EMI problem of inductively coupled non-contact IC card system
Radio frequency identification (RFID) technology has developed rapidly in recent years and has been widely used. However, as a kind of radio frequency technology, its electromagnetic compatibility (EMC) performance has also received more and more attention. RFID covers a wide range of frequencies, including multiple frequency bands below 135 kHz, 13.56 MHz, 433 MHz, 860-960 MHz, 2.45 GHz, and 5.8 GHz. This paper only introduces and analyzes the electromagnetic interference (EMI) problem of inductively coupled non-contact RFID cards in the two frequency bands below 135 kHz and 13.56 MHz in combination with relevant international standards.
Electromagnetic compatibility of electronic products
Electromagnetic compatibility of electronic products EMC has two aspects: one is electromagnetic interference EMI, and the other is anti-electromagnetic interference capability EMS. EMI refers to any electromagnetic effect produced by an electronic product that may degrade the performance of other devices, equipment, systems, or may adversely affect organisms and substances. EMS refers to the ability of electronic products to resist deterioration in an electromagnetic environment.
The strict restrictions on electronic products EMI are reflected in many international standards and related national standards. Representative institutions and organizations that develop these standards are: International Radio Interference Special Committee 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 the relevant EMI standards or it cannot be sold and used in the region or country.
Operating frequency range and standard for inductively coupled non-contact IC cards
Generally, inductively coupled non-contact IC cards operate at frequencies below 135 kHz and 13.56 MHz. The contactless IC card operating at 13.56 MHz can be further divided into a near-coupled IC card (PICC) and a loosely coupled card (VICC). Its readers are also known as PCD and VCD.
Frequencies below 135 kHz are primarily suitable for lower cost answer tag chips. The system's readers can provide higher power. This band has a high penetration depth for non-metallic materials and water, so it has a wide range of applications in biometrics, water meters and other fields. Due to its low carrier frequency, although the power consumption of the transponder is low, the data transmission rate is not high. Currently, ISO/IEC 18000-2 gives the air interface standard for such systems.
13.56MHz is the worldwide industrial, scientific and medical frequency band (ISM). RFID operating in this frequency band can use a microprocessor in the transponder because of the high carrier frequency, so that the smart contactless IC card function can be realized. In addition, at this carrier frequency, the antenna loop of the transponder can realize the resonant circuit of on-chip capacitor and printed inductor, which is very convenient for its application.
The main standards for 13.56MHz radio frequency identification are: 1ISO/IEC 14443, which is the standard for near-coupling IC card system. It is divided into TYPE A and TYPE B; 2ISO/IEC 15693, which is the standard for the loosely coupled IC card system; 3ISO /IEC18000-3 standard, it has two modes, namely MODE 1 and MODE 2, MODE 1 and ISO/IEC 15693 standards are compatible, and MODE 2 gives new methods such as phase jitter modulation (PJM).
EMI standard for contactless IC cards below 135 kHz
In the non-contact IC card below 135 kHz, more chips are used at 125 kHz. The power amplifier of the reader circuit is a class B or class D circuit, and an antenna transmitter with an inductive loop is used, which belongs to low power and micro power transmission. For RFID systems in this frequency range, the following specifications apply:
Allowable maximum field strength FCC standard
The FCC Standard Part 15 Section 209 (FCC part 15.209) specifies the allowable field strength of its carrier, given by the maximum electric field strength E, ie E is not greater than (2400/f) μV/m@300m. Among them: f is 9~490kHz, @300m means the distance is 300m. Therefore, for a frequency of 125 kHz, its E should be no greater than 19.2 μV/m @ 300 m. If it is converted to dBμV, E = 19.2 mV/m is E = 20 log (19.2 / 1) = 25.66 dBμV / m.
Other Standards In other standards, the maximum field strength allowed is given in the H field. For example EN300330 and the German standard 17TR2100. The E field and H field representation values ​​can be extrapolated using the following equations, namely:
H[dBμA/m ]=E[dBμV/m]-51.5dB
In the 17TR2100, the specified limit is H = 13.5dBμA/m@30m. Convert this value to the above equation to get E=65 dBμV/m@30m.
If we convert the 300 m distance of the FCC standard to 30 m and increase by 40 dB/ten times (the attenuation value of the free field), 25.66 dBμV/m@300 m can be expressed as 65.66 dBμV/m@30 m. From the above calculations, the limits of the two standards are similar. If this is expressed as a field value at 10 m, then about 40 log (30/10) + 13.5 = 32.5 dBμA/m can be obtained.
Modulation Bandwidth and Spurious Emissions During data communication in a contactless IC card system, load modulation is employed by the transponder to the reader. The gap between the reader and the transponder is often modulated by gap amplitude shift keying (ASK). These require a certain modulation bandwidth.
Parasitic emissions refer to emissions that do not contribute to the carrier frequency or modulation sidebands. In a contactless IC card system, the reader amplifier and modulation circuitry are the main components that generate harmonics and other spectrum. The field strength and spurious emissions of the non-contact IC card system in the modulation band cannot exceed the allowable limits specified by the EMI standards.
13.56MHz EMI standard
Allowable maximum field strength The non-contact IC card standard specifies that the field strength generated by its PCD is between 1.5A/m_7.5A/m in the 13.56MHz ISO/IEC 14443 standard. The action field strength of its transponder (PICC) is Hmin1.5A/m. In the ISO/IEC 15693 standard, the field strength generated by VCD is 115 mA/m to 7.5 A/m, and the field strength of the transponder (VICC) is Hmin 1.5 m A/m.
About the EMI standard 1FCC standard
FCC part 15.225 specifies that the RFID system carrier frequency range is 13.56 MHz ± 7 kHz, and the carrier field strength is 10 mV/m at 30 m.
2EN300330 (9kHz~25MHz) standard
The EN300330 standard specifies the carrier power limit for a first type of transmitter (referred to as a transmitter with an inductive loop antenna, the antenna consisting of a winding with one or several coils). The direction in which the H field has the largest value is measured and performed in free space. The limit specified by EN300330 is 42dBμA/m@10m.
Comparison of several standards. We have made some transformations and normalized the several criteria given above, and then compared them.
1ISO/IEC 14443 standard Generally, the working distance of the near-coupling IC card system is less than 10cm, and from the above, it can be known that Hmin1.5A/m at this time, then we can approximate the H field value of 1.5m (10cm) from the antenna. A/m. Generally, in the non-contact IC card system, the near field is at a distance of λ/2π (for a frequency of 13.56 MHz, λ/2π=3.5 m), and the attenuation is 60 dB/ten times; the distance greater than λ/2π is In the far field, the attenuation is 20dB/ten times the distance. Therefore, it can be estimated that the H field at 1 m is 1.5 m A/m, and the attenuation value of the H field at 3.5 m is less than 60 log (3.5 m/1 m) = 32 dB. 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 has a working distance of about 50cm, with Hmin=115m A/m at this time, 13.56MHz, and a near-field range of 3.5m. The field strength attenuation at 3.5m is 60log (3.5m/0.5m). ) = 50.7 dBmA. The field is 3.5m to 10m, and the attenuation is 20log (10m/3.5m) = 9dBμA. Therefore, the dBμA value at 10 meters is: 20log115mA/m-50.7dBuA-9dBμA=41dBμA
3FCC standard
30m is 10m V/m, converted to a dB value of 80dBμV/m, and then converted to 10m, the amount of increase is 40log (30m/10m) = 19dB. The conversion to dBμA/m is: H=80+19-51=48dBμA/m@10m.
The above results are listed in Table 1. From Table 1, it can be seen that if the H field is designed according to the RFID standard, it can meet the relevant standards of EMI.
Modulation Bandwidth and Harmonic Emissions ISO/IEC 14443 Modulation Method In TYPE A, PCD uses 100% ASK modulation of the modified Miller code for PICC communication. Its PICC uses Manchester encoding for PCD communication and ASK modulation after subcarrier modulation. In TYPE B, PCD uses ASK modulation of NRZ code for PICC communication. PICC uses BPSK subcarrier modulation (847 kHz) for NRZ code (106 kbps) for PCD communication, and then transmits it to PCD with ASK modulation.
ISO/IEC 19653
When the VCD communicates with the VICC, it takes a long distance to use the 10% ASK encoded by "1 in 256". For short distances, 100% ASK modulation encoded by "4 out of 1" is used. The VICC uses Manchester encoding for VCD communication, modulation with subcarriers (either ASK or frequency shift keying FSK), and then ASK modulation of the carrier with modulated subcarriers.
As can be seen from the above communication modes, these modulations require a modulation bandwidth, so care must be taken to control the emission spectrum. The FCC specifies that harmonic power should be limited by 50 dB below the carrier.
Measures to suppress EMI
Measures to suppress EMI can be considered from the following aspects:
The RF front-end circuit design of the reader/writer should consider the requirements of device selection, PCB layer number, size, circuit layout, wiring, grounding and grounding configuration, shielding, filtering, etc. when designing the RF circuit of the reader.
For example, in the design of the reader, a crystal oscillator is required. Most quartz crystal oscillators (XO) do not provide inherent EMI suppression. Therefore, designers use shielding, filtering or special printed board layout technology to make products. Pass the EMI assessment. However, MAXIM's DS108X series of silicon oscillators use spread spectrum technology to reduce peak EMI by more than 20dB, which provides new ideas for the choice of frequency source.
In the circuit design and adjustment of the inductively coupled non-contact IC card system, the following aspects should be noted: 1 The antenna loop parameters should be accurately tuned to the carrier frequency. 2 For systems with Class D amplifiers (below 135 kHz) or Class E amplifiers (13.56 MHz) for the final stage power amplifier, the circuit operation state should be carefully adjusted to reduce the generation of spurious 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 magnitude and the communication bandwidth should be carefully adjusted.
The choice of new modulation methods In ISO/IEC 14443, ISO/IEC 19653 and non-contact IC card systems below 135 kHz, the data transmission of the reader to the transponder uses 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 average signal power. It has not been adopted in the aforementioned standards, mainly because PSK demodulation can only use a relatively complex coherent demodulation technique, and cannot use a simple envelope detection method, and the coherent demodulation circuit is relatively complicated.
MODE 2 in the ISO/IEC 18000-3 standard proposes a phase jitter modulation (PJM) method in the 13.56 MHz system. In the communication between the reader/writer and the transponder, the carrier is PJM modulated by the modified frequency coding (MEM). Usually, the two phase angles of the binary modulation of PSK are 0 degrees and 180 degrees, and a carrier phase transition occurs when the coding changes. The larger phase transition causes the spectrum to broaden, making the power spectrum's side lobes larger and slower. The two phase angles of the PJM are defined in the range of 2 degrees, see Figure 1. Therefore, the change in phase, that is, the decrease in jitter, makes the side spectrum of the signal spectrum smaller and the attenuation is faster. In addition, another advantage of PJM is that in the communication between the reader and the transponder, there is no gap in the energy field due to the pause of the ASK modulation, and full-duplex communication can be supported.
Therefore, the research and implementation of the new modulation method will also greatly improve the EMI of the inductively coupled non-contact IC card.
Conclusion
In this paper, the EMI problem of the inductively coupled non-contact IC card system is expounded. It should be noted that its EMS capability is obviously weak, and it is limited by the space, and no more explanation is given. In view of the fact that China's RFID standards have not been officially promulgated, this article does not cover.
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