Wireless Integrated System Laboratory

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Research

 RF-powered, wireless temperature sensor tag chip

We investigated the design of an RF-powered, wireless temperature sensor tag chip which is suitable for environmental temperature monitoring. The transponder generates its power supply from UHF-band (900 MHz) RF signal using voltage multiplier based on custom Schottky diodes. Thanks to the highly efficient voltage multiplier, the tag chip can collect its energy from a base station located in 10 m distance.

UHF-band passive RFID tag

We present a fully integrated long-range UHF-band passive RFID tag chip fabricated in 0.35-μm CMOS using Titanium (Ti/Al/Ta/Al)-Silicon Schottky diodes. The diodes showed low turn-on voltages of 95 and 140 mV for diode currents of 1 and 5 μA, respectively. In addition, the Schottky diodes exhibited low resistive loss, and a high Q-factor design approach was exploited to achieve a long read range for the tag IC.

 

 

We investigated the characteristics of Schottky diode voltage multiplier fabricated using a CoSi2–Si junction in a 0.18 μm CMOS process for producing high sensitivity UHF-band passive RFID tag chips. The voltage multiplier based on the Schottky diodes resulted in superior voltage sensitivity compared to ones based on low-threshold MOSFETs.

 

An important issue in the passive RFID tag chip is generating a stable system clock for its internal digital and memory circuits. The tag IC sets up its internal oscillator frequency according to the timing information sent by a reader. Inside the tag chip, the DC supply voltage changes depending on the RF power received from the reader, and thus, the system clock generated internally without an on-board battery is susceptible to voltage and temperature variations. Therefore, a power efficient technique to generate and calibrate the system clock is necessary. A new architecture for generating a stable system clock (2.2 MHz) for the tag IC was employed to deal with supply voltage and temperature variations. Measurements showed that the clock generator had an error of 0.91% from the center frequency thanks to an 8-bit digital calibration scheme

 

 Before communicating with a reader, tag adjusts its system clock frequency to 2.2 MHz according to oscillator calibration signal sent by the reader. Because the system clock generated internally using a ring oscillator is susceptible to voltage and temperature variation, it needs digital correction

 

 The radio frequency identification (RFID) are growing rapidly with a good deal of promising features in technology and applications, especially in the UHF band for its suitability in the middle to long range communication link between a reader and tag. The figure shows a block diagram of RFID tag using backscatter modulation. The tag consists of tag antenna and tag chip. The tag chip includes analog block (voltage multiplier, ASK demodulator, power-on-reset, system clock generator, and modulator), digital block, and non-volatile memory.

 

We investigated the design trade-off in the development of UHF-band RFID tag for increased reading range. Using the quality factors of the tag antenna and tag chip as design parameters, the effects of the quality factors on the turn-on voltage of the tag chip and the backscattered power from the tag were examined. The design equations from the analysis indicate two regions of good impedances, one for providing high turn-on voltage for the tag chip, and the other resulting in increased radar cross section (RCS) for the antenna

 

The inductively-coupled RFID tag antenna operating at UHF band is very compact (4044mm), and the resistance and inductive reactance of the proposed antenna can be almost independently controlled with a simple adjustment of two parameters, in the complex conjugate region of chip impedances of common practice. The measured detection distance in anechoic chamber is 5.3m for -75 dBm reader sensitivity.  

 

To be used with the tag chip, a meander line antenna using an inductively coupled structure shown in figure was designed. The feeding part is inductively coupled to the radiation part of a meander line shape. The coupling depends on the separation L1 between the radiation and the feeding part, and the loop length L2.

 

The designed tag antenna was tested in anechoic chamber using Alien 9800 multi-protocol reader and 6 dBi linearly polarized reader antenna. A commercially available Class-1 Gen-2 tag chip, which has ZL = 11−j127 Ω, was attached to the fabricated antenna. Detection distance was determined as the maximum range where the RFID reader can detect correct EPC codes.

 

we present several issues for the design of a UHF-band near-field RFID tag chip. The power management of the analog block includes voltage multiplier, RF limiter, and regulator. The signal processing part includes ASK modem, clock generator, low voltage detector, analog random number generator, and power-on-reset.

 

The digital control for the tag chip is based on the EPCglobal Gen-2 protocol. Analog block includes a high dynamic range regulator with no voltage drop limiter. The tag chip was implemented using Hynix 0.18 Ám CMOS process. The tag chip includes 4kb EEPROM to store relative large information needed for security function. The chip size synthesized using 6 metal process was about 1 x 1 mm2.

 

The main control is a finite state machine that determines the appropriate response to a command and issues its own command to the encoder. The encoder interprets the tag’s command and transmits serialized data to the analog front-end. The nonvolatile memory holds or stores information required by the reader and can be only accessed when the tag and the reader establishes one to one connection through a number of verifications such as the use of handlers and passwords. For the nonvolatile memory, 4Kb EEPROM memory was used to store a relative large information needed for security function and the memory controllers embedded in the main control block.

 

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