Micore Reader IC Family; Directly Matched Antenna Design

Simplification due to symmetry
Due to the symmetrical circuitry of the whole antenna for one coil it is required that:

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As this symmetry is a general requirement for the proper function of the antenna, it can be used to simplify the whole circuitry. In the following only the upper half of the complete antenna circuit is used to calculate the matching network and the external resistor(s). The lower half looks and behaves exactly the same.

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Fig 7. One half of the symmetrical antenna

Although this simplification can be done in theory, the practical measurement cannot be done with this simplified model, because of the coupling between La and Lb. So all the impedance measurements have to be done with the complete antenna circuit with both sides (between T1 and T2 or A and B) and NOT between T1(or A) and GND or T2 (or B) and GND.

The external resistor
Together with the general definition of the Q-factor of the coil HF 13.56MHz Mifare 4K Cards ,NXP Mifare 4k Pre-printed cards,NXP Mifare 4K Printed Cards,NXP Mifare 4k Offset Printing Cards

the overall resistor (that specifies the overall Q) HF 13.56MHz Proximity Cards,Mifare 4K Card,NXP Mifare 4k Offset Printing Cards,Mifare 4k Printing Cards

and the requirements for   Q=304

the external resistor RSa and RSb can be estimated: Mifare 4K RFID Cards,NXP Mifare 4k Offset Printing Cards,Mifare 4K Proximity Cards,NXP Mifare 4k Pre-printed cards

Neglecting the influence of all the other components on the Q-factor, this calculation only gives an estimation of the later used value of RS, but this estimation is necessary to do the calculation of the matching capacitors in the next step.  The Q-factor has to be checked and adjusted later on as described in section 3.7 to get an exact value for the external resistor.

 The parallel and serial capacitors
With these values of the coil La, R (including the external resistor) and the required impedance Zant, now the parallel capacitor C2 and serial capacitor C1 can be calculated:

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Although the estimated external resistor may vary from the really needed one, the calculated capacitor values should be taken as start values for the following tuning procedure. 

Tuning procedure

With the given and calculated values for L, RSa, RSb, C1a, C1b, C2a and C2b the complete antenna circuit has to be tuned to equalize measurement and calculation uncertainty and tolerances. Fig 8 shows the tuning procedure. C1 and C2 each mean both the corresponding symmetrical values. The use of an impedance analyzer is recommended, but there is also an easy method described in the appendix 6.2 to tune the antenna circuit (without impedance analyzer) using only an oscilloscope and a signal generator.

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Remark: It is useful to start the tuning procedure with smaller capacitance values than
calculated. On one hand measuring the antenna coil disregards stray capacitance also
influencing the tuning. On the other hand it is much easier to increase capacitance (by
adding extra capacitors) during the tuning procedure than reducing the capacitance.
The Excel sheet can also be used to check and “get a feeling” for the relation between
capacitor value changes and the impedance changes. A few pF may change the whole
matching!

Checking the Q-factor and output current
As the Q-factor has a direct influence on the edges of the modulation shape, this should be used to check the Q-factor.

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Fig 10. Setup to check the Q-factor

An oscilloscope with a bandwidth of at least 50 MHz shall be used and two probes shall be connected as shown in Fig 10:
CH1: Form a loop with the ground line at the probe to enable inductive signal coupling. Hold the probe loop closely above the antenna.
CH2: Connect probe to the MFout signal at Pin 4 of the reader IC, Trigger source = CH2.
The MFoutSelect register (26h) has to be set to
“2″ (Modulation Signal (envelope) from internal coder, Miller coded)
or “3″ (Serial data stream, not Miller coded)

An example of these test signals is shown in Fig 11.

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Fig 11. Test Signals of the Micore Antenna

It is recommended to check the pulse shape and compare the scope plot to Fig 12. The related values are given in Table 4:.
Remark:
The absolute measured voltage in CH1 depends on the coupling (= distance) between the probe loop and the reader antenna.
The influence of the coupling on the shape can be neglected.
The complete antenna tuning and Q-checking is done without any card (unloaded).
However, the complete PCD has to be checked against the ISO/IEC14443 (see ref [8]) using the reference PICCs acc. ISO10373-6 (see ref [9]). That requires e.g. to check the pulse shapes unloaded and under load conditions (using the  eference PICC as defined in ref [9]).