(eBook - PDF - Electronics) Antenna Circuit Design for RFID Applications(Lee 2003)

Author: Youbok Lee, Ph.D. >$
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Microchip Technology Inc. (!nkv^]  
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50PAGE &zJI~R  
X+;[Gc}(W  
ENGLISH t neTOj  
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INTRODUCTION 6HY): M&?  
Passive RFID tags utilize an induced antenna coil ^o3,YH  
voltage for operation. This induced AC voltage is %l7[eZ{Y  
rectified to provide a voltage source for the device. As bCw{9El!K4  
the DC voltage reaches a certain level, the device tY"eoPme  
starts operating. By providing an energizing RF signal, *2zp>(%  
a reader can communicate with a remotely located e#Cv*i_<  
device that has no external power source such as a cT'Bp)a  
battery. Since the energizing and communication RM|J |R  
between the reader and tag is accomplished through @N Yl4N  
antenna coils, it is important that the device must be y&$n[j  
equipped with a proper antenna circuit for successful }:#WjH^  
RFID applications. Cb;6yE)!Z  
An RF signal can be radiated effectively if the linear {fXkbMO|  
dimension of the antenna is comparable with the )s $]+HQs  
wavelength of the operating frequency. However, the e"04jd/  
wavelength at 13.56 MHz is 22.12 meters. Therefore, D;Qx9^.  
it is difficult to form a true antenna for most RFID applications. P-\f-FS  
Alternatively, a small loop antenna circuit that .#yg=t1C  
is resonating at the frequency is used. A current ~n]5iGz  
flowing into the coil radiates a near-field magnetic field !vwio!  
that falls off with r-3. This type of antenna is called a cZh0\DyU  
magnetic dipole antenna. &ys>z<Z  
For 13.56 MHz passive tag applications, a few u<S`"MR:J  
microhenries of inductance and a few hundred pF of DU!T#H7  
resonant capacitor are typically used. The voltage [{7#IZL  
transfer between the reader and tag coils is accomplished aN0[6+KP;  
through inductive coupling between the two t$!zgUJ  
coils. As in a typical transformer, where a voltage in the dOeM0_o  
primary coil transfers to the secondary coil, the voltage !zE{`Ha~  
in the reader antenna coil is transferred to the tag arL>{mj  
antenna coil and vice versa. The efficiency of the P=n_wE  
voltage transfer can be increased significantly with high Ce}`z L  
Q circuits. @Go_5X(  
This section is written for RF coil designers and RFID >-MnB  
system e .. bB_LL  
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