Micore Reader IC Family; Directly Matched Antenna Design

Required design steps
1. Design a coil, measure L and R or L and Q .
2. Calculate the resonance capacitors to design a resonance circuitry together with the coil.
3. Tune this resonance circuitry together with the EMC low pass filter to the required impedance
4. Connect the resonance circuitry to the Micore output, check the ITVDD and if necessary retune the components for optimum performance.
5. Check & adjust the Q-factor.
6. Check & adjust the receive circuitry

Impedance Matching & Resonance
The principle of the antenna matching is the same as shown before in section 3.6, but now the EMC low pass filter has to be included into the matching and tuning procedure.

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Fig 16. Matching the antenna

So based on the same design the whole circuit as shown in Fig 16 has to be matched to an impedance of approximately 40 Ω between Tx1 and Tx2, using the following values as proposed in Ta ble 6:

Q-factor
In any case of designing a Micore reader antenna, the Q-factor has to be checked. The overall Q-factor of a Micore antenna œ supporting higher bit rates œ is limited to
Q ≤ 22
and shall be checked in principle as given in section 3.7 . Th is value is valid for mifare and I-Code (proximity) designs.
The lower Q factor compared to a standard mifare reader design is related to the pulse shape requirements of higher bit rates according to [8] . In addition to that the lower Qfactor increases the overall stability and the robustness against environmental changes.
So in addition to section 3.7 for higher bit rates the relevant pulse shapes shall be checked, too. Refer to the application note [2] for details.

Additional design hints
Antenna functionality
In each of the design steps the three functions of a reader antenna should be considered:
Transmit power: The radiated magnetic field has to be maximized considering the radiation and datasheet limits, especially the limits for the radiation of the harmonics (up to 1GHz).
Transmit data: The coded and 10% or 100% ASK modulated data signal has to be transmitted in a way, that every card is able to receive it. The signal shape and timing
(i.e. the Q-factor) has to be considered.
Receive data: The card‘s answer has to be delivered to the receive input of the Micore considering the datasheet limits.
If one of these functions is not completely provided, the overall function of the antenna is disturbed or at least the performance is reduced. So if a supposed overall performance is not achieved with a specific design, each of these 3 functions shall be checked separately.

Layout
Even though this document does not replace any relevant RF design documents and it does not cover EMC related topics in detail, some general recommendations can be given to simplify a proper design.
The Micore IC itself drives the 13.56 MHz carrier with a signal, which is almost a square signal. This leads to many harmonics up to the GHz range, which have to be suppressed sufficiently to meet all the relevant EMC regulations8 . Th e most critical part of the overall analog layout is the circuits directly connected to the Micore IC: the EMC low pass filter as well as the connection of the supply voltage pin TVDD.
So on one hand an additional EMC filter for the supply voltage might be usefull.
On the other hand the layout of L0 and C0 shall be considered carefully. The overall layout and placement area of TX1, TX2, L0, C0 and TVSS shall be kept as small as possible. A proper and short GND connection is required! One proper GND plane is recommended!
A 2-layer board reference layout is shown in Fig 17 and Fig 18 with the corresponding schematic in Fig 20 and Ta ble 7: .

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Table 7: Bill of Material
Part Value Remark
C20 10pF NPO
C10 100pF NPO
C11 n.a.  
C16, C17 15pF NPO
CA4 10µF  
R8 820Ω  
R9 2.2kΩ