WPI NXP PCF7991 tuning

1 . Introduction to PCF7991

As the “big brother” of the NXP PEPS architecture LF chip, PCF7991 (codenamed ABIC1) is designed to send data to the key (Transponer for short) in the 125KHz frequency band, combined with amplitude shift keying (ASK for short) technology; at the same time, it is also based on amplitude modulation/phase modulation (AM/PM).The principle of reading data from the key. As 图 1.1shown in Figure 1.1.

 

Figure 1.1 基站端与钥匙Data communication between the base station and the key LF

In NXP's introduction to PCF7991, it involves more IMMO communication methods, and in this chapter we will mainly explain the PKE function. In other words, 125KHz data transmission from the base station to the key will be the focus of this time.

Because the data transmission of the base station uses amplitude shift keying, this will involve amplitude modulation. In PCF7991, the modulation depth of amplitude shift keying is 100%, so it can be called a special amplitude shift keying-binary amplitude keying (OOK). And this modulation can be achieved through the control of the PCF7991 serial port by the main control chip. Specifically, it is to write PCF7991 Configuration Pages through the three serial ports of SCLK, DOUT, and DIN, then calculate the sampling phase, make general settings, and then enable transparent transmission through WRITE_TAG(_N). After that, the user can turn off and on the antenna drive by controlling the DIN level change, which completes the OOK modulation.

 

Second, the reason for tuning

Tuning in this part of PEPS LF is a very necessary part, and it is no exception for PCF7991. When the USER performs an operation similar to Figure 2.1 on this low-FREQUENCY chip, HE will definitely notice that the status bit ANTFAIL returned by GET_CONFIG_PAGE2 and GET_CONFIG_PAGE3 has been set. Part of the reason for this is that there is a problem with the choice of LF antenna, or more precisely, the antenna is detuned.

Figure 2.1 PCF7991 configuration process

Figure 2.2 shows the respective responses after the GET_CONFIG_STAGE command is sent. If the bit 4 of the PAGE2 and PAGE3 responses is 1, it means that the antenna may be detuned.

 

图 2.2 GET_CONFIG_PAGE 响应

Antenna detuning means that the low-frequency data will not be modulated onto the 125KHz carrier, which means that under normal conditions with the receiving antenna, the communication between the two parties may be affected. In addition, for PCF7991, antenna detuning can also cause antenna failure. This also results in the antenna current and the voltage of the Tap point cannot be in a normal working state, specifically, the current and voltage values are low, as 图 2.3shown in Figure 2.3.

 

Figure 2.3 In the case of detuning, the antenna current ( CH4 , green) is very low

The low current value can be understood as the limited power consumption of the base station's LF signal transmission, which will affect the communication distance of the PKE. If this problem needs to be solved, LF antenna tuning is very necessary.

 

Third, the tuning method

1. Tuning using a network analyzer

During the tuning process, the user can use a network analyzer to determine whether 125KHz is a resonance point. Not only that, the network analyzer can also measure the real Rant of the antenna impedance including Rrf and Rcopper. This parameter is mainly determined by 图 3.1the Ra in Figure 3.1, and it will determine the maximum current of the antenna..

 

Figure 3.1 包含Antenna circuit with EMI protection

During the debugging process, you can first disconnect this part of the circuit from the TX1, TX2, and RX pins on the chip side, and then connect the antenna circuit to the ends of TX1 and TX2 to the network analyzer (note that the antenna needs to be connected to form a complete loop), and then you only need to adjust La or Ca (Of course, it is recommended to determine the La first, and then adjust the Ca capacitance value) to ensure that the imaginary part of the 125KHz impedance is as 0 as possible, and the real part is Rant, as shown in Figure 3.2 .

 

Figure 3.2 The network is resistive at 125KHz,  Rant = 32.375Ω (the image is for illustration only)

2. In-system tuning

For antenna tuning, you can also use an oscilloscope. The user can place the probe at the Tap point, because within half the cycle of the sine wave, there will be a 5V-step formed by driving the switch. If the 5V-step is located outside the peak and trough, the antenna is detuned,As shown in Figure 3.3 ; on the contrary, it is proved that the antenna has been tuned, as shown in Figure 3.4 .

 

Figure 3.3 5V-step is not in the peaks and troughs ( CH2 , blue), the antenna is detuned

 

Figure 3.4 5V-step is located in the peak and trough of the wave ( CH1 , yellow), and the antenna is tuned

 

Fourth, precautions

In the process of antenna tuning, it is recommended to determine the inductance value of the antenna first. For most applications with a peak current of 200mA, it is recommended to choose 400uH~500uH, while 200uH~250uH can be used in Burst Mode.

After the inductance is determined, you can start to adjust Ca, and Ca and its series capacitor Cs together form a resonant capacitor. Cs can choose a larger capacitance value, and the accuracy requirements can not be too high. However, Ca needs to use high-precision, NP0 capacitors (excluding the interference of soldering iron welding and other conditions on the capacitance value). Since the capacitance value belongs to the nF level, in order to make the material selection more convenient, the Ca package is also as large as possible (above 0603).

五、 参考文献

• 1) f7991at.pdf
• 2) AN99075-V2.pdf
• 3) Circuit design guide.pdf