CLRC632 Datasheet, PDF (141/163 Pages) NXP Semiconductors – Multiple Protocol Contactless Reader IC

Philips Semiconductors
Product Specification Rev. 3.0; May 2003
Multiple Protocol Contactless Reader IC
CL RC632
20.2.3 RECEIVING CIRCUIT
The internal receiving concept of the CL RC632 makes use of both side-bands of the sub-carrier load
modulation of the card response. No external filtering is required.
It is recommended to use the internally generated VMID potential as the input potential of pin RX. This DC
voltage level of VMID has to be coupled to the Rx-pin via R2. To provide a stable DC reference voltage a
capacitance C4 has to be connected between VMID and ground.
Considering the (AC) voltage limits at the Rx-pin the AC voltage divider of R1 + C3 and R2 has to be
designed. Depending on the antenna coil design and the impedance matching the voltage at the antenna coil
varies from antenna design to antenna design. Therefore the recommended way to design the receiving
circuit is to use the given values for R1, R2, and C3 from the above mentioned application note, and adjust
the voltage at the Rx-pin by varying R1 within the given limits.
Note: R2 is AC-wise connected to ground (via C4).
20.2.4 ANTENNA COIL
The precise calculation of the antenna coils’ inductance is not practicable but the inductance can be
estimated using the following formula. We recommend designing an antenna either with a circular or
rectangular shape.
L₁
[nH]
=
2
⋅
l
1
[cm
]
⋅



ln^

l₁


D₁ 
−

K

N 1,8
1
l₁............... Length of one turn of the conductor loop
D₁ ............. Diameter of the wire or width of the PCB conductor respectively
K............... Antenna Shape Factor (K = 1,07 for circular antennas and K = 1,47 for square antennas)
N₁ ............. Number of turns
ln .............. Natural logarithm function
The actual values of the antenna inductance, resistance, and capacitance at 13.56 MHz depend on
various parameters like:
• antenna construction (Type of PCB)
• thickness of conductor
• distance between the windings
• shielding layer
• metal or ferrite in the near environment
Therefore a measurement of those parameters under real life conditions, or at least a rough measurement
and a tuning procedure is recommended to guarantee the optimum performance. For details refer to the
above mentioned application notes.
141
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