Radio Frequency (RF) is the abbreviation of Radio Frequency, which refers to the electromagnetic frequency that can radiate into space. The frequency range is from 300kHz to 300GHz.RF is RF current, which is a high-frequency AC changeelectromagnetic waveShort name of.Change less than 1000 times per secondalternating currentIt is called low-frequency current, and it is called low-frequency current if it is greater than 10000 timeshigh-frequency currentRF is such a high-frequency current.RF (300K-300G) is the higher frequency band of high frequency (more than 10K), and microwave band (300M-300G) is the higher frequency band of RF.
stayelectronicsIn theory, when the current flows through the conductormagnetic field；Alternating current will form around the conductor when it passes through the conductorelectromagnetic field, called electromagnetic wave.When the electromagnetic wave frequency is lower than 100kHz, the electromagnetic wave will be absorbed by the ground and cannot form effective transmission. However, when the electromagnetic wave frequency is higher than 100kHz, the electromagnetic wave can spread in the air andatmospherePeripheralthe ionosphereReflect to form long-distance transmission capability.We put the high frequency with long-distance transmission capabilityelectromagnetic waveIt is called radio frequency.RF technology is widely used in the field of wireless communication，Cable TV systemIs to adoptRF transmissionMode.[1]
The most basic RFID system consists of three parts:
1. Tag (radio frequency card): consists of coupling elements and chips. The tag contains a built-in antenna for communication with the radio frequency antenna;
2. Reader: device that reads (can also be written into the read-write card) label information;
3. Antenna: transmit RF signals between tags and readers.Some systems also connect with the external computer (host computer system) through the RS232 or RS485 interface of the reader for data exchange.
RF classification application
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The working frequencies of RFID products are defined as different products that meet different standards in the frequency range of low frequency, high frequency and very high frequency, and RFID products in different frequency bands will have different characteristics.Among them, there are passive and active sensors.[2]
Low frequency
In fact, RFID technology was first widely used and promoted in low frequency (from 125kHz to 134kHz).This frequency works mainly through inductive coupling, that is, there is transformer coupling between the reader writer coil and the inductor coil. The voltage induced in the inductor antenna by the alternating field of the reader writer is rectified and can be used as power supply voltage.The magnetic field region can be well defined, but the field strength drops too fast.[2]
characteristic:
The general working frequency of the inductor working at low frequency ranges from 120kHz to 134kHz, and the working frequency of TI is 134.2kHz.The wavelength of this frequency band is about 2500m;
1. In addition to the influence of metal materials, generally low frequency can pass through any material without reducing its reading distance.
2. There are no special licensing restrictions for readers and writers working at low frequencies in the world.
3. Low frequency products have different packaging forms.Good packaging is too expensive, but it has a service life of more than 10 years.
4. Although the magnetic field area of this frequency drops rapidly, it can produce relatively uniform read and write areas.
5. Compared with RFID products in other bands, the data transmission rate in this band is relatively slow.
6. The price of the sensor is relatively expensive compared with other bands.
Main applications:
Management system of animal husbandry;Application of car anti-theft and keyless door opening system;Application of marathon running system;Automatic parking fee collection and vehicle management system;Application of automatic refueling system;Application of hotel door lock system;Access control and security management system.
high frequency
High frequency (working frequency is 13.56MHz) inductors at this frequency no longer need coils for winding, and antennas can be made by etching or printing.Inductors generally work through load modulation.That is to say, the voltage on the antenna of the reader writer is changed by the connection and disconnection of the load resistance on the sensor, so that the amplitude modulation of the antenna voltage is realized by using a remote sensor.If people control the connection and disconnection of load voltage through data, then these data can be transmitted from the sensor to the reader writer.
characteristic:
1. The operating frequency is 13.56MHz, and the wavelength of this frequency is about 22m;
2. Except for metal materials, the wavelength of this frequency can pass through most materials, but it often reduces the reading distance.The sensor needs to be some distance away from the metal;
3. This frequency band is recognized worldwide without special restrictions;
4. The inductor is generally in the form of electronic label;
5. Although the magnetic field area of this frequency drops rapidly, it can produce relatively uniform reading and writing areas;
6. The system has anti-collision characteristics and can read multiple electronic labels at the same time;
7. Some data information can be written into the tag;
8. The data transmission rate is faster than low frequency, and the price is not very expensive.
Main applications:
Application of library management system;Management and application of liquefied gas cylinders;Management and application of garment production line and logistics system;Three meter pre charging system;Management and application of hotel door locks;Large conference personnel access system;Fixed assets management system;Management and application of pharmaceutical logistics system;Management of intelligent shelves.
VHF
VHF (operating frequency between 860MHz and 960MHz) system transmits energy through electric field.The energy of the electric field does not drop very quickly, but the read area is not well defined.The reading distance of this frequency band is far, and the passive range can reach about 10m.It is mainly realized by capacitive coupling.
characteristic:
1. In this frequency band, the global definition is not very same - the frequency defined in Europe and some Asia is 868MHz, the frequency band defined in North America is 902MHz to 905MHz, and the frequency band recommended in Japan is 950MHz to 956MHz.The wavelength of this frequency band is about 30cm.
2. Unified definition of power transmission in this frequency band (4W in the United States, 500mW in Europe).
3. The VHF radio waves cannot pass through many materials, especially water, dust, fog and other suspended particles.Compared with high-frequency electronic tags, electronic tags in this frequency band do not need to be separated from metal.
4. The antenna of the electronic tag is generally long and tag shaped.The antenna has linear and circular polarization designs to meet the needs of different applications.
5. This frequency band has a good reading distance, but it is difficult to define the reading area.
6. It has a high data transmission rate and can read a large number of electronic tags in a short time.
Main applications:
Management and application in the supply chain;Management and application of production line automation;Management and application of air parcel;Container management and application;Management and application of railway parcels;Application of logistics management system;Application of large-scale personnel access management.
Active RFID technology
Active RFID technology (2.45GHz, 5.8GHz) Active RFID has the characteristics of low transmission power, long communication distance, large amount of data transmission, high reliability and good compatibility. Compared with passive RFID, it has obvious advantages in technology.It is widely used in highway toll collection, port freight management, personnel positioning management and other applications.However, the use of this frequency band has strong directionality, and if there are metal objects in the receiving area, the metal objects will refract and reflect the RF of this frequency band, thus affecting the signal reading and writing of the RF receiver.[2]
working principle
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The basic work flow of the system is: the reader sends a certain frequency of RF signal through the transmitting antenna. When the RF card enters the working area of the transmitting antenna, it generates induced current, and the RF card is activated to obtain energy;The RF card transmits its own code and other information through the built-in sending antenna of the card;The system receiving antenna receives the carrier signal sent from the RF card, which is transmitted to the reader through the antenna regulator. The reader demodulates and decodes the received signal and then sends it to the background main system for related processing;The main system judges the validity of the card according to logical operation, makes corresponding processing and control for different settings, and sends command signals to control the action of the actuator.
In terms of coupling mode (inductance electromagnetic), communication flow (FDX, HDX, SEQ), data transmission method from RF card to reader (load modulation, backscattering, high-order harmonics), frequency range, etc., different non-contact transmission methods have fundamental differences, but all readers are very similar in functional principle and design structure,All readers can be simplified into two basic modules: high-frequency interface and control unit.The high-frequency interface comprises a transmitter and a receiver, and its functions include: generating high-frequency transmission power to start the RF card and provide energy;Modulate the transmission signal to transmit data to the RF card;Receive and demodulate the high-frequency signal from the RF card.DifferentRFID systemThere are some differences in the design of high frequency interface.
The functions of the control unit of the reader include: communicating with the application system software and executing the commands sent by the application system software;Control the communication process with the RF card (master slave principle);Signal encoding and decoding.For some special systems, there are additional functions such as implementing anti-collision algorithms, encrypting and decrypting the data to be transmitted between the RF card and the reader, and performing authentication between the RF card and the reader.
Radio frequency identification systemThe read/write distance of is a key parameter.Long distance RFID systems are expensive, so it is important to find ways to improve their reading and writing distance.The factors that affect the read-write distance of the RF card include the operating frequency of the antenna, the RF output power of the reader, the receiving sensitivity of the reader, the power consumption of the RF card, the Q value of the antenna and the resonant circuit, the antenna direction, the coupling degree between the reader and the RF card, and the energy obtained by the RF card itself and the energy for sending information.The read distance and write distance of most systems are different, and the write distance is about 40%~80% of the read distance.
Calculation unit
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Absolute power
Introduction to RF common calculation units
Various common RF computing units are one of the necessary basic knowledge for in-depth understanding of RF concepts;DB representation of absolute power[3]RF signalAbsolute power commondBm、dBWIndicates that its conversion relationship with mW and W is as follows:
For example, when the signal power is xW and expressed in dBm, its size is:
For example, 1W equals 30dBm and 0dBW.
1mW is equal to 0dBm.
Relative power
RF transceiver core circuit
The dB of the relative power represents the relative power of the RF signal in two forms: dB and dBc. The difference is that:
DB is the logarithmic representation of the ratio of any two powers;DBc is the logarithmic representation of the ratio of the output power of a certain frequency point to the output power of the carrier frequency.
For example: 30dBm - 0dBm=30dB
Antenna gain
Antenna gain is generally expressed by dBi or dBd.DBi refers to the power of antenna relative to non directional antennaenergy density The ratio, dBd refers to the ratio of the power energy density relative to the half wave oscillator Dipole. The gain of the half wave oscillator is 2.15dBi, so 0dBd=2.15dBi.
Other units
RF Principle
Resistance: an object or substance that blocks the passage of electric current, thus converting electric energy into heat energy or other forms of energy, unit:Ohms，Ω
Voltage: potential or potential difference, unit: V
Current: the number of charges passing through a certain point on the circuit in unit time, in amperes, A
Inductance: something around a coil, usually a wire, becauseelectromagnetic inductionThe coil can produce electromotive potential energy, unit: Henry, H
Capacitance: the potential maximum charge rate of a charged insulating conductive object, in Farad, F
application area
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RF communication
ThroughoutRF communicationIt mainly includes the following frequencies: transmission frequency, reception frequency, intermediate frequency and baseband frequency.Baseband frequency is used to modulate datasignal frequency 。The real transmission frequency is much higher than the baseband frequency. The general spectrum range is 500MHz to 38GHz, and data signals are transmitted at this high frequency.Generally speaking, the RF system has a very powerful function of transmitting modulated signals. Even in the case of interference signals and blocking signals [z2], the system can transmit with the highest quality and receive modulated signals with the best sensitivity.There are two kinds of blocking signals: in band blocking signals and out of band blocking signals.Out of band blocking signals refer to those distributed inSignal spectrumOther irrelevant signals, such as data signals generated by other wireless transmission technologies.In band blocking signals are distributed within the signal spectrum of interest, such as data signals generated by the same wireless transmission technology at other terminals.For wireless communication, in order to successfully realize the RF reception function, it is necessary to filter out these two blocking signals.Intermediate frequency is often used as the transition between transmission/reception frequency and baseband frequency, and this transmission mode is the basis of superheterodyne structure.In general, out of band blocking signals can be filtered by the antenna's own filter.The presence of IF gives us the opportunity to filter out the blocking signal in the band before the signal is mixed to the baseband frequency and digital processing is performed.On the other hand, at the transmitter, IF is often used to filter out all possible pseudo data andNoise。
Another implementation method using superheterodyne structure is to use IF sampling to reduce the number of devices on the signal chain.This method chooses to sample the signal at the intermediate frequency instead of mixing the signal to the baseband before sampling.In the first superheterodyne structure, the conversion process from IF to baseband requires the following devices: local phase-locked loop, intelligent demodulator (mixer) and bidirectional ADC (analog digital converter).If you choose to sample at intermediate frequency, these three devices can be replaced by a high-performance ADC.This can not only reduce the complexity of the signal chain, but also improve the quality of signal demodulation.
However, if a high-quality intelligent demodulator is used in the downlink baseband converter, a very good communication effect can also be obtained.If the leakage of the local PLL and RF devices is small enough, the baseband DC offset can be minimized.In addition, the demodulator'sphaseThe separation function can achieve very accurate 90 degree phase separation, which will ensure that the value of the error vector will not deteriorate or only deteriorate a little when the signal is demodulated.Finally, if we are usingIntelligent demodulatorAt the same time, use aphase noise The phase-locked loop (PLL) of will ensure the low noise of baseband output signal, and therefore obtain a good bit error rate (BER).
Because ADC needs to work at higher and higher frequencies, the power consumption of IF sampling structure becomes higher and higher than that of the first superheterodyne structure, and therefore becomes more and more expensive. This is the main disadvantage of IF sampling structure.For this reason, RF structures based on IF sampling are often more suitable for relatively low or intermediate frequency applications, after all, these frequency bands have little impact on cost.But with the development of technology, especiallyCMOS processWith the introduction of, the price of integrated high-performance devices and circuits is getting lower and lower. In the near future, the IF sampling structure will no longer be an expensive choice.
The third structure used in RF communication directly mixes baseband signals and RF signals in the same process as the direct conversion structure, which makes the signal link of this structure the simplest and requires the least components.Different from the other two structures, it will not need intermediate frequency processing and soundsurface wave(SAW) filter.
The main advantages of direct conversion structure are: low price, miniaturization, low power consumption, and no IF conversion related devices.These advantages make this structure very suitable for low power consumption, portable terminal applications.However, the use of some high-performance devices has opened the door for direct conversion structure applications in the high-end market.In fact, it is the use of these high-performance devices that makes the direct conversion structure receive more and more attention.
Since there is no IF processing unit in the direct conversion structure, the power of the in band blocking signal will be directly transferred to the mixer and the analog-to-digital converter (if the signal link contains an analog-to-digital converter).The mixer with low noise will ensure that weak signals will not be submerged by noise and blocking signals.In addition, because the mixer has high output swing and low distortion, the blocking signal will neither drive the whole system nor modulate to our needscarrier signalOn.
For baseband superheterodyne receivers, if there is a leakage path between the local PLL and the RF input, DC offset will certainly occur.For andGlobal Mobile Communication SystemFor some RF applications that support frequency hopping, frequency hopping will lead to the change of local PLL leakage, and eventually lead to the jump of DC offset of the whole system.If it is to be corrected, a DC offset compensation loop must be introduced into the system.However, in those applications that do not require frequency hopping, the leakage of the local PLL is constant, so the compensation of dynamic DC offset is of little significance.
At the transmission end, because the in band noise and distortion cannot be effectively reduced, the RF with direct conversion structure is adoptedtransmitterIt must be composed of components with large dynamic range.
In the related applications of base stations, the direct conversion structure is particularly promising because the area and channel density should be taken into account.From the perspective of the BTS, the in band blocking signal does not exist (that is, the BTS will process the in band blocking signal itself). Therefore, even if the direct conversion structure lacks the function of filtering out the in band blocking signal, it is acceptable.
Of course, the choice of RF circuit structure should be determined by the market application.These factors guiding design include: time from design to product entering the market, cost, shape, functional indicators, flexibility, and whether it can support a variety of differentApplication modewait.How to select a suitable RF structure for a certain application is beyond the scope of this article.But it is clear that some RF device manufacturers can now provide various targeted services to help us design appropriate RF systems. In the whole process of structural design, they can even provide several experienced engineers to answer our questions.[4]
recognition system
RFID technologyIt can be divided into low frequency system and high frequency system according to different frequencies;According to whether batteries are installed in the electronic tags to power them, they can be divided into active systems and passive systems;The information injected from the electronic tag can be divided into three types: integrated circuit fixed mode, field wired rewriting mode and field wireless rewriting mode;According to the technical means of reading electronic tag data, it can be divided into three categories: broadcast transmission type, frequency multiplication type and reflection modulation type.[4]
1. Low frequency system generally refers to its operating frequency less than 30MHz. Typical operating frequencies include 125KHz, 225KHz, 13.56MHz, etcRFID systemGenerally, they are supported by corresponding international standards.Its basic characteristics are that the cost of electronic labels is low, the amount of data stored in the labels is small, and the reading distance is short (passive, typical reading distance is 10cm). The electronic labels have various shapes (card, ring, button, pen), and can be readAntenna directivityNot strong, etc.
2. High frequency system generally refers to its operating frequency greater than 400MHz. Typical operating frequency bands include 915MHz, 2450MHz, 5800MHz, etc.High frequency systems are also supported by numerous international standards in these bands.The basic characteristics of the high-frequency system are that the cost of electronic tags and readers is high, the amount of data stored in tags is large, the reading distance is long (up to several meters to ten meters), the performance of adapting to high-speed movement of objects is good, the shape is generally card shaped, and the reading antenna and electronic tag antenna have strong directionality.
3. The battery is installed in the active electronic tag, which generally has a long reading distance. The disadvantage is that the battery life is limited (3~10 years);passive tag There is no battery inside. After receiving the microwave signal sent by the reader (readout device), it converts part of the microwave energy intoDCFor self work, maintenance free is generally available.Compared with the active system, the passive system can read distance and adapt objectsMovement speedThere are slight restrictions in this regard.
RF card classification
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Many companies that produce RFID products adopt their own standards, and there is no unified standard in the world.ISO18000。The most widely used standards are ISO14443 and ISO15693, both of which are composed of four parts: physical characteristics, RF power and signal interface, initialization and anti-collision, and transmission protocol.
According to different ways, RF cards are classified as follows:
1. It can be divided into active card and passive card according to power supply mode.Active means that there is a battery in the card to provide power, which has a long operating distance, but has limited life, large size, high cost, and is not suitable for working in harsh environments;There is no battery in the passive card. It uses the beam power supply technology to convert the received RF energy into DC power supply to supply power for the circuit in the card. Its operating distance is shorter than that of the active card, but its service life is longer and the requirements for the working environment are not high.
2. Presscarrier frequency It is divided into low-frequency RF card, intermediate frequency RF card and high-frequency RF card.There are mainly two kinds of low-frequency radio frequency cards: 125kHz and 134.2kHz. The frequency of intermediate frequency radio frequency cards is 13.56MHz, and the frequency of high-frequency radio frequency cards is 433MHz, 915MHz, 2.45GHz, 5.8GHz, etc.Low frequency system is mainly used in short distance and low-cost applications, such as most access control, campus card, animal supervision, cargo tracking, etc.IF system is used foraccess control And application systems that need to transmit large amounts of data;High frequency systems are used in situations where long read/write distance and high read/write speed are required. The antenna beam direction is narrow and the price is high. They are used in train monitoring, highway toll collection and other systems.[5]
3. It can be divided into active type and passive type according to different modulation modes.The active RF card uses its own RF energy to actively send data to the reader writer;The passive RF card transmits data in the modulation scattering mode, and it must use thecarrierThis kind of technology is suitable for use in access control or traffic applications, because the reader writer can ensure that only a certain range of RF cards are activated.In the case of obstacles, the energy of the reader writer must pass through the obstacles twice in the modulated scattering mode.However, the signal transmitted by the active RF card only passes through the obstacle once, so the active RF card is mainly used in applications with obstacles, with a longer distance (up to 30 meters).[5]
4. According to operating distance, it can be divided into close coupling card (operating distance is less than 1 cm), close coupling card (operating distance is less than 15 cm), loose coupling card (operating distance is about 1 m) and long distance card (operating distance is from 1 m to 10 m, or even more).
5. It is divided into read-only card, read-write card andCPUCard.[5]
Terminological knowledge
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1. Power/level (dBm): output capacity of the amplifier, generally expressed in w, mw, dBm.
2．gain(dB): the magnification, expressed in decibels (dB).
Note: dBm is the absolute power level expressed in decibels with 1mw as the reference value.Conversion formula:
Level (dBm)=10lgmw
5W → 10lg5000=37dBm
10W → 10lg10000=40dBm
20W → 10lg20000=43dBm
It is easy to see from the above that the level value increases by 3dB every time the power doubles
That is, dB=10lgA (A is the power magnification)
3．Insertion loss: attenuation increased when a device or component is connected to the transmission circuit, expressed in dB.
4．selectivity: Measure workfrequency bandInternal gain and out of band radiation suppression capability-The 3dB bandwidth is the bandwidth when the gain decreases by 3dB. The same is true for - 40dB and - 60dB.
5．Standing wave ratio(Return loss): ratio of antinode voltage to nodal voltage (VSWR) in traveling and standing wave state
Attachment: Standing wave ratio return loss comparison table:
Note: According to the calculation of return loss (RL), the standing wave ratio (SWR) formula is SWR=[10 ^ (RL/20)+1]/[10 ^ (RL/20) - 1]].
6．Third-order intermodulation: If there are two sine signalsω1 andωBecause of the nonlinear effect, many intermodulation components will be generated, of which 2ω1-ω2 and 2ω2-ω1 The two frequency components are called third-order intermodulation components, whose power P3 and signalω1 orωThe power ratio of 2 is called third-order intermodulation coefficient M3.
That is, M3=10lg P3/P1 (dBc)
7. Noise figure: generally defined as the ratio of input signal-to-noise ratio to output signal-to-noise ratio, which is converted into decibels in actual use.The unit is dB.
8. Coupling degree: power ratio between coupling port and input port, in dB.
9. Isolation: the ratio of the power of local oscillator or signal leaked to other ports to the original power, in dB.
10. Antenna gain (dB): refers to the ability of the antenna to concentrate the transmission power in a specified direction.Generally, the maximum radiation direction of the antennafield strengthE and IdealIsotropyThe uniform radiation field intensity E0 of the antenna is compared withpower density The multiple of increase is defined as gain.Ga=E2/ E02
11. AntennaPattern: refers to the range of electromagnetic wave radiated by antenna in free space.The width of pattern generally refers toMain lobe widthThat is, the included angle between two points when the maximum value drops by half.
Generally, the wider the pattern, the lower the gain;The narrower the pattern, the higher the gain.
12. Antennas front to rear ratio: refers to the ratio of maximum forward gain to maximum reverse gain, expressed in decibels.
13. Simplex: also known as single frequency simplex system, that is, the same frequency is used for receiving and transmitting. Since the same frequency is used for receiving and transmitting, the receiving and transmitting cannot be carried out at the same time, which is called simplex.
14. Duplex: also known as cross frequency duplex system, that is, two different frequencies are used for sending and receiving, so that either party can receive the other party's speech while sending.Simplex and duplex belong to the working mode of mobile communication.
15．amplifier: (amplifier) The circuit used to realize signal amplification.
16．wave filter: (filter) A component or device that suppresses unwanted frequency signals through useful frequency signals.
17. Attenuator: (attenuator) A type of attenuator with zero phase shift and frequency independent attenuation and characteristic impedance in a relatively wide frequency bandconstantConsisting of resistance elementsFour terminal networkIts main purpose is to adjust the signal size in the circuit and improve impedance matching.
Power divider: device for power distribution.There are two, three, four...... Power divider;There are three types of joints: N head (50 Ω), SMA head (50 Ω), and F head (75 Ω).
18. Coupler: a device that extracts part of the signal from the trunk channel.According to the coupling degree, it can be divided into 5, 10, 15, 20...... DB different specifications;frombase stationHigh power coupler (300W) can be used to extract signal, and its coupling degree can be selected from 30~65dB;Coupler joints are mostly N head.
19. Load: the terminal is in a certain circuit (such asamplifier）Or electrical output port, the element/device, component or device receiving electrical power is collectively referred to as load.The most basic requirements for the load are impedance matching and the power it can withstand.
20. Circulator: device that enables signal to be transmitted in one direction.
21. Adapter: a device for connecting different types of transmission lines.
22. Feeder: It is a transmission line that transmits high-frequency current.
23. Antenna:WaveguideThe energy of form is transformed intoelectromagnetic waveAnd emit it in the specified direction or reduce the electromagnetic wave from a certain direction to high-frequency current.
