Detailed explanation of NFC specifications and testing

In recent years, due to the rapid growth of mobile phone functions and popularity, early electronic wallets have had opportunities to be promoted. The evolution of NFC is derived from the specific frequency band of RFID. Due to the market application of mobile phones, NFC can obtain standard interfaces and platforms at a relatively fast time. This article will discuss the architecture and specifications of NFC.

NFC Applications

In the electronic payment system, the most complete solution currently used in mobile phone systems is based on NFC (near field communication). There are already related products circulating in the market, such as NOKIA3220, Samsung SGH-X700, VISAWAVE and Taipei Metro EasyCard, all of which are compatible with the NFC system. In addition to personal identification and electronic payment systems, NFC also has some attractive functions in data transmission and exchange, such as: data download of electronic posters (including admission tickets, exhibition information). In addition, NFC can also be used as a simplified function for Bluetooth device pairing and password input. If the user has a Bluetooth device with NFC, he can place NFC close to two sets of Bluetooth devices with NFC respectively, so that they can be quickly connected without the pairing process of Bluetooth search and password input. At present, South Korea, China, Europe and the United States have conducted multiple verifications in the public transportation payment system under the cooperation of telecommunications operators, chip manufacturers and mobile phone manufacturers, and NFC is expected to grow rapidly.

NFC Architecture

The operating frequency of NFC is 13.56MHz, and the operating distance is about within 10cm. The NFC specification is based on the RFID 13.56MHz frequency band. Earlier applications of this frequency band include Philips MiFARE (ISO1443A), ISO1443B, ISO15693, ISO18000-3 and Sony Felica. Since contactless cards are used in personal data identification or electronic payment systems, they emphasize security mechanisms, and near-field communication, which is close to the card reader system, integrates the 13.56MHz medium- and short-range systems. Therefore, what we see on the market are Philips MiFARE (ISO1443A) and Sony Felica. In the early days, the two systems developed independently and were incompatible with each other. It was not until recent years that the two specifications were merged and the NFC specification ECMA340/ISO18092 (NFCIP1, NFCInterfaceandprotocol1) was formulated. This specification is compatible with the existing Philips MiFARE (ISO1443A) and Sony Felica.

NFC operates at 13.56MHz, ASK modulation, and the transmission rate can be divided into three types: 106kbps/212kbps/424kbps. The communication mode can be divided into active mode and passive mode. The active mode means that both the initiator and the target can generate RFfield with their own power supply, while in the passive mode, the initiator supplies its own power to generate RFfield; and the target uses a full-wave rectifier circuit to convert the energy of the initiator's RFfield into DC to supply its own power. It is worth mentioning that in order to meet the power saving requirements in the passive mode, load modulation is adopted, and this modulation method can achieve the effect of power saving.

In terms of usage, NFC is usually used during peak periods, and in order to avoid data link errors caused by different initiators or targets communicating at the same time, NFC adopts a mechanism called listenbeforetalk. This mechanism allows the initiator device to detect the external magnetic field strength before sending an inquiry signal to determine whether there are other devices communicating. The implementation of this mechanism is called RFCollisionAvoidance (RFCA). Its action behavior is to detect the external magnetic field every time the initiator sends an inquiry signal. When the magnetic field strength exceeds the threshold strength (Hthreshold=0.1875A/m), the inquiry will be stopped until the external strength drops below the threshold value. If it is below the critical value, the inquiry command will be issued. The detection time is TIDT+nTRFW, and n is a probability sampling of 0 to 3: TRFW=512/fc (RFwaitingtime), TIDT>4096/fc (initialdelaytime). When the initiator does not detect a magnetic field exceeding the threshold strength within TIDT+nTRFW, it will first transmit the unmodulated RFfield of TIRFG and then send an inquiry signal, where TIRFG must be greater than 5ms.

NFC specification changes

Here we will describe NFCRFinterface. First of all, we will introduce the two standards EMCA340 and EMCA356. EMCA340 describes the NFC related protocols. Here we will introduce data sealing for discussion. NFC data subpacketization is divided into two types, one is 106kpbs and the other is 212/424kbps.

Since NFC106kbps is 100% ASK modulation, the entire High/Low signal packet structure has a very detailed definition. Several parameters include the time from 100% to 5%Am (t1), the duration of 5%Am (t2), and the time from 5%Am to 60%Am (t4), which is the range of overshoot. 212/424kbps has a modulation rate of 8% to 30%.

RF Test Kit

1. Calibration coil: The function of the coil is to verify whether the signal emitted by the object under test is of the correct strength and modulation during the test. This coil is a simple antenna structure. Of course, EMCA also specifies all the dimensions in detail. The value measured by this coil is 0.32V (RMS) = 1A/m (magnetic field strength).
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2. Field Generating Antenna: The kit is used to transmit the magnetic field. The figure also includes a set of antenna matching circuits.

3. Sensecoil: The sensecoil is used to measure the magnetic field strength and modulation of the object under test.

4. Reference device: reference device is used to test the standard parts of DUT.

RF Test Procedure

1. Initiate device power test:

This test item tests whether the magnetic field strength supplied by the initiator can provide sufficient operating power to the target.

Adjust the signal from the generating antenna to transmit, and the strength measured by the calibration coil on the right is Hmax (7.5A/m). At this time, adjust C2 with the reference device to make the line resonance point at 19MHz (this part is not detailed in the specification for adjusting to 19MHz. It is inferred that if 19MHz can achieve 3V output, then when the target is 13.56MHz, its voltage will definitely exceed 3V, so this should be the lower limit). Place it at the DUT position and adjust line R2 so that the voltage value obtained by C3 is 3V. At this time, the reference device is completed. Then use this card to measure the initiator device. Put this card in the over-operation range marked by the initiator device. The voltage value (C3) measured at any position in this over-operation range cannot exceed 3V. As for the Hmin test, it is roughly the same as the max test, except that the reference device resonance frequency is adjusted to 13.56MHz and the measured voltage value must exceed 3V.

2. Load modulation test of target object:

(1) Passive mode

●106kbps: This test verifies that the target object can modulate the waveform correctly. First, place the calibration coil at the lower outer edge to confirm that the waveform and strength emitted by the generating antenna are correct. Then place the target object to be tested at the upper outer edge and edit a SENS_REQ waveform defined by ECMA340 to be emitted by the generating antenna. The target object to be tested will then send back a SENS_RES signal. In this way, the signal can be measured through two senseoils. This measurement architecture uses the voltage difference between the two senseoils for calculation because the load modulation signal transmitted back is weak. Due to compatibility issues, MiFARE continues to use subcarrier modulation as the passive target's return signal at 106kbps, so the measurement points should be at fc+fs and fc-fs (fc=13.56MHz, fs=fc/16).

●212/424kbps: The high-speed modulation signal measurement method is very similar to 106kbps, except that the measurement capture position is changed to fc, because subcarrier modulation is not used in these two transmission speeds.

(2) Active mode

The active mode test is not much different from the passive mode test. Since it is the active mode, the target object's RFfield emission time, the command issuance time, etc. are also tested.

3. Load modulation test of the initiating device: This test is to verify the modulation mechanism of the initiating device, which can be divided into active mode transmission and passive mode reception.

(1) Active mode transmission: Place the calibration coil at any position defined by the initiator, and the measured waveform must comply with the specifications set by ECMA340.

(2) Passive mode reception: This is to test whether the initiator can correctly receive the signal sent back by the passive target. Using the reference device made by the load modulation test circuit in Figure 7-2, first calibrate the relationship between the corresponding C3 voltage and distance according to the structure in Figure 8, and then measure the DUT of the initiator with this card to test the reception of the modulated signal sent by the reference device at the DUT. Only some of the test items can be discussed here, and please refer to ECMA for detailed tests.

NFC system and chip development

Currently, the most active NFC manufacturers in the market include Philips, NOKIA, Sony and Samsung. These manufacturers all have products on the market, including Philips PN511/PN531 and NOKIA3220. RFMD also plans to release two single chips with NFC functions, RF4100 and RF4113. RF4100 is a system chip integrating Bluetooth and NFC, with a highly integrated mobile phone application interface, while RF4113 is an NFC system chip. The size of RF4100 is 5mm×5mm BGA and 3.7mm×3.7mm WLCSP. It is expected that NFC will usher in a new era for mobile phone applications.