Micore Reader IC Family; Directly Matched Antenna Design

Pulse shape for the SL RC400 design

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Fig 13. Pulse shape for the SL RC400

The I-Code pulse shall be switched to 100% ASK to check the Q-factor as shown in Fig 13. The time t1 describes the time span, in which the signal falls from 90% down below 5 % of the signal amplitude. As the pulse length of Micore is accurate enough, only the time t2 has to be checked: the signal has to remain below 5% for the time t2.

 

Table 5: Pulse duration for the SL RC400
Pulses length t2 [μs]
T1 MAX 8.7
T1 MIN 7.2

To guarantee a correct antenna tuning and Q-factor the following shall be checked:

I) The signal has to fall below the 5 % value.

II) The time t2 shall be in the limit: 7.2µs < t2 < 8.7µs

If t2 < 7.2µs, the Q-factor is too high (Q > 35). Rext has to be increased.

If t2 > 8.7µs, the Q-factor is too low and the operating distance will be dissatisfying. Rext has to be decreased.

Receiving circuitry

When all the transmit design issues (sections 3.2 to 3.7 ) ha ve been taken care of, the reader antenna radiates the maximum possible magnetic field and correctly transmits the TX-data according to the specified protocol and coding. Now the receive circuitry has to be connected and adjusted.

The antenna circuitry should be assembled with the components as given in section 3.1:

 

nFC 13 = (Ceramic NP0, tolerance ≤ ± 10%)
nFC 1004 = (Ceramic X7R, tolerance ≤ ± 10%)
ΩΩ= kR …4.74701 (For determining the exact value: see below)
Ω= 8202R  

As the matching of the antenna (TX-way) provides a maximum of power coupled into the antenna depending on its impedance, the voltage at the antenna (at node T1 as given in Fig 3 ) is slightly different from antenna coil to antenna coil. The Rx input pin (PIN29) of the Micore is high-impedance, so a voltage is coupled back into the Rx-input of the Micore.

So two rules have to be fulfilled:

I) DC-voltage level at the Rx input pin has to be kept at Vmid. (That is why R2 and C4 are required.)

II) AC-voltage level at the Rx input pin has to be kept within the following limit:

1.5 Vpp < VRx < 3 Vpp

If VRx > 3 Vpp, R1 has to be increased. If VRx < 1.5 Vpp, R1 has to be decreased. The Rx input voltage shall be checked with and without a card in the field with minimum

and maximum operating distance.

Remarks: Do not exceed the limit of VRx = 3Vpp AC at the Rx-input pin!! A higher input voltage may not destroy the chip, but results in a receiving failure.

The RX-Input may either be connected to node T1 or T2.