Sunday, March 26, 2017

RFID Tag and Reader Antenna Design Techniques

RFID stands for Radio-Frequency Identification. The RFID device provides a unique identifier for that object and just as a bar code or magnetic strip the RFID device must be scanned to retrieve the identifying information.

RFID System Working Principal

A RFID system has three parts:
  1. A scanning antenna
  2. A transceiver with a decoder to interpret the data
  3. A transponder – the RFID tag – that has been programmed with information
RFID Systems
In most of RFID system, tags are attached to all items that are to be tracked. These tags are made from a tiny tag-chip that is connected to an antenna. The tag chip contains memory which stores the product’s electronic product code (EPC) and other variable information so that it can be read and tracked by RFID readers anywhere. An RFID reader is a network connected device (fixed or mobile) with an antenna that sends power as well as data and commands to the tags. The RFID reader acts like an access point for RFID tagged items so that the tags’ data can be made available to business applications.

RFID Frequency Band Allocation

There are a number of RFID frequencies, or RFID frequency bands that systems may use. There are a total of four different RFID frequency bands or RFID frequencies that are used around the globe.
rfid4.jpg

RFID Antennas

As part of the design of the RFID antenna, parameters such as the radiation resistance, bandwidth, efficiency, and Q all need to be considered, so that the resulting design for the RFID antenna meets the requirements and allows the required level of performance to be achieved. RFID antennas are tuned to resonate only to a narrow range of carrier frequencies that are centered on the designated RFID system frequency.
The RFID antenna propagates the wave in both vertical and horizontal dimensions.  The field coverage of the wave and also its signal strength is partially controlled by the number of degrees that the wave expands as it leaves the antenna.  While the higher number of degrees means a bigger wave coverage pattern it also means lower strength of the signal. Passive RFID tags utilize an induced antenna coil voltage for operation. This induced AC voltage is rectified to provide a voltage source for the device. As the DC voltage reaches a certain level, the device starts operating. By providing an energizing RF signal, a reader can communicate with a remotely located device that has no external power source such as a battery. According to the different functions in the RFID system, the RFID antennas can be divided into two classes: the tag antenna and the reader antenna.

rfid_tag.jpg

Tag Antenna

Tag antennas collect energy and channel it to the chip to turn it on. Generally, the larger the tag antenna’s area, the more energy it will be able to collect and channel toward the tag chip, and the further read range the tag will have. Tag antennas can be made from a variety of materials; they can be printed, etched, or stamped with conductive ink, or even vapor deposited onto labels.  The tag antenna not only transmits the wave carrying the information stored in the tag, but also needs to catch the wave from the reader to supply energy for the tag operation. Tag antenna should be small in size, low-cost and easy to fabricate for mass production. In most cases, the tag antenna should have omnidirectional radiation or hemispherical coverage. Generally, the impedance of the tag chip is not 50 ohm, and the antenna should realize the conjugate match with the tag chip directly, in order to supply the maximum power to the tag chip.  Tag antenna may be the signal turn or multiple turns as shown here.
rfid3

Reader Antenna

Reader antennas convert electrical current into electromagnetic waves that are then radiated into space where they can be received by a tag antenna and converted back to electrical current.

RFID Antenna Design

This design RFID system is used to track object placed on the storage rack. In this system, there is two component of RFID.
rfid6RFID Reader: this component is fit on the shelf  and connected to data base computer system
RFID Tag: this component along with planar antenna is placed in tracking objects placed in store
When a particular object placed on the shelf or removed from the shelf, information of that object is automatically updated in database computer. The antenna is optimized to increasing the read accuracy and shortening the optimization phase. One more RFID transponder antenna designed at 13.5 MHz is shown below.
rfid7

Planar Antenna for Ultra High Frequency (UHF) RFID Handheld Reader

This antenna consists of a microstrip-to-coplanar stripline transition, a meandered driven dipole, a closely-coupled parasitic element, and a folded finite-size ground plane. This Antenna is suitable for RFID handheld readers.
rfid8


rfid9

Wednesday, March 15, 2017

Helical (Helix) Antenna Design

The helix antenna is a travelling wave antenna, which means the current travels along the antenna and the phase varies continuously. Helix antennas (also commonly called helical antennas) invented by John Kraus give a circular polarized wave.  Helix antennas are referred to as axial-mode helical antennas. The benefits of this helix antenna are it has a wide bandwidth, is easily constructed, has real input impedance, and can produce circularly polarized fields. There are two mode of circular polarization in helix antenna.
Left handed helix antenna: In left handed helix antenna if you curl left hand fingers around the helix, thumb would point up. The waves emitted from this helix antenna are Left Hand Circularly Polarized.
Right handed helix antenna: In right handed helix antenna if you curl right hand fingers around the helix, thumb would point up. The waves emitted from this helix antenna are Right Hand Circularly Polarized
The minimum number of turns for a helix is between 3 and 5. There are many online tools available to calculate design parameter of helical antenna.

Design parameters for Helical Antenna

  • D – Diameter of a turn on the helix antenna.
  • C – Circumference of a turn on the helix antenna (C=pi*D).
  • S – Vertical separation between turns for helical antenna.
  • α  – Pitch angle, which controls how far the helix antenna grows in the z-direction per turn
  • N – Number of turns on the helix antenna.
  • H – Total height of helix antenna, H=NS.

Broad-Band Helix Antenna Design

Below is example of one Helix antenna that work at central frequency 1.35 GHz.
helix1.jpg
Radius-20 mm
Wire Radius – 2 mm
Number of Turn – 12
Height – 400 mm
Rotation- RHS
Helix antennas of at least 3 turns will have close to circular polarization in the +z direction when the circumference C is close to a wavelength.
 helix2.jpg
The helix antenna functions well for pitch angles between 12 and 14 degrees. Typically, the pitch angle is taken as 13 degrees.

Simulated Result

Antenna is simulated using Finite-Difference Time domain (FDTD) technique. Below is simulated return loss and field plot.
helix3.jpg
Helix_gainAntenna radiation pattern ( Far Field data plot) is shown below . Antenna Gain is around 15 dB .
There are many other form of Helix antenna like  quadrifilar helix antenna.The Quadrifilar Helix Antenna has 4 excitations and  each element driven a progressive 90 degrees in phase. Then  Bifilar helix is constructed using two volutes with an equal number of turns, and their starting points positioned 180° apart. The ends of the volutes are connected with a shorting wire which adds to the structural integrity of the antenna.

Helix Antenna Array


Sunday, March 5, 2017

Signal Integrity Analysis of USB 3.0 Data Bus

USB 3.0
USB(Universal Serial Bus) is the most popular connection used to connect a computer to devices such as digital cameras, printers, scanners, and external hard drives. USB is a cross-platform technology that is supported by most of the major operating systems. UART is a computer hardware device that translates data between parallel and serial forms (SerDes). A dual UART (Universal asynchronous receiver/transmitter), or DUART, combines two UARTs into a single chip. The universal asynchronous receiver/transmitter (UART) takes bytes of data and transmits the individual bits in a sequential fashion. At the destination, a second UART re-assembles the bits into complete bytes. Each UART contains a shift register, which is the fundamental method of conversion between serial and parallel forms. Serial transmission of digital information (bits) through a single wire or other medium is less costly than parallel transmission through multiple wires. Below is USB 3.0 onboard interconnect and stack of ground and power nets.
USB SI-PI Setup
SERDES is a high-speed serial data link used in integrated circuits (ICs) to serialize the parallel data and transfer it at a much faster rate. A typical SERDES architecture looks like a communication set-up with a transmit and a receive side. At transmit side, a PLL generates the fast clock necessary to drive the serializer. A clock and data recovery (CDR) circuit recovers a clock from the transmitted serial data and retimes the data at the receive side. One advantage of using SERDES is reduced clock skew, so data can be sent at the GHz rate. The main disadvantage in SERDES is timing jitter, the deviation of the actual signal transition from the expected transition in time. Timing skew is not a problem in serial interface because in each data lane, there is only one differential signal in each direction, and there is no external clock signal since clocking information is embedded within the serial signal itself.
usbs2.jpgEM simulated data is extracted for USB data group signals and combined with USB connector EM Data. Simulated result of USB connector to USB controller chip is shown here. This data is combined with other channel components for full channel simulation. From this analysis, important factors from a Signal Integrity point of view (e.g., impedance matching, reflection, attenuation, impedance mismatch, propagating delay, crosstalk, and alignment shapes of connectors) are analyzed.
usbs3
As shown here, without any equalization eye is closed and after applying RX equalization eye is open and meeting USB specification. The idea behind equalization is to use the voltage levels of the other bits to correct the voltage level of the current bit. Due to the inter-symbol interference (ISI) from the frequency dependent loss of the channel, the eye of the received signals is totally closed, and the clock and data cannot be recovered from the severely distorted signals. After the equalizer, the eye of the equalized signals is opened.