RFID PROTOCOL TERMS AND CONCEPTS

Technical jargon develops around any new technology, and RFID is no exception. Some of these terms are quite useful, serving as a convenient way to communicate concepts needed to describe other concepts that will appear in the pages that follow. These terms include:

Singulation

This term describes a procedure for reducing a group of things to a stream of things that can be handled one at a time. For example, a subway turnstile is a device for singulating a group of people into a stream of individuals so that the system may count them or ask them for access tokens.

This same singulation is necessary when communicating with RFID tags, because if there is no mechanism to enable the tags to reply separately, many tags will respond to a reader at once and may disrupt communications.

Singulation also implies that the reader learns the individual IDs of each tag, thus enabling inventories. Inventories of groups of tags are just singulation that is repeated until no unknown tags respond.

Anti-collision

This term describes the set of procedures that prevent tags from interrupting each other and talking out of turn. Whereas singulation is about identifying individual tags, anti-collision is about both regulating the timing of responses and finding ways of randomizing those responses so that a reader can understand each tag amidst the plethora of responses.

Identity

An identity is a name, number, or address that uniquely refers to a thing or place. "Malaclypse the Elder" is an identity referring to a particular person. "221b Baker Street London NW1 6XE, Great Britain" is an identity referring to a particular place, just as "urn:epc:id:sgtin:00012345.054322.4208" is an identity referring to a particular widget

ADVANTAGES OF RFID OVER OTHER TECHNOLOGIES BASIC INFORMATION

There are many different ways to identify objects, animals, and people. Why use RFID? People have been counting inventories and tracking shipments since the Sumerians invented the lost package. Even some of the earliest uses of writing grew from the need to identify shipments and define contracts for goods shipped between two persons who might never meet.[*] Written tags and name badges work fine for identifying a few items or a few people, but to identify and direct hundreds of packages an hour, some automation is required.

The bar code is probably the most familiar computer-readable tag, but the light used to scan a laser over a bar code imposes some limitations. Most importantly, it requires a direct "line of sight," so the item has to be right side up and facing in the right direction, with nothing blocking the beam between the laser and the bar code.

Most other forms of ID, such as magnetic strips on credit cards, also must line up correctly with the card reader or be inserted into the card reader in a particular way. Whether you are tracking boxes on a conveyor or children on a ski trip, lining things up costs time.

Biometrics can work for identifying people, but optical and fingerprint recognition each require careful alignment, similar to magnetic strips. Facial capillary scans require you to at least face the camera, and even voice recognition works better if you aren't calling your passphrase over your shoulder.

RFID tags provide a mechanism for identifying an item at a distance, with much less sensitivity to the orientation of the item and reader. A reader can "see" through the item to the tag even if the tag is facing away from the reader.

RFID has additional qualities that make it better suited than other technologies (such as bar codes or magnetic strips) for creating the predicted "Internet of Things."[*] One cannot, for instance, easily add information to a bar code after it is printed, whereas some types of RFID tags can be written and rewritten many times. Also, because RFID eliminates the need to align objects for tracking, it is less obtrusive. It "just works" behind the scenes, enabling data about the relationships between objects, location, and time to quietly aggregate without overt intervention by the user or operator.

[*] This term was originally attributed to the Auto-ID Center. We will discuss both this term and the Auto-ID Center in more detail later in this book.

To summarize, some of the benefits of RFID include the following:

Alignment is not necessary

A scan does not require line of sight. This can save time in processing that would otherwise be spent lining up items.

High inventory speeds

Multiple items can be scanned at the same time. As a result, the time taken to count items drops substantially.

Variety of form factors

RFID tags range in size from blast-proof tags the size of lunch boxes to tiny passive tags smaller than a grain of rice. These different form factors allow RFID technologies to be used in a wide variety of environments.

Item-level tracking

Rewritability

Some types of tags can be written and rewritten many times. In the case of a reusable container, this can be a big advantage. For an item on a store shelf, however, this type of tag might be a security liability, so write-once tags are also available.

AMPLITUDE MODULATED RADIO-FREQUENCY BANDS CLASSIFICATION

Amplitude modulated radio frequencies are grouped into three bands according to the wavelength of their carrier frequencies. The carrier frequency chosen depends to a large extent on the distance between the broadcasting station and the target listeners.

1. Long wave (low frequency).

All transmission whose carrier frequencies are less than 400 kHz are generally classified as long wave. At a frequency of 100 kHz, a quarter-wavelength antenna is 750 meters high.

Such an antenna poses several problems such as vulnerability to high winds and danger to low flying aircraft. Long wave broadcasting stations therefore use an electromagnetically short antenna which necessarily limits their reach to a few tens of kilometers because the short antenna has only the ground wave.

2. Medium wave.

Carrier frequencies in the range 300 kHz to 3MHz are regarded as medium wave. The height of the antenna becomes more manageable and the possibility of using the sky wave to reach distant audiences is a reality. Generally, it is used for local area broadcasting.

3. Short wave.

Short wave generally refers to carrier frequencies between 3MHz and 30MHz. The wavelengths under consideration are between 100 meters and 1 meter. Antenna structures can be constructed to give specified directional properties.

Most of the energy can be put into the sky wave and the signal can be bounced off the ionosphere (the layer of ionized gas that surrounds the Earth) to reach receivers halfway round the world. A very severe problem is encountered in short wave transmission, that is, the signal tends to fade from time to time.

This phenomenon is caused by the multiple paths by which the signal can reach the receiver. It is clear that if two signals reach the receiver by different paths such that their phase angles are 180 degrees apart they will cancel each other.

The ionosphere sometimes experiences severe turbulence due mainly to radiation from the Sun. Short wave transmission is therefore at its best during the hours of darkness.

RADIO FREQUENCY TRANSDUCERS BASIC INFORMATION

The term radio-frequency (RF) transducer is a fancy name for an antenna. Antennas are so common that you probably don’t think about them very often. Your car radio has one.

Your portable headphone radio, which you might use while jogging on a track (but never in traffic), employs one. Cellular and cordless telephones, portable television receivers, and handheld radio transceivers use antennas.

Hundreds of books have been written on the subject. There are two basic types of RF transducer: the receiving antenna and the transmitting antenna.

A receiving antenna converts electromagnetic (EM) fields, in the RF range from about 9 kHz to several hundred gigahertz, into ac signals that are amplified by the receiving apparatus. A transmitting antenna converts powerful alternating currents into EM fields, which propagate through space.

There are a few significant differences between receiving antennas and transmitting antennas designed for a specific radio frequency. The efficiency of an antenna is important in transmitting applications, but not so important in reception.

Efficiency is the percentage of the power going into a transducer that is converted into the desired form. If the input power to a transducer is Pin watts and the output power is Pout watts, the efficiency in percent, Eff%, can be found using the following equation:
Eff% �= 100 Pout /Pin


In a transmitting antenna, 75 W of RF power are delivered to the transducer, and 62 W are radiated as an EM field. What is the efficiency of the transducer?

To solve this problem, plug the numbers into the formula. In this particular case, Pin � 75 and Pout � 62. Therefore, Eff% �= 100 � 62/75 � 100 � 0.83 � 83 percent

Another difference between transmitting and receiving antennas is the fact that, for any given frequency, transmitting antennas are often larger than receiving antennas. Transmitting antennas are also more critical as to their location.

Whereas a small loop or whip antenna might work well indoors in a portable radio receiver for the frequencymodulation (FM) broadcast band, the same antenna would not function well at the broadcasting station for use with the transmitter.

Still another difference between transmitting and receiving antennas involves power-handling capability. Obviously, very little power strikes the antenna in a wireless receiver; it can be measured in fractions of a microwatt.

However, a transmitter might produce kilowatts or even megawatts of output power. A small loop antenna, for example, would get hot if it were supplied with 1 kW of RF power; if it were forced to deal with 100 kW, it would probably melt.

FREQUENCY HOPPING SYSTEM BASIC INFORMATION AND TUTORIALS

Frequency hopping is the simpler of the two spread-spectrum techniques. A frequency synthesizer is used to generate a carrier in the ordinary way.

There is one difference, however: instead of operating at a fixed frequency, the synthesizer changes frequency many times per second according to a preprogrammed sequence of channels.

This sequence is known as a pseudo-random noise (PN) sequence because, to an outside observer who has not been given the sequence, the transmitted frequency appears to hop about in a completely random and unpredictable fashion.

In reality, the sequence is not random at all, and a receiver which has been programmed with the same sequence can easily follow the transmitter as it hops and the message can be decoded normally.

Since the frequency-hopping signal typically spends only a few milliseconds or less on each channel, any interference to it from a signal on that frequency will be of short duration. If an analog modulation scheme is used for voice, the interference will appear as a click and may pass unnoticed.

If the spread-spectrum signal is modulated using digital techniques, an errorcorrecting code can be employed that will allow these brief interruptions in the received signal to be ignored, and the user will probably not experience any signal degradation at all. Thus reliable communication can be achieved

in spite of interference.

Sample Problem

A frequency-hopping spread-spectrum system hops to each of 100 frequencies

every ten seconds. How long does it spend on each frequency?

SOLUTION

The amount of time spent on each frequency is

t = 10 seconds/100 hops

= 0.1 second per hop

If the frequency band used by the spread-spectrum system contains known sources of interference, such as carriers from other types of service, the frequency-hopping scheme can be designed to avoid these frequencies entirely.

Otherwise, the communication system will degrade gracefully as the number of interfering signals increases, since each new signal will simply increase the noise level slightly.

RADIO FREQUENCY CIRCUITS BASIC AND TUTORIALS

Radio-frequency circuits are represented here by only a few general examples, because the circuits and design methods that have to be used are fairly specialized, particularly for transmission; the reader who wishes more information on purely RF circuits is referred to the excellent amateur radio publications.

At one time, a reference book would have shown discrete circuits for RF and IF receiver stages, but for conventional analogue radio reception these functions are now invariably carried out by ICs.

The Philips TEA5711 is an IC, now quite old (1992) and established, that integrates all the functions of an AM/FM radio from front end to AM detector and FM stereo output in a 32-pin DIL package.

Figure 7.37 shows a suggested application from the datasheet, using a separate TDA5070 output chip. The TEA5711 chip allows a wide range of supply voltage, from 1.8 V to 12 V, and has a low current consumption of 15 mA on AM and 16 mA on FM.

fig 7.37

The input sensitivity for FM is 2.0 μV, with high selectivity, and the FM input uses a high impedance MOSFET. The main applications are in portable radios.


The Chorus FS1010 from Frontier Silicon is a 179-pin BGA package that implements the most difficult sections of a DAB digital radio receiver, needing only an external RF stage, audio D to A, flash memory, keypad and display for a complete radio.

The chip incorporates 16 K of ROM, 384 K of RAM, and two 8 K cache memories. It is likely that some day we shall have all of these functions on one chip, but until DAB radios sell in more significant numbers and until gaps in transmitting areas are filled in this is not likely to happen rapidly.  One significant difference from radio as we used to know it is that there is no chance of using discrete components.


Much more specialized devices are used for microwave frequencies, and a specialist in semiconductors for these ranges is Tquint Semiconductors. As example, the Tquint TGC1430G multiplier is intended as a ×3 multiplier with an output in the range of 20–40 GHz using stripline architecture with GaAs semiconductors.

FREQUENCY MODULATION (FM) BROADCASTING BASIC TUTORIALS

WHAT IS FREQUENCY MODULATION (FM) BROADCASTING?

The monophonic system of FM broadcasting was developed to allow sound transmission of voice and music for reception by the general public for audio frequencies from 50 to 15,000 Hz, all to be contained within a +/−75-kHz RF bandwidth.

This technique provided higher fidelity reception than was available with standard broadcast AM along with less received noise and interference. FM broadcasting in the U.S. is allocated the 88–108 MHz frequency band.

Pre-emphasis is employed in anFMbroadcast transmitter to improve the received signal-to-noise ratio. The pre-emphasis upper-frequency limit is based on a time constant of 75 μs as required by the FCC for FM broadcast transmitters.

Audio frequencies from 50 to 2120 Hz are transmitted with normal FM; whereas audio frequencies from 2120 Hz to 15 kHz are emphasized with a larger modulation index. There is significant signal-to-noise improvement at the receiver, which is equipped with a matching de-emphasis circuit.

INTERMODULATION PRODUCTS OF TWO DIFFERENT FREQUENCIES

FREQUENCIES INTERMODULATION INFORMATION


Understanding the dynamic performance of the receiver requires knowledge of intermodulation products (IP) and how they affect receiver operation. Whenever two signals at frequencies F1 and F2 are mixed together in a non-linear circuit, a number of products are created according to the mF1 ± nF2 rule, where m and n are either integers or zero (0, 1, 2, 3, 4, 5, . . .).



Mixing can occur in either the mixer stage of a receiver front end, or in the RF amplifier (or any outboard preamplifiers used ahead of the receiver) if the RF amplifier is overdriven by a strong signal. It is also possible for corrosion on antenna connections, or even rusted antenna screw terminals to create IPs under certain circumstances.

One even hears of alleged cases where a rusty downspout on a house rain gutter caused re-radiated intermodulation signals. The order of an IP product is given by the sum (m + n). Given input signal frequencies of F1 and F2, the main IPs are:

Second order: F1 ± F2 2F1 2F2
Third order: 2F1 ± F2 2F2 ± F1 3F1 3F2
Fifth order: 3F1 ± 2F2 3F2 ± 2F1 5F1 5F2


When an amplifier or receiver is overdriven, the second-order content of the output signal increases as the square of the input signal level, while the third-order responses increase as the cube of the input signal level.

Consider the case where two HF signals, F1 = 10 MHz and F2 = 15 MHz are mixed together. The second order IPs are 5 and 25 MHz; the third-order IPs are 5, 20, 35 and 40 MHz; and the fifth-order IPs are 0, 25, 60 and 65 MHz.

If any of these are inside the passband of the receiver, then they can cause problems. One such problem is the emergence of ‘phantom’ signals at the IP frequencies. This effect is seen often when two strong signals (F1 and F2) exist and can affect the front-end of the receiver, and one of the IPs falls close to a desired signal frequency, Fd.

If the receiver were tuned to 5 MHz, for example, a spurious signal would be found from the F1–F2 pair given above. Another example is seen from strong in-band, adjacent channel signals. Consider a case where the receiver is tuned to a station at 9610 kHz, and there are also very strong signals at 9600 kHz and 9605 kHz. The near (in-band) IP products are:

Third-order: 9595 kHz ( F = 15 kHz)
9610 kHz ( F = 0 kHz) (ON CHANNEL!)
Fifth-order: 9590 kHz ( F = 20 kHz)
9615 kHz ( F = 5 kHz)

Note that one third-order product is on the same frequency as the desired signal, and could easily cause interference if the amplitude is sufficiently high. Other third- and fifth order products may be within the range where interference could occur, especially on receivers with wide bandwidths.

The IP orders are theoretically infinite because there are no bounds on either m or n. However, because the higher order IPs have smaller amplitudes only the second-order, third-order and fifth-order products usually assume any importance.

Indeed, only the third-order is normally used in receiver specification sheets because they fall close to the RF signal frequency.

There are a large number of IMD products from just two signals applied to a non-linear medium. But consider the fact that the two-tone case used for textbook discussions is rarely encountered in actuality. A typical two-way radio installation is in a signal rich environment, so when dozens of signals are present the number of possible combinations climbs to an unmanageable extent.