Micore Reader IC Family; Directly Matched Antenna Design

Pulse shape according to ISO14443A
For the antenna design for the MF RC500, the MF RC530, the MF RC531 and the CL RC632 the pulse shape (Q-Factor) shall be checked according to the ISO14443A. For the SL RC400 the shape is described in section 3.7.2

Mifare 4K Proximity Contactless Cards,NXP Mifare 4k Offset Printing Cards,Mifare Classic 4K Card

Table 4: Pulse duration in [μs] compliant with ISO/IEC 14443A
Pulses length t1 t2 min t3 max t4 max
T1 MAX 3.0 0.7 1.0 0.4
T1 MIN 2.0 0.7 1.0 0.4

The time t1-t2 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 times t2 and t4 have to be checked: the signal has to remain below 5% for the time t2.

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: 0.7µs < t2 <1.4µs

If t2 < 0.7µs, the Q-factor is too high (Q > 35). Rext has to be increased.

If t2 > 1.4µs, the Q-factor is too low and the operating distance will be dissatisfying. Rext has to be decreased.

Full parameter design
The full parameter design needs some basic knowledge about RF design. Some parameters, that are fixed in section 3, he re are free to be changed or adapted to special requirements to achieve more flexibility, but of course the design needs some more design steps and detailed design work. The complete Micore reader antenna design principle is discussed with the same reference hardware as shown in Fig 2 .
So the antenna design principle is the same as given in section 3 , and the circuit (see Fig 14 ) its elf is the same as above (compare with Fig 3 ). T o get improve performance the EMC low pass filter is included into the matching.  

HF 13.56MHz Mifare 4K Cards ,NXP Mifare 4k Pre-printed cards,NXP Mifare 4K Printed Cards,NXP Mifare 4k Offset Printing Cards

Fig 14. Circuit of the directly matched antenna

Design requirements
Filtering the supply voltage

Even though it is not required, an EMC filter connected to the TVDD pin as shown in Fig 14 might help to improve the overall performance:
a) It suppresses noise coming from the supply voltage coupling into the analog part of the antenna circuit, and
b) it suppresses harmonics coming from the transmitter to be radiated into the environment (and the rest of the circuit).
A similar filter might be used for the AVDD and even DVDD.

Resonance frequency of the EMC filter
In the basic design the resonance frequency of the given EMC low pass filter is approximately 13.56 MHz to make the design easy.
To get a better performance the resonance frequency of the EMC filter itself should be around 14.4MHz (= fc + 847.5kHz). Proposed values for this EMC filter can be found in Table 6:. Th is should be for two reasons:
a) It increases the signal to noise ratio for the receive signal, and improves the receive performance.
b) It decreases the overshoots of the transmit pulses, and improves the signal quality of the transmit signal.

Table 6: Proposed value of EMC low pass filter components
Component Value
L0 1µH (e.g. TDK NL322522T-1R0J)
C01 68pF each (Ceramic NP0, tolerance ≤ ±2%)
C02 56pF each (Ceramic NP0, tolerance ≤ ±2%)

Of course this resonance frequency requires the EMC low pass filter to be considered in the matching of the antenna.

Matching requirement

In addition to the general design rules based on the Mifare interface principle, as given in the application note in ref. [1] , the basic requirement for the antenna design is reduced to the minimum requirement of the Micore.

The Micore delivers a square signal of

UTxAC ≈± 2.5 V pp (square)5 with f0 = 13.56 MHz and a maximum output current of

ITVDD ≤ 150mA

That means, the TX output toggles between VL = 0V and VH ≈ 5V at a frequency of 13.56 MHz. Tx1 and Tx2 usually have a 180 degrees phase shift, depending on he setting of 5 Additionally there is a mean DC voltage of UTxDC = 2.5 V!   

TX2Inv (bit 3 of the Tx-Control Register). Please refer also to the related Micore datasheet.

Therefore four main requirements can be specified:

I) The TX-output current must not exceed the given limit: ITVDD ≤150mA II) The harmonics have to be suppressed to meet the regulation rules.6 III) The receive signal has to be coupled back into Rx input of the Micore.

Limiting the radiation of harmonics is not the specific goal of this application note. However, the basic guidance provided herein shows, that an easy design is possible that also meets the general EMC rules. The use of a low pass filter directly connected to the TX outputs is recommended.

Remark: As mentioned in the beginning of the full parameter design, some knowledge is required to design a Micore reader antenna different from the recommendation in section

3. This includes the knowledge of the EMC behavior at RF outputs. Usually highly efficient RF outputs generate a great number of harmonics, which have to be suppressed sufficiently to meet government restrictions. Due to the limited Q-factor and some parasitic effects of the passive components the whole antenna circuitry might resonate at some frequencies above 100MHz, or behave like a second (unwanted) antenna at a certain frequency above 13.56 MHz. This has to be checked very carefully during the design.

Layout hint: The most critical part of the antenna circuit is the EMC low pass filter, so the component area of this filter shall be as small as possible, and a proper GND connection of this filter shall be directly connected to the TVSS pin.

With the given voltage and current, the first requirement can be formed into the following (see Fig 15 ):

Ia) The minimum load impedance connected to a TX-output, shall be at least

Z = ZloadTx = 20 .7     

HF 13.56MHz Proximity Cards,Mifare 4K Card,NXP Mifare 4k Offset Printing Cards,Mifare 4k Printing CardsRemark: Of course all the general rules, like maximum power, a correct Q-factor, resonance, antenna size, receive circuit, etc. have to be considered, too.

The mean DC voltage at each TX pin has to be considered, too, but usually a matching capacitor decouples the DC voltage anyway.

Although the TX-output current basically is AC (@13.56 MHz), the specified supply current ITVDD is DC that easily can be measured and checked at the TVDD pin of the Micore continuously during the whole design.

Z always means a complex impedance, consisting of resistance and reactance:

Z = R + jX = Z ⋅ejϕ