Questions and Answers about EMI in Inductively Coupled Contactless IC Card Systems-EEWORLD

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The application of radio frequency identification (RFID) is becoming more and more extensive and in-depth, and the electromagnetic interference (EMI) problem of RFID has also attracted much attention. This article only introduces and analyzes the EMI problem of inductively coupled contactless IC cards in combination with relevant international standards.

Introduction

Radio frequency identification (RFID) technology has developed rapidly in recent years and has been widely used. However, as a wireless radio frequency technology, its electromagnetic compatibility (EMC) performance has also attracted more and more attention. RFID involves a wide range of frequencies, including multiple frequency bands below 135kHz, 13.56MHz, 433MHz, 860-960MHz, 2.45GHz, 5.8GHz, etc. This article only introduces and analyzes the electromagnetic interference (EMI) problem of inductively coupled contactless RFID cards in the two frequency bands below 135kHz and 13.56MHz in combination with relevant international standards.

1 Electromagnetic compatibility of electronic products

Q1-1. What is electromagnetic compatibility?

The electromagnetic compatibility (EMC) of electronic products includes two aspects: one is electromagnetic interference (EMI), and the other is electromagnetic interference resistance (EMS). EMI refers to any electromagnetic effect generated by electronic products that may reduce the performance of other devices, equipment, and systems, or may have adverse effects on organisms and materials. EMS refers to the ability of electronic products to resist deterioration of their performance in a certain electromagnetic environment.

Q1-2. Are there any restrictions on EMI for electronic products?

There are strict restrictions on EMI for electronic products, which are reflected in many international standards and relevant national standards. Representative institutions and organizations that formulate these standards include: International Special Committee on Radio Interference CISPR, International Organization for Standardization ISO, International Electrotechnical Commission IEC, Federal Communications Commission FCC, European Telecommunications Standards Institute EISI, etc. An electronic product must comply with relevant EMI standards, otherwise it cannot be sold or used in the market of the region or country.

2 Operating frequency range and standards

Q2-1What is the operating frequency range and standard of inductive coupling contactless IC cards?

Typically, the operating frequency of an inductively coupled contactless IC card is below 135kHz and 13.56MHz.

(1) The 135kHz frequency is mainly suitable for low-cost transponder tag chips. The system's reader can provide higher power, and this frequency band has a higher penetration depth for non-metallic materials and water, so it is widely used in biometrics, water meters and other fields. Due to its low carrier frequency, although the power consumption of the transponder is also low, the data transmission rate is not high. At present, ISO/IEC18000-2 provides the air interface standard for this type of system.

(2) The 13.56MHz contactless IC card can be divided into close coupling IC card (PICC) and remote coupling IC card (VICC). Its reader is also called PCD and VCD.

13.56MHz is the world's industrial, scientific and medical frequency band (ISM). RFIDs operating in this frequency band can use microprocessors in transponders due to their high carrier frequency, thus realizing the functions of smart contactless IC cards. In addition, at this carrier frequency, the antenna loop of the transponder can realize the resonant circuit of on-chip capacitors and printed inductors, which greatly facilitates its application.

The main standards for 13.56MHz RFID are: ①ISO/IEC 14443, which is the standard for close-coupled IC card systems, and is divided into TYPE A and TYPE B; ②ISO/IEC 15693, which is the standard for sparsely coupled IC card systems; ③ISO/IEC18000-3 standard, which has two modes, MODE 1 and MODE 2. MODE 1 is compatible with ISO/IEC 15693 standard, and MODE 2 provides new methods such as phase jitter modulation (PJM).

3 EMI standards for contactless IC cards

Q3-1. What is the EMI standard for contactless IC cards below 135kHz?

Among the contactless IC cards below 135kHz, 125kHz chips are mostly used. The power amplifier of its reader circuit is a Class B or Class D circuit, and an antenna transmitter with an inductor loop is used, which belongs to low-power and micro-power transmission. For the RFID system in this frequency range, there are the following specifications:

Maximum allowed field strength:

(1) FCC standards

FCC standard Part 15, Section 209 (FCC part15.209) stipulates the permissible field strength of its carrier, which is given by the maximum electric field strength E, that is, E is not greater than (2400/f)?V/m@300m. Among them: f is 9~490kHz, @300m means the distance is 300m. Therefore, for a frequency of 125kHz, its E should not be greater than 19.2μV/m@300m. If it is converted to dBμV, then E=19.2mV/m is E=20log(19.2/1)=25.66dBμV/m.

(2) Other standards

In some other standards, the maximum allowable field strength is given in H field. For example, EN300330 and the German standard 17TR2100. The E field and H field values can be calculated from each other using the following formula: H[dBμA/m]=E[dBμV/m]-51.5dB. In 17TR2100, the limit value is H= 13.5dBμA/m@30m. Converting this value according to the above formula, we can get E=65 dBμV/m@30m.

If we convert the 300m distance of the FCC standard to 30m, and increase it by 40dB/ten times the distance (the attenuation value of the free field), then 25.66dBμV/m@300m can be expressed as 65.66dBμV/m@30m. From the above calculation, the limit values of the two standards are similar. If this is expressed as the field value at 10m, it can be obtained that it is about 40log(30/10)+13.5=32.5 dBμA/m.

Q3-2. What are modulation bandwidth and parasitic emission?

In the data communication process of the contactless IC card system, the communication from the transponder to the reader/writer adopts load modulation. The communication from the reader/writer to the transponder often adopts gap amplitude shift keying (ASK) modulation. These all require a certain modulation bandwidth.

Parasitic emission refers to the emission that does not contribute to the carrier frequency or modulation sideband. In the contactless IC card system, the reader power amplifier and modulation circuit are the main parts that generate harmonics and other spectra. The field strength and parasitic emission of the contactless IC card system in the modulation frequency band cannot exceed the allowable limit values specified in the relevant EMI standards.

Q3-3. What is the EMI standard for 13.56MHz?

Maximum allowed field strength

(1) Contactless IC card standards

The 13.56MHz ISO/IEC 14443 standard stipulates that the field strength generated by the PCD is between 1.5A/m and 7.5A/m. The operating field strength of the transponder (PICC) is Hmin1.5A/m. The ISO/IEC 15693 standard stipulates that the field strength generated by the VCD is 115mA/m~7.5A/m, and the operating field strength of the transponder (VICC) is Hmin1.5m A/m.

(2) EMI standards

①FCC Standards

FCC part 15.225 stipulates that the carrier frequency range of the RFID system is 13.56MHz±7kHz, and the carrier field strength is 10mV/m at 30m.

②EN300330 (9kHz~25MHz) standard

The EN300330 standard specifies the carrier power limit values for the first category transmitters (transmitters with inductive loop antennas, which consist of windings with one or more coils). The measurement is performed in the direction where the H field has a maximum value and in free space. The limit value specified in EN300330 is 42dBμA/m@10m.

Q3-4. What are the differences when comparing several standards?

We make some transformations and normalize the standards given above, and then compare them.

①ISO/IEC 14443 standard

Usually, the working distance of the close-coupled IC card system is less than 10cm. From the above, we know that Hmin is 1.5A/m at this time, so we can roughly estimate that the H field value at 0.1m (10cm) from the antenna is 1.5A/m. Generally, in a contactless IC card system, the distance within λ/2π (for 13.56MHz frequency, λ/2π=3.5m) is the near field, and its attenuation is 60dB/ten times the distance; the working distance greater than λ/2π is the far field, and its attenuation is 20dB/ten times the distance. Therefore, it can be estimated that the H field at 1m is 1.5m A/m, and the attenuation value of the H field at 3.5m is about 60 log (3.5m/1m)=32dB compared to the H field at 1 meter. From 3.5m to 10m, it can be considered to enter the far field, and the attenuation value of this section is 20log(10m/3.5m)=9dB. Therefore, the dBμA value at 10m is 20log1.5mA/m-32-9=22dBμA/m.

②ISO/IEC 15693 standard

Its effective distance is about 50cm. Assuming Hmin= 115m A/m at this time, for 13.56MHz, the near field range is 3.5m, then the field strength at 3.5m is attenuated by 60log(3.5m/0.5m)=50.7dBmA. 3.5m to 10m is the far field, and the attenuation is 20log (10m/3.5m)=9dBμA. Therefore, the dBμA value at 10 meters is: 20log115mA/m-50.7dBuA-9dBμA=41dBμA

③FCC Standards

30m is 10m V/m, converted to dB value is 80dB?V/m, and then converted to 10m, the increase is 40log(30m/10m)=19dB. Converted to dB?A/m: H=80+19-51=48dBμA/m@10m.

The above results are listed in Table 1. It can be seen from Table 1 that if the H field is designed according to the RFID standard, it can meet the relevant EMI standards.

Table 1 Comparison of various standards

standard	ISO/IEC14443	ISO/IEC15693	FCC	EN300330
H field strength at 10m dBμA/m	twenty two	41	48	42

Q3-5. What should I pay attention to regarding modulation bandwidth and harmonic emission methods?

①Modulation method of ISO/IEC 14443

In TYPE A, PCD to PICC communication uses 100% ASK modulation of modified Miller code. PICC to PCD communication uses Manchester coding, and ASK modulation is performed after subcarrier modulation. In TYPE B, PCD to PICC communication uses NRZ code ASK modulation, and PICC to PCD communication uses BPSK subcarrier modulation (847kHz) of NRZ code (106kbps), and then ASK modulation is used to transmit to PCD.

②ISO/IEC 15693

When VCD communicates with VICC, 10% ASK with "1 out of 256" coding is used for long distance. 100% ASK modulation with "1 out of 4" coding is used for short distance. Manchester coding is used for communication between VICC and VCD, and the subcarrier is modulated (ASK or frequency shift keying FSK can be used), and then the modulated subcarrier is used to perform ASK modulation on the carrier.

From the above communication modes, it can be seen that these modulations require modulation bandwidth, so it is necessary to pay attention to controlling the emission spectrum. The FCC stipulates that the harmonic power should be limited to 50dB lower than the carrier.

4 Measures to suppress EMI

Q4-1 What measures can be taken to suppress EMI?

(1) Reader/writer RF front-end circuit design

When designing the RF circuit of a reader/writer, many issues that need to be taken into consideration include device selection, number of PCB layers, size, circuit layout, wiring, grounding and ground wire configuration, shielding, filtering, etc.

For example, when designing a reader, a crystal oscillator is required. Most quartz crystal oscillators (XOs) do not provide inherent EMI suppression measures, so designers use shielding, filtering or special printed board layout techniques to make the product pass EMI assessment. However, MAXIM's DS108X series silicon oscillators use spread spectrum technology to reduce their peak EMI by more than 20dB, which provides a new idea for the selection of frequency sources.

(2) The following aspects should be noted in the circuit design and adjustment of the inductively coupled contactless IC card system:

①The antenna loop parameters should be accurately tuned to the carrier frequency.

② For systems that use a Class D amplifier (less than 135kHz) or a Class E amplifier (13.56MHz) as the final power amplifier, the circuit operating state should be carefully adjusted to reduce the generation of parasitic signals. At this time, the Q value of the antenna loop can regulate the current flowing through the coil, and the relationship between the current and the communication bandwidth should be carefully adjusted.

(3) Selection of new modulation methods

In ISO/IEC 14443, ISO/IEC 19653 and contactless IC card systems below 135kHz, the data transmission from the reader to the transponder adopts the ASK carrier modulation mode, but in fact, the phase shift keying PSK modulation has better performance than ASK in terms of bit error rate and signal average power. It is not adopted in the above standards, mainly because the demodulation of PSK can only use the more complex coherent demodulation technology, rather than the simple envelope detection method, and the coherent demodulation circuit is relatively complex.

MODE 2 in the ISO/IEC 18000-3 standard proposes a phase jitter modulation (PJM) method in a 13.56MHz system. In the communication from the reader to the transponder, the carrier is modulated by PJM using modified frequency coding (MEM). Usually, the two phase angles of PSK binary modulation are 0 degrees and 180 degrees, and a jump in the carrier phase occurs when the coding changes. A large jump in phase widens the spectrum, making the side lobes of the power spectrum larger and the decay slower. The two phase angles of PJM are defined in the range of 2 degrees.

Therefore, the change of phase, i.e. the reduction of jitter, makes the side lobe of the signal spectrum smaller and decays faster. In addition, another advantage of PJM is that in the communication between the reader and the transponder, there will be no gap in the energy field due to pauses like ASK modulation, and it can support full-duplex communication. Therefore, the research and implementation of new modulation methods will also greatly improve the EMI of inductively coupled contactless IC cards.

Conclusion

This article discusses the EMI issues of the inductively coupled contactless IC card system. It should be noted that its EMS capability is obviously very weak. Due to space limitations, no further explanation is given. Since my country's RFID standards have not yet been officially promulgated, this article does not involve them.

Reference address：Questions and Answers about EMI in Inductively Coupled Contactless IC Card Systems

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