Abstract:
An ASK demodulator for use in an RFID transponder having a limiter circuit associated with the antenna circuit and converting the ASK antenna fieldstrength modulation into an ASK limiter current modulation by limiting the antenna voltage to a fixed value and thereby causing the limiter current to be substantially proportional to the ASK antenna field strength, and a current discriminator circuit that discriminates the ASK limiter current modulation. By converting the fieldstrength modulation into a proportional limiter current and discriminating that limiter current, a linear relationship and a stable demodulator sensitivity are achieved. The current discrimination can be made accurately under low-voltage conditions.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an ASK demodulator for use in an RFID transponder. 
       BACKGROUND OF THE INVENTION 
       [0002]    The analog front end of an RFID transponder includes a resonant LC antenna circuit. In high frequency RFID transponders the resonant frequency is typically 13 MHz. The antenna captures the modulated RF field from an interrogator (ASK downlink modulation) giving rise to a fairly high antenna voltage with voltage swings in the range of 8V. An antenna voltage in the range of 8V allows a straight forward envelope detection for demodulation with a conventional CMOS circuit used in the RFID transponder. 
         [0003]    However, advanced CMOS technologies allow only small drain-source and gate voltages. A voltage swing in a range of 8V is too high for integrated CMOS circuits implemented in an advanced CMOS technology, especially when a low cost process is used which does not support high voltage transistors or high precision capacitors for use in a resonant circuit. The antenna voltage needs to be limited by some voltage limiting circuit to comply with the specifications of the CMOS circuit. When a voltage limitation to a maximum of 3.6V is needed, the minimum needed supply voltage limits the antenna voltage down to 1.6V plus a (rectifier) diode voltage of 0.8V i.e. 2.4V. Taking into account inevitable CMOS process variations, the antenna voltage must be limited so as not to exceed a voltage range of 2.4V to 3.6V. Accordingly, the voltage swing from an ASK downlink modulation will be severely clamped, and a direct envelope demodulation will not work any more. 
         [0004]    However, the ASK information is present in the limiter current. A possible approach to demodulation is to detect the gate voltage of the limiter transistors in the voltage limiting circuit since it also contains the modulation information. But the gate voltage of the limiter transistors is a quadratic function of the RF field and has to be linearized to ensure a defined sensitivity. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    The present invention takes a different approach. The inventive idea is to demodulate the ASK information contained in the limiter current with a current discriminator circuit that discriminates the ASK limiter current modulation. Preferably, the current discriminator has two current mirrors that both mirror the limiter current but have different PMOS and NMOS ratios. 
         [0006]    Specifically, the present invention provides an ASK demodulator for use in an RFID transponder, comprising a limiter circuit associated with the antenna circuit and converting the ASK antenna field strength modulation into an ASK limiter current modulation by limiting the antenna voltage to a fixed value and thereby causing the limiter current to be substantially proportional to the ASK antenna field strength, and a current discriminator circuit that discriminates the ASK limiter current modulation. By converting the field strength modulation into a proportional limiter current and discriminating that limiter current—or a copy of that current—, a linear relationship and a stable demodulator sensitivity are achieved. The current discrimination can be made accurately under low-voltage conditions. 
         [0007]    One aspect of the current discriminator can be easily implemented with the following components:
   a first current mirror that mirrors the ASK limiter current into a first pair of series-connected complementary MOS transistors,   a second current mirror that mirrors the ASK limiter current into a second pair of series-connected complementary MOS transistors,   and a latch circuit with a set input coupled to the interconnection node of the first pair of complementary MOS transistors and a reset input coupled to the interconnection node of the second pair of complementary MOS transistors. With this configuration, the output of the latch circuit directly provides the demodulation signal as a serial stream of detected “0” and “1” bit values.   
 
         [0011]    Further advantages and features will ensue from the following description of a preferred embodiment with reference to the appending drawings. In the drawings: 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a block diagram of a transponder with a voltage limiter circuit and an ASK demodulator; 
           [0013]      FIG. 2  is a circuit diagram of the ASK demodulator; 
           [0014]      FIG. 3  is a circuit diagram of a basic current discriminator used in the demodulator of  FIG. 2 ; 
           [0015]      FIG. 4  is a signal diagram illustrating the underlying principles of current discrimination; 
           [0016]      FIG. 5  is a slightly modified circuit diagram of the basic current discriminator; 
           [0017]      FIG. 6  and  FIG. 7  are signal diagrams further illustrating the underlying principles of current discrimination; and 
           [0018]      FIG. 8  is a signal diagram illustrating the operation of the demodulator in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION  
       [0019]    With reference first to  FIG. 1 , a transponder has an external LC resonance circuit and an analog front end with input terminals Ant 1  and Ant 2  connected to the LC resonance circuit. Each of the input terminals Ant 1  and Ant 2  has an associated limiting transistor T 1 , T 2  the gates of which are interconnected and driven by a limiter control block which is also connected to both input terminals Ant 1  and Ant 2 . Each of the transistors T 1  and T 2  draws half of the limiter current, I LIM / 2 . A transistor T 3  mirrors the limiter current I LIM / 2  to I LIM /x and a further transistor T 4  mirrors the current I LIM /x to I DEMOD     —     IN , which is the input current I IN  to the ASK demodulator circuit of  FIG. 2 . The output signal R×D of the ASK demodulator corresponds to the output signal V OUT  in  FIG. 2 . 
         [0020]    The ASK demodulator in  FIG. 2  receives as its input I IN  the copied limiter current I DEMOD     —     IN  from the voltage limiter circuit in  FIG. 1 . The input stage of the demodulator is a diode-connected NMOS transistor NM 1  that receives the copy of the limiter current I IN  on its drain and has its source connected to ground. The current I IN  is mirrored by a second NMOS transistor MN 2  at a ratio of 1:1. Transistor MN 2  is connected in series with a first PMOS transistor MP 1  which is diode-connected between the supply terminal V DD  and the drain of transistor MN 2 . The interconnected drains of transistors MN 2  and MP 1  provide a bias potential pbias. The limiter current I IN  is also mirrored by a third NMOS transistor MN 3  which has its gate at the common bias level nbias from transistors MN 1  and MN 2 , and is connected in series with a second PMOS transistor MP 2  so that MP 2  and MN 3  have a common drain voltage compZERO. Transistor MP 2  has its gate coupled to the drain of transistor MN 2  and the gate of transistor MP 1  through an RC lowpass filter that provides bias potential pbias_slow, and thus mirrors the current through transistors MN 2  and MP 1  with a delay that is a multiple of the bit width of the signal to be demodulated, ensuring that the current through the PMOS transistors is nearly like a DC current. In parallel to transistor pair MP 2  and MN 3 , a further pair of complementary transistors MP 3  and MN 4  mirror the same currents and provide a common drain voltage compONE on their interconnected drains. 
         [0021]    In the transistor pair MP 2 , MN 3  of the first current mirror branch, the NMOS ratio is greater than the PMOS ratio, the relation between the two ratios being e.g. 12:11 in a specific implementation. In the transistor pair MP 3 , MN 4  of the second current mirror branch, the NMOS ratio is only slightly greater than the PMOS ratio, e.g. 12.4:12 in a specific implementation. The relation NMOS ratio to PMOS ratio (e.g. 12:11) is greater in the first current mirror branch than in the second current mirror branch (e.g. 12.4:12). Due to the different transistor ratios between the two current mirror branches, the corresponding common drain voltages compZERO and compONE will be different. 
         [0022]    A latch circuit LATCH has a set input connected to the interconnected drains of the first transistor pair MP 2 , MN 3  at the common drain voltage compZERO and a reset input connected to the interconnected drains of the second transistor pair MP 3 , MN 4  at the common drain voltage compONE. The output of the latch circuit LATCH directly provides an output signal V OUT  as the demodulated signal consisting of successive ONEs and ZEROs. 
         [0023]    With reference now to  FIG. 3 , the main underlying idea is a PMOS and a NMOS current mirror with a common current, connected together. The common drain voltage then is strongly dependent on the PMOS and NMOS transistor ratios. The PMOS ratio is 1:P and the NMOS ratio 1:N. The output voltage depending on the transistors ratios is shown in the graphs of  FIG. 4 . 
         [0024]    Now setting N=1 and P&lt;1. The PMOS mirror should be very slow like shown in the slightly modified diagram of  FIG. 5 . It should be understood that  FIG. 5  is a partial circuit of the demodulator intended to explain the principles of the invention. The behaviour for a transient decreasing input current looks like the graph of  FIG. 6 . 
         [0025]    The transient behaviour in the graph of  FIG. 7  shows the functionality of the demodulator principle.  FIGS. 6 and 7  should be considered in relation to the partial circuit in  FIG. 5 . When the input current decreases by more than the same ratio as the PMOS transistor ratio, the output voltage changes from ‘0’ to ‘1’. Now, however, the demodulator will be sensitive to noise in the low current region. Therefore, a second mirror pair with an NMOS ratio slightly above 1 has been inserted, as shown in  FIG. 2 . With the latch circuit at both outputs, the functionality of a current demodulator now is as illustrated in  FIG. 8 . 
         [0026]    With reference to  FIG. 8 , due to the different transistor ratios in the two current mirror branches, the output voltages V CompZERO  and V CompONE  have different threshold values. As long as both output voltages V CompZERO  and V CompONE  are low, the latch is in a RESET state. When V CompONE  goes high, the latch reset is released and the latch is prepared for switching. However, switching of the latch only occurs when output voltages V CompZERO  goes high, as seen in  FIG. 8 . A similar hysteresis (similar to a Schmitt trigger) is observed in  FIG. 8  for the switching of the latch to the opposite condition. 
         [0027]    The input current in the analog front end is the mirrored limiter current, and the correlation between H-field and limiter current is given: 
         [0000]    
       
      
       I 
       LIM 
       =H 
       (d) 
       ·K−I 
       0 
      
     
         [0028]    With: I LIM : Limiter current
   H (d) : Electromagnetic Field strength at the Tag&#39;s antenna (at distance d from a readers antenna)   K: constant factor, depends on antenna parameter   I 0 : Current that is needed in the resonant circuit, to build up the supply voltage
       1. (Includes also the IC&#39;s current consumption)   
       
 
         [0033]    For H (d) *K&gt;I 0 , there is a linear correlation given between limiter current and field strength. 
         [0034]    The ASK modulation index m normally is given by: 
         [0000]    
       
         
           
             m 
             = 
             
               
                 
                   I 
                   unmod 
                 
                 - 
                 
                   I 
                   mod 
                 
               
               
                 
                   I 
                   ummod 
                 
                 + 
                 
                   I 
                   mod 
                 
               
             
           
         
       
     
         [0035]    This means a constant ratio between the modulated (I mod ) and unmodulated limiter current (I unmod ). 
         [0000]    
       
         
           
             
               I 
               mod 
             
             = 
             
               
                 I 
                 unmod 
               
               · 
               
                 
                   1 
                   - 
                   m 
                 
                 
                   1 
                   + 
                   m 
                 
               
             
           
         
       
     
         [0036]    By adjusting the PMOS ratios at the CompZero node to the same size, a specified ASK modulation size can be demodulated. 
         [0037]    The sensitivity now is only depending on the transistors mismatch, which is handled and documented very well in CMOS processes. No absolute influence on component parameters is given. 
         [0038]    As a result this circuit allows realizing a controlled sensitivity. 
         [0039]    Furthermore this circuit has a build in dynamic range, the sensitivity is always a function of the current ratio and not depending on the absolute current size. 
         [0040]    A third advantage is the dynamic current consumption of the circuit. The current consumption is always a part of the limiters current. 
         [0041]    At low field strength (means e.g. far distances), the current is small and when much current is available (e.g. at near distances), the circuits current consumption is also high.