For too many times I've seen wrong usage of terms like VLF, ELF and similar. As a matter of fact if you go and look over the web in articles connected with VLF, you'll see that almost everyone uses different term for something that is ELF, or SLF or vice versa. In some article you will read that 4kHz is ELF, while the other article you'll read that 4kHz it is VLF. So where is this 4kHz anyway? And what is difference between ELF, ULF and VLF?

Each frequency range has a band designator and each range of frequencies behaves differently and performs different functions. The frequency spectrum is shared by civil, government, and military users of all nations according to International Telecommunications Union (ITU) radio regulations. For communications purposes, the usable frequency spectrum now extends from about 3Hz to about 300GHz. There are also some experiments at about 100THz where research on laser communications is taking place but we won't discuss this now. This range from 3Hz to 300GHz has been split into regions. The good thing is that once this range has been split it remained that way and became standard. And it is up to you if you want to accept this standard or not. Frequency band standard is described in International Telecommunications Union radio regulations. And it looks as follows.

Designation		Frequency	Wavelength
ELF	extremely low frequency	3Hz to 30Hz	100'000km to 10'000 km
SLF	superlow frequency	30Hz to 300Hz	10'000km to 1'000km
ULF	ultralow frequency	300Hz to 3000Hz	1'000km to 100km
VLF	very low frequency	3kHz to 30kHz	100km to 10km
LF	low frequency	30kHz to 300kHz	10km to 1km
MF	medium frequency	300kHz to 3000kHz	1km to 100m
HF	high frequency	3MHz to 30MHz	100m to 10m
VHF	very high frequency	30MHz to 300MHz	10m to 1m
UHF	ultrahigh frequency	300MHz to 3000MHz	1m to 10cm
SHF	superhigh frequency	3GHz to 30GHz	10cm to 1cm
EHF	extremely high frequency	30GHz to 300GHz	1cm to 1mm

However, to simplify things about VLF a good idea would be to use term audio frequency range. Imagine doing a project covering 20 Hz to 20 kHz, it would be annoying to write ELF/SLF/ULF/VLF all the time. But again it would not be correct if you just write VLF because there is much more in that range. Right? Instead, simply use term 'audio frequency range'. As I said in the beginning there are already dozens of articles with wrong frequency designations and a good thing would be to correct and minimize mistakes.

On some documents that describe electromagnetic spectrum you might see terms like LW, MW and SW. In a lot of occasions you might see them mixed in the same context as frequency range we mentioned above. For example here is a quote from one webpage: ''...extends thru LW, MW, HF and VHF.'' This is wrong! We talk apples and oranges here. LW is not LF, and MW is not MF, therefore you can't put them together in same context with HF and VHF.

LW, MW and SW are frequency designations of AM broadcast radio stations, and thats about it. They have nothing to do with ITU's band designations we mentioned in the table above. Some countries don't even have LW, so you should not mix LW, MW or SW with HF, VHF etc. unless you are talking about AM broadcast stations. To be exact MW and HF should never be mixed together in the the same context. Here is the frequency table for AM broadcast bands:

Designation	Name	Frequency
LW	long wave	153 - 279 kHz
MW	medium wave	531 - 1620 kHz
SW	short wave	2310 - 25820 kHz

source: http://www.vlf.it/frequency/bands.html

What is the best RFID frequency?
Every RFID frequency has advantages and disadvantages. Higher RFID frequencies, such as the UHF (800-1000 MHz) and microwave bands are easier to shield and reflect, and thus have more difficulty penetrating dielectric materials such as liquids or the human body; however, over short distances it is possible to use UHF or microwave RFID tags with loop antennas (similar to 13.56 MHz) which can overcome this problem (see next question). Higher frequencies can also be transmitted more easily over long distances with small antennas. RFID systems at lower frequencies can be made very inexpensively and have less problems with materials, but generally require larger antennas and higher RF power (several Watts or more) to operate at distances greater than 12 inches or so.

What is Near-Field UHF?
All antennas, regardless of the frequency, generate electromagnetic fields that have both magnetic and electric components. Part of the electromagnetic field produced by an antenna is a radiating field which will propagate over long distances covering many wavelengths. This is known as the 'far field.' Examples include broadcast radio station antennas, and also cell phone antennas. At distances near the antenna, however, there exist other electromagnetic field components that decay much more quickly with distance and, as a result, are only active near the antenna. Depending on the resonant properties of the antenna and its geometric design, it is possible to design antennas (such as loop antennas or coils) which have a strong magnetic near-field component. UHF tags are now comercially available with loop antennas that can couple to this near-field. Although these 'near-field' UHF tags can only be used near the antenna (within a half-wavelength or so = 50 cm for UHF) such tags are of interest for applications which require penetration through liquids or dielectrics. There are also other tag designs which contain a 'hybrid antenna' which can operate with both the near-field and far-field components produced by the reader antenna. With few exceptions, most lower-frequency RFID tags operating at 13.56 MHz and 125 KHz are all near-field RFID systems, with the reading range roughly limited by the diameter of the reader antenna.

How small can you make an RFID Tag?
Although the electronic chip used in most RFID tags can be smaller than a grain of sand, a tag requires an antenna. The size of an RFID tag is generaly contrained by this antenna design. At higher frequencies the antenna is usually designed to be some small fraction of a wavelength, such as quarter-wavelength (roughly 4 inches at 900 MHz and 1 inch at 2.4 GHz); if the antenna is made smaller, then the reading distance of the tag will be greatly reduced. If long reading distance is not necessary, it is also possible to use inductive coupling for the UHF tags as well (near-field UHF), which can enable resonably small (1 cm) tags with loop antennas. In lower-frequency RFID systems, such as 13.56 MHz or 125 KHz, inductive coupling is used almost exclusively, and requires antennas in the form of coils, however at these lower frequencies the coils require a large number of turns. Small antenna coils wound on ferrite (to increase magnetic flux and range) are commonly used for tagging animals; these tags are the size of a grain of rice. Smaller tags can be made by electroforming the antenna directly onto the silicon chip, but the reading range is necessarily small.

What determines the reading range of a passive RFID Tag?
It is important to understand that there are many types of RFID technologies and each type has its own set of factors which determine range. At lower frequencies (125 KHz, 13.56 MHz), the tag antenna is electromagnetically coupled to the reader antenna, so the reading range of the RFID tag is determined by the sensitivity of the reader, the transmit power of the reader, the Q-factor of the tag, the power consumption of the tag IC chip, the size of the reader antenna, and the ratio of the tag antenna size to the reader antenna size. A crude rule of thumb is to say that the reading range is limited to 1.5 times the diameter of the reader antenna. At higher frequencies, such as UHF or microwave, the tag communicates through backscatter modulation, so the reading range is primarily determined by the reader sensitivity, the reader power, the power consumption of the tag IC chip, the radar scattering cross section of the antenna, the efficiency of the antenna, and the gain of the reader antenna and tag antenna. Some of these factors are regulated by the local governments. Present-day commercial UHF RFID systems have a range of 5-8 meters in free space using US power regulations, which allow 1 Watt of RF power and 4 Watts radiated power (EIRP).

What is chipless RFID?
It is important to understand chipless RFID is not one single technology, but rather it is a category of RFID that does not require the use of an IC chip. Some chipless RFID tags are designed to function as sensors (e.g. temperature or humidity) but others are dedicated to ID-only. Because no chip is used, the ID or sensor information is encoded in the analog electromagnetic signal of the tag, such as the resonant freuqency or harmonics. There are dozens of different chipless RFID technologies (too many to discuss here) ranging from very low frequency magnetic materials (such as library book security systems) to HF, UHF and microwave resonant, harmonic or backscatter systems (such as those used in retail anti-theft systems). Time-domain chipless tags are now also available, which echo back pulses transmitted from the reader. Although chipless RFID tags have less functionality than chip-based tags, the chipless technologies are generally 10X-100X lower in cost and can function at extreme temperatures, high radiation levels and other environments where chip-based technologies cannot operate.

What is the lowest cost RFID Tag?
We must first define what is an RFID tag. If we define a tag as a chip with an antenna on a paper or plastic substrate (with no adhesive or topcoat or printing), then this is known as an "inlay." Simple 96-bit UHF-frequency RFID tag inlays can be purchased today (2006) at a cost of approximately 10 cents (US $) at quantities over 50,000. The antennas for lower-frequency tags are a bit more expensive since the electromagnetic penetration depth is longer whcih requires a thicker metal layer; also, most tags at lower frequencies require a coil antenna, which generally requires a 2-layer metal process a cross-over linkfor the coil. If more memory or functionality is required, such as encryption, then the cost of the RFID chip itself can be significantly higher. RFID tags used for payment cards, for example, can store several kilobits of memory and support encryption and JAVA applications. There also exists other forms of RFID, such as chipless RFID which do not use a chip or can use special printed inks. Other families of RFID technologies are emerging as well which will have other cost structures. Some of these are shown in the figure below.

For more information, please contact us directly.

source:http://www.tagsense.com/ingles/faq/faq.html

Automatic Identification

In recent years automatic identification procedures (Auto ID) have become very popular in many service industries, purchasing and distribution logistics, industry, manufacturing companies and material flow systems. Automatic identification procedures exist to provide information about people, animals, goods and products.

The omnipresent barcode labels that triggered a revolution in identification systems some considerable time ago, are being found to be inadequate in an increasing number of cases. Barcodes may be extremely cheap, but their stumbling block is their low storage capacity and the fact that they cannot be reprogrammed.

The technically optimal solution would be the storage of data in a silicon chip. The most common form of electronic data carrying device in use in everyday life is the chip card based upon a contact field (telephone chip card, bank cards). However, the mechanical contact used in the chip card is often impractical. A contactless transfer of data between the data carrying device and its reader is far more flexible. In the ideal case, the power required to operate the electronic data carrying device would also be transferred from the reader using contactless technology. Because of the procedures used for the transfer of power and data, contactless ID systems are called RFID systems (Radio Frequency Identification).

Radio Frequency Identification - RFID

An RFID system is always made up of two components:

the transponder, which is located on the object to be identified,the detector or reader , which, depending upon design and the technology used, may be a read or write/read device.

Picture: The reader and the transponder are the main components of every RFID system

A reader typically contains a high frequency module (transmitter and receiver), a control unit and a coupling element to the transponder. In addition, many readers are fitted with an additional interface (RS 232, RS 485, ...) to enable it to forward the data received to another system (PC, robot control system, ...).

The transponder, which represents the actual data carrying device of an RFID system, normally consists of a coupling element and an electronic microchip. When the transponder, which does not usually possess its own voltage supply (battery), is not within the response range of a reader it is totally passive. The transponder is only activated when it is within the response range of a reader. The power required to activate the transponder is supplied to the transponder through the coupling unit (contactless) as is the timing pulse and data.

Operating principles of RFID systems

There is a huge variety of different operating principles for RFID systems. The picture below provides a short survey of known operation principles (The numbers refer to the relating chapters in the book).

The most important principles - 'inductive coupling' and 'backscatter coupling' are described more detailed below.

Picture: The allocation of the different operating principles of RFID systems into the chapters of the English eddition.

Inductive Coupling (3.2.1)

An inductively coupled transponder comprises of an electronic data carrying device, usually a single microchip and a large area coil that functions as an antenna.

Picture: Iductive coupled RFID-System

Inductively coupled transponders are almost always operated passively. This means that all the energy needed for the operation of the microchip has to be provided by the reader. For this purpose, the reader's antenna coil generates a strong, high frequency electro-magnetic field, which penetrates the cross -section of the coil area and the area around the coil. Because the wavelength of the frequency range used (< 135 kHz: 2400 m, 13.56 MHz: 22.1 m) is several times greater than the distance between the reader's antenna and the transponder, the electro-magnetic field may be treated as a simple magnetic alternating field with regard to the distance between transponder and antenna (see the chapter "Physical Principles – Transition from Near Field to Far Field" (4.2.1.1.) for further details).

A small part of the emitted field penetrates the antenna coil of the transponder, which is some distance away from the coil of the reader. By induction, a voltage Ui is generated in the transponder's antenna coil. This voltage is rectified and serves as the power supply for the data carrying device (microchip). A capacitor C1 is connected in parallel with the reader's antenna coil, the capacitance of which is selected such that it combines with the coil inductance of the antenna coil to form a parallel resonant circuit, with a resonant frequency that corresponds with the transmission frequency of the reader. Very high currents are generated in the antenna coil of the reader by resonance step-up in the parallel resonant circuit, which can be used to generate the required field strengths for the operation of the remote transponder.

The antenna coil of the transponder and the capacitor C1 to form a resonant circuit tuned to the transmission frequency of the reader. The voltage U at the transponder coil reaches a maximum due to resonance step-up in the parallel resonant circuit.

Picture: Operation principle of an inductive coupled system

As described above, inductively coupled systems are based upon a transformer-type coupling between the primary coil in the reader and the secondary coil in the transponder. This is true when the distance between the coils does not exceed 0.16 l, so that the transponder is located in the near field of the transmitter antenna (for a more detailed definition of the near and far fields, please refer to the chapter "Physical Principles").

If a resonant transponder (i.e. the self-resonant frequency of the transponder corresponds with the transmission frequency of the reader) is placed within the magnetic alternating field of the reader's antenna, then this draws energy from the magnetic field. This additional power consumption can be measured as voltage drop at the internal resistance in the reader antennae through the supply current to the reader's antenna. The switching on and off of a load resistance at the transponder's antenna therefore effects voltage changes at the reader's antenna and thus has the effect of an amplitude modulation of the antenna voltage by the remote transponder. If the switching on and off of the load resistor is controlled by data, then this data can be transferred from the transponder to the reader. This type of data transfer is called load modulation.

To reclaim the data in the reader, the voltage measured at the reader's antenna is rectified. This represents the demodulation of an amplitude modulated signal. An example circuit is shown in the chapter "Reader – Low Cost Layout".

Picture: sample circuit of the power supply and load modulator in a transponder

Picture above: If the additional load resistor in the transponder is switched on and off at a very high elementary frequency fH, then two spectral lines are created at a distance of ±fH around the transmission frequency of the reader, and these can be easily detected (however fH must be less than fREADER). In the terminology of radio technology the new elementary frequency is called a subcarrier. Data transfer is by the ASK, FSK or PSK modulation of the subcarrier in time with the data flow. This represents an amplitude modulation of the subcarrier.

Backscatter Coupling (3.2.2)

We know from the field of RADAR technology that electromagnetic waves are reflected by objects with dimensions greater than around half the wavelength of the wave. The efficiency with which an object reflects electromagnetic waves is described by its reflection cross-section. Objects that are in resonance with the wave front that hits them, as is the case for antenna at the appropriate frequency for example, have a particularly large reflection cross-section.

Picture: Operation principle of a backscatter transponder

Power P1 is emitted from the reader's antenna, a small proportion of which (free space attenuation) reaches the transponder's antenna. The power P1' is supplied to the antenna connections as HF voltage and after rectification by the diodes D1 and D2 this can be used as turn on voltage for the deactivation or activation of the power saving "power-down" mode. The diodes used here are low barrier Schottky diodes, which have a particularly low threshold voltage. The voltage obtained may also be sufficient to serve as a power supply for short ranges.

A proportion of the incoming power P1' is reflected by the antenna and returned as power P2. The reflection characteristics (= reflection cross-section) of the antenna can be influenced by altering the load connected to the antenna. In order to transmit data from the transponder to the reader, a load resistor RL connected in parallel with the antenna is switched on and off in time with the data stream to be transmitted. The amplitude of the power P2 reflected from the transponder can thus be modulated (à modulated backscatter).

The power P2 reflected from the transponder is radiated into free space. A small proportion of this (free space attenuation) is picked up by the reader's antenna. The reflected signal therefore travels into the antenna connection of the reader in the "backwards direction" and can be decoupled using a directional coupler and transferred to the receiver input of a reader. The "forward" signal of the transmitter, which is stronger by powers of ten, is to a large degree suppressed by the directional coupler.

The ratio of power transmitted by the reader and power returning from the transponder (P1 / P2) can be estimated using the radar equation (for a more detailed explanation, please refer to the chapter 4 "Physical Principles" of the RFID-handbook).

source:http://rfid-handbook.de/about-rfid.html?showall=1&limitstart=