Application of the hottest zero if receiving techn

The application of zero if receiving technology in RFID design

RFID technology

RFID (radio frequency identification) technology is an automatic identification technology that has developed rapidly in recent years. The distance of RFID can range from tens of centimeters to several meters, which is determined by the working frequency of the system and the type of tag (active and passive); According to the reading and writing methods of electronic labels, they are divided into read-only labels and readable and writable labels; For a single or multiple writable label, it depends on its memory size. You can input several or even thousands of bytes of information; At the same time, electronic labels also have high confidentiality. The application fields of RFID technology mainly include: material tracking, vehicle and shelf identification and other occasions requiring non-contact data acquisition and exchange, especially those requiring frequent changes in data content

uhf RFID reader refers to the RFID reading system working in the UHF frequency band. The internationally used UHF frequency band is the common frequency band of ISM (industry, science, medicine), such as 865_ 868MHz、902_ 928mhz and other frequency bands are applicable to different countries or regions. The former is the frequency band used in Europe, while the latter is mostly used in the United States, Canada and other countries. The basic characteristics of the high-frequency system are that the cost of the electronic tag and the card reader is high, the amount of data stored in the tag is large, the reading distance is long, the performance of adapting to the high-speed movement of objects is good, the shape is generally card shaped, and the reading antenna and the electronic tag antenna have strong directionality

at present, there are many international standards on RFID system, which are not unified. The main standards include iso/iec 15693 (13.56MHz), iso/iec 18000, and iso/iec 18000 Part 6 stipulates the air interface protocol of UHF band RFID. There are other standards organizations, such as American standards organizations (auto ID, global EPC) and Japanese standards organizations

China is also developing Chinese standards that meet the needs of its own market

typical RFID system composition and working principle

typical RFID system composition

this paper introduces the design principle of UHF Passive card reader and the system composition of typical RFID. As shown in Figure 1, the composition of RFID system generally includes at least two parts:

(1) electronic tag

(2) card reader

among them, electronic tag is often referred to as tag, RF card, tag and transponder, and card reader is also often referred to as card reader, reader and interleaver. Of course, a complete RFID system must also include the main PC, card reader antenna, interface cable, terminal monitoring software, etc. sometimes an RFID system also needs to be connected to the Internet to obtain remote data information or connect to multiple RFID systems

Figure 1 a typical RFID system

electronic tags generally store electronic data in the agreed format. In practical applications, electronic tags are attached to the surface of the object to be identified. The card reader can read and recognize the electronic data stored in the electronic tag without contact, so as to achieve the purpose of automatic identification of objects. Further through the computer and network to achieve the object recognition information collection, processing and remote transmission and other management functions

electronic labels are extremely small (<1mm2 = chip is fixed with an antenna of moderate size. Generally, the tag antenna is a frequency limited device. Considering the requirements of working frequency band and gain, the antenna cannot be made very small. The tag antenna can be a single antenna structure or a multi antenna, that is, a polarization isolation structure. The card reader also includes at least one antenna to communicate with the tag. The antenna of the card reader is generally not limited in size, and its polarization mode is also based on the requirements It is designed for different occasions. If the antenna of the card reader is in the form of circular polarization, there is no requirement for the installation method of the label

rfid system working principle

at present, most UHF RFID electronic tags in the world are passive tags. For passive electronic tags, the necessary power supply is obtained by detecting the RF signal transmitted by the card reader antenna. When an object with an electronic tag enters the effective coverage area of the card reader antenna, because the working frequency of the tag antenna is the same as that of the card reader antenna, the tag antenna receives a RF signal with sufficient power, and the charge pump circuit at the front end of the tag chip begins to work. It converts this part of the signal received by the antenna into a DC voltage for use by the circuit at the back end

the antenna of the card reader transmits am wave. Part of the AM signal received by the tag antenna enters the detection circuit (charge pump), and the other part enters the demodulation circuit. The demodulation circuit outputs the envelope signal of AM wave, including clock correction, synchronization and other instructions sent by the card reader to the tag. After analyzing and executing various instructions of the card reader, the tag modulates the unique ID information stored in ROM to the transmission signal of the card reader. The most commonly used modulation technology is backscatter modulation. The modulated signal backscattered by the tag is very weak. It is received by the antenna of the card reader and sent to the receiving and demodulation circuit, and then processed by DSP to obtain the correct tag ID information

uhf RFID card reader design principle

the system works in half duplex mode, following the ITF (card reader speech priority) principle, that is, whether the tag needs to send back signals is based on whether the command sent by the card reader is received and correctly demodulated, that is, when the system starts to work, the card reader first sends a series of tag reading instructions, and when the tag enters the effective coverage area of the card reader antenna, it will be activated by electromagnetic fields, Begin to demodulate the instruction of the card reader. Only after the correct reading instruction is obtained, the tag will send its own ID information and other data back to the card reader through the backscatter modulation

this paper introduces the application of multi-channel zero intermediate frequency detection technology, focusing on the introduction of receiving circuit, and does not elaborate on the generation of transmitted signal (power source) compared with glass fiber composite, transmission modulation circuit, DSP signal processing, control circuit, etc

the schematic diagram of the successfully developed 900MHz RFID card reader system is shown in Figure 2. The synthesizer generates the 902~928mhz frequency signal required for the operation of the system, which is divided into two channels after appropriate buffer amplification. One enters the modulation and transmission circuit, and the other enters the receiving circuit as the local oscillator signal, while the tag echo signal from the antenna also enters the receiving circuit, and acts on the multi-channel zero IF demodulation circuit together with the local oscillator signal

figure 2schematic diagram of UHF RFID card reader system

mcu controls the frequency synthesizer to generate the RF signal required by the system. The RF signal is divided into two channels and sent to the modulator&pa circuit and the receiver circuit respectively. The transmission data sent from MCU is modulated into RF signal to become am amplitude modulation wave, which is radiated by the transmitting antenna (ANT). When the tag enters the effective identification area of the card reader antenna, it starts to communicate with the card reader wirelessly. The tag back signal is very weak, which is received by the card reader receiving antenna and sent to the receiving circuit. Finally, the data information sent back by the tag is demodulated and sent back to MCU. MCU is connected to the host computer through serial port or Ethernet interface, and the card reader can be operated through control software, such as card reading, card writing and other commands

zero if technology application

when selecting the receiver circuit scheme, the design complexity, cost, power consumption and so on need to be considered first. The two common receiver design principles include superheterodyne and homodyne. Superheterodyne receiver is not only complex in circuit, but also very expensive in cost. In contrast, homodyne receiver (also known as zero IF receiver) can directly obtain demodulated signal (i.e. baseband signal) with only one level of same frequency mixer, which not only greatly reduces the cost, but also has very simple structure and convenient debugging and testing

Figure 3 channel zero if receiving circuit

the design principle of UHF RFID card reader receiving circuit is shown in Figure 3, including 4-channel zero if receiving circuit. Among them, 900MHz phase-shifting circuit is realized by using discrete component inductance and capacitance, which greatly reduces the area of PCB layout compared with the traditional microstrip phase-shifting circuit

a is continuous wave, using VLA (T) =acos ω 0t means that a is the signal amplitude. After three-stage phase shift, the local oscillator signal forms the following signals at points B, C and D: point B VLB (T) =acos（ ω 0t+ π/4), point C VLC (T) =acos（ ω 0t+2 π/4), point D VLD (T) =acos（ ω 0t+3π/4)。

the tag echo signal received by the antenna is am wave, assuming that the received signal at point D is VRD (T) =b (T) cos（ ω 0t+ φ)， Where B (T) is the data information of the tag, which is generally unipolar binary data, φ It refers to the phase difference between the transmitted and received signals, which is related to the distance between the antenna and the tag

after three-stage phase shift, the received signal forms the following signals at points c, B, and a: point C VRC (T) =b (T) cos（ ω 0t+ φ+ π/4), point B VRB (T) =b (T) cos（ ω 0t+ φ+ 2 π/4), point a VRA (T) =b (T) cos（ ω 0t+ φ+ 3π/4)。

for each diode mixing circuit, the input signal includes local oscillator and received signal, and the output signal is the mixing result of two input signals, such as ACOS at A'point ω 0tx B(t)cos( ω 0t+ φ+ 3π/4)=AB(t)/2cos(2 ω 0t+ φ+ 3π/4)+ AB(t)/2cos( φ+ 3π/4)。

each signal passes through a low-pass filter to convert the high-frequency component 2 ω 0t filter it out and get the tag echo signal B (T):

a''point AB (T)/2cos（ φ+ 3π/4)； B''point AB (T)/2cos（ φ+ π/4)； C''point AB (T)/2cos（ φ- π/4)； D''point AB (T)/2cos（ φ- 3π/4)。

it is not difficult to see that ab (T)/2cos（ φ+ 3 π/4) and ab (T)/2cos still need a long time of experimental testing to ensure their effectiveness and stability（ φ- π/4) phase difference π, ab (T)/2cos（ φ+ π/4) and ab (T)/2cos（ φ- 3 π/4) phase difference π enters into diff1 and diff2 of differential amplifier respectively. The output signals of the two differential amplifiers are:

point I: G1 (AB (T)/2cos（ φ+ 3π/4)- AB(t)/2cos( φ- π/4))= -G1 AB(t) sin( φ+ π/4)= G1 AB(t) cos( φ+ 3π/4)；

Q point: G2 (AB (T)/2cos（ φ+ π/4)- AB(t)/2cos( φ- 3π/4))= -G2 AB(t) sin( φ- π/4)= G2 AB(t) cos( φ+ π/4)；

where G1 and G2 are the gains of diff1 and diff2 differential amplifiers respectively. Obviously, the output signals of the two differential amplifiers only follow φ Relevant, and orthogonal. After amplification and comparison, the IQ signal at TTL level is finally obtained and sent to DSP for processing

i-data and q-6. After the test piece is broken, the data is an orthogonal signal, which is only related to the distance between the card reader antenna and the label (the phase difference between the transmitted and received signals), that is, when the label is located in the coverage area of the card reader antenna, I-data and q-data are uniquely determined at each determined time, and because the orthogonality between I-data and q-data determines that these two signals cannot be zero at the same time, that is to say, There will always be a non-zero signal (I-data or q-data) sent to DSP, which ensures that DSP can always receive an effective tag back signal

conclusion

this paper adopts the 4-channel zero intermediate frequency receiving technology, which greatly simplifies the design of uh fast and slow f RFID reader, with low cost and high cost performance compared with similar products. The system design meets the requirements of FCC electromagnetic standard, that is, the output RF power is less than 1W, the identification of electronic labels is stable, and the maximum identification distance is 6 meters. (end)