set for the resonant frequency of the smart card itself. Therefore, objectively, the diversity of resonant frequencies of the currently circulating smart card is caused. At present, according to the form of smart cards, there are mainly two methods for measuring the resonant frequency of smart cards commonly used in the industry:

1: LCR bridge or impedance analyzer measurement; (measured L, C value, then use the formula to calculate the resonant frequency)

2: Spectrum analyzer or network analyzer measurement. (Measure the resonant frequency of the sealed smart card)

First introduce how to measure the electrical parameters of each part, and then use the formula to calculate the resonant frequency. Smart card in the physical structure, mainly composed of three parts, 1: IC chip, 2: coupling antenna, 3: packaging materials, as shown in Figure 1, wherein the packaging material is usually insulated material, do not introduce electrical parameters, so this article does not In-depth analysis.

The resonance frequency fres formula of the smart card is as follows: It can be seen that fres depends on the inductance and capacitance in the equivalent circuit.

Looking from the left to the right of the dashed line La/Lb in Figure 1, the electrical parameters related to the resonance frequency of the IC chip port, Rab is the sum of IC chip port resistances, and Cic is the sum of IC chip port capacitances, Cmount The meaning is the capacitance value introduced when the IC chip is packaged into a module. If the chip does not need to be packaged, the Cmount can be ignored. Looking from the right side to the left side of dotted line La/Lb in Fig. 1, it is the electrical parameters of the coupling antenna section and the resonant frequency. Lcoil is the inductance of the coupled antenna, Rcoil is the resistance of the coupled antenna, and Ccoil is the capacitance of the coupled antenna. The meaning of Cpack is the value of the package capacitance introduced by the coupling antenna during the card manufacturing process. The value of the Cpack is related to various factors in the card manufacturing process, depending on the specific circumstances.

According to the equivalent circuit structure of Figure 1, we expand the calculation formula of smart card fres as follows:

When we have a detailed calculation formula, can we calculate the exact fres? The actual situation is not the case. Next, we introduce measurement methods for each of the L and C parameters, as well as sources of error. At present, XOA2 and COB are two types of module packages commonly used in IC chips, and because Cmount is affected by the comprehensive factors such as the technical level, application materials, and electrostatic protection of each module processing plant, modules produced by each module processing plant are There are differences in Cmount, and it is impossible to give accurate values. At this point, the first parameter error is introduced using the fres calculation formula of the smart card. At the same time, in the card production of the smart card, Cpack will receive the technical level, materials and With the addition of electrostatic protection and other comprehensive factors, the cards produced by each value card factory are also different in Cpack, and cannot give accurate values, thus introducing the second parameter error. In the actual calculation, the above two parameters usually use empirical values, and the calculated fres will have errors. Therefore, when we use fres, we need to clarify the error range. It is particularly emphasized that the experience values ​​of the above two parameters are not universal for smart cards processed under different conditions.

The Agilent 16047E with the Agilent 4285A (LCR Meter) is used to measure the Cic, Lcoil, and Ccoil in the equivalent circuit. The overall measurement platform is shown in Figure 2.

Figure 2 Agilent 4285A (LCR Meter) and measurement fixture Agilent 16047E

Since the parasitic parameters of the coupled antenna and the IC chip will bring errors to the measurement results, selecting an appropriate equivalent circuit model can effectively reduce the influence of parasitic parameters. Usually Lcoil is a small inductor. The effect of series parasitic resistance Rs is obvious. Therefore, when measuring Lcoil, Ls ~ Rs model is used; while Cic is large, parallel parasitic capacitance Rp has obvious influence, so when measuring Cic, use Cp ~ Rp model .

After the above measurement conditions are determined, according to the instrument's use procedure, after warm-up and calibration, we use the following method to measure the Lmil and 30 MHz coupling antenna inductance Lm, and then calculate the Ccoil through Lcoil and Lm.

1: Select the measurement model: Ls to Rs.

2: Set the measurement voltage: 1Vrms.

3: Set the measurement frequency: 1MHz.

4: Record the measurement result Ls, which is Lcoil.

5: Set the measurement frequency: fm = 30MHz.

6: Record the measurement result Ls, which is Lm. Calculate the Ccoil of the coupled antenna by the following formula.

We measured the coupled antenna sample with the module base as shown in Fig. 3. To illustrate the impact of the module base on the measurement results, we measured the Lcoil and the Ccoil of the coupled antenna with the module base and the removal module base, respectively. As shown in Table 1. (The data in the table are the average value after 10 measurements. The effective number of digits is retained to 2 decimal places, the same below.) Comparing the data in Table 1, it can be found that the module base exists for the coupled antenna sample. Lcoil has no effect, but it will increase Ccoil by 0.16 pf.

Figure 3 Coupled antenna sample with module base

Table 1 Inductance and Capacitance of Coupled Antennas

Sample status

Lcoil/uh

Lm/uh

Rcoil/ohm

Ccoil/pf

Coupled antenna + module base

5.30

22.92

9.16

4.08

Coupling antenna

5.32

20.45

8.70

3.92

Difference

-0.02

2.47

0.47

0.16

Next, we discuss how to measure the port capacitance of the IC chip Cic. The sample is shown in Fig. 4. The selected chip is NXP S50. The left side is the module base (same as the base module in Fig. 3), and the right side is the complete module package (XOA2) The appearance of the sample after this, so the capacitance value obtained below is constituted as "Cic + Cmount (C module contains C module base)".

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