Radio Frequency Identification Technology (3)-energy and Modulation

Non-contact and non-contact IC cards have two major characteristics compared with contact IC cards. Passive means that there is no power on the card, and no contact means that the read and write operations on the card do not need to be in contact with the reader. The non-contact smart card is also an IC card, and the IC on the card, that is, the IC must be powered when working. The card itself has no power and does not come into contact with the reader. Where does the power come from?

In fact, it is very simple to answer this question, that is, electromagnetic induction. The reader generates an electromagnetic field. The antenna on the card is an LC oscillating circuit, and the resonance frequency of this oscillating circuit is the same as that of the reader's electromagnetic field. When the card enters the reader's RF field, the oscillating circuit on the card starts to vibrate. The circuit oscillation means that there is electronic flow, and the electronic flow can be used as a diode to accumulate electrons, with the accumulation of electronics, voltage is formed and smart cards with voltage can work. This is generally the way cards get energy. If you don't want to do the cards yourself, you don't have to go into details. You just need to understand the principles. If you still don't understand it, think about the transformer. The original and secondary sides of the transformer are also insulated, but the energy can be transferred from the original side to the secondary side, the power obtained by RF cards is equivalent to the secondary edge of the transformer.

With energy, how do readers and cards communicate with each other? Since it is free of contact, it is only through radio transmission. Radio Transmission rarely directly sends signals. Generally, a carrier of a certain frequency is selected, that is, a sine wave. The sender modulated the useful signal on the carrier, and the receiver demodulated the received signal, remove the carrier to obtain useful signals. Regardless of television, broadcast, mobile communication, or RF Card, the radio communication principle is the same.

The carrier (sine wave) has three elements: amplitude, frequency, and phase. The corresponding useful signals also have three types of modulation for the carrier: Amplitude Modulation, frequency modulation, and phase modulation. Amplitude modulation is a useful signal for changing the carrier's amplitude record, FM is a useful signal for changing the carrier's frequency record, and phase modulation is a useful signal for changing the carrier's phase record. The amplitude modulation circuit is simple and vulnerable to interference. The structure of the FM and phase modulation is more complex, but the anti-interference capability is strong. A simple example is that the FM ratio of your radio is AM) sound nice. In addition, the amplitude modulation and frequency modulation require the carrier frequency to be much greater than the maximum frequency of useful signals, usually 10 times or more, while the phase modulation does not have this requirement. The carrier frequency can be higher than or equal to (2 BPSK) it is even less than (4 BPSK, 16 BPSK) the frequency of useful signals.

Based on the above three modulation features, RF Card communication distance is only a few meters, usually close to the card (PICC) maximum distance is 10 centimeters, when a relatively strong local RF field is formed within such a short distance, interference is almost unnecessary. There is no power on the RF Card, especially for cost considerations. Naturally, the simpler the circuit on the card, the better. Therefore, although the amplitude modulation is susceptible to interference, the circuit structure is simple and becomes the first choice for RF Card modulation.

Modulation has an indicator called modulation coefficient, that is, to measure the modulation of the carrier amplitude of a useful signal. The modulation coefficient is 0, which is equivalent to no modulation. If the modulation coefficient is 1, the amplitude of the carrier is adjusted to 0. Generally, the modulation coefficient is between 0.1 and 0.9. Obviously, because the RF card needs to obtain energy from the magnetic field, if the modulation coefficient is close to 1, it means that the magnetic field is closed and the time is short, and the power supply on the card will inevitably disappear after a long time, the basic working conditions of the card are none, but the modulation coefficient is strong in anti-interference ability and easy to demodulated. On the contrary, if the modulation coefficient is small, the card can obtain stable energy supply, but the anti-interference capability is weak and the demodulation is difficult.

In the process of information exchange between the reader and the card, the reader generates an RF field and modulated the RF field generated by the reader when sending data to the card. This is easy to understand. However, cards are passive, not only do they not produce RF fields, but they also need to obtain energy from the reader's RF field. How can they send data back to the reader through modulation RF fields? The RFID technology uses a method called load modulation. The principle is also very simple. For example, we all have this common sense. If the transformer's secondary side is suddenly connected to a large load, the original transformer side voltage will instantly decrease, after the load is removed, the original side voltage of the transformer will be restored immediately. As mentioned above, the RF Card is equivalent to the secondary side of the transformer, and there is also such a load in the card. When the load is connected, the oscillation amplitude of the RF field will be reduced, the load will be removed, and the amplitude of the RF field will be restored. The card uses load access to indicate the sent data. The reader detects the amplitude change of the RF field to know what data the card sends. This is the principle of Card delivery data.