CR95HF Antenna Design issues.

[achou.10]

Hello All,

I have some CR95HF Antenna Design Queries

1) what is actual difference between in performance of 424nH antenna and 1120nH antenna used in  M24L-DISCOVERY kit and NUCLEO-NFC03A1 respectively?

2) In CR95HF-A demo Board Antenna impedance design guidelines document,  mentioned formula to calculate antenna inductance using frequency 13.56MHZ

they have given Ra=0.5 Ohm and La=36.6 * w

where w= 2 * pi * f

so w= 2 * pi * 13.56 = 85.2

so as per calculation La= 36.6 * 85.2 = 3118.3

but they have give La= 430nH

can you explain this ?

3) In CR95HF chip datasheet I have read that ZOUT differential impedance between TX1 and TX2(1) 27 Ω and Small signal differential input resistance (Rx1/Rx2) 100 kΩ , what this actual means?

Is this parameters taken in consideration without onboard mounting or with components?

because PCB manufacturer considered it without mounting components.

from above parameters TX1 and TX2 and (Rx1/Rx2) I have checked all matching circuit component in M24L-DISCOVERY kit and NUCLEO-NFC03A1 board both have same (Rx1/Rx2) component 330 Ohm but TX1 and TX2 matching components are different on both the circuit,

I want to know why it is different? is it because of different antenna inductance or anything else?

[Henry Crane · Accepted solution]

​Hello,

here are some some answers to your questions here above.

1) Antenna of CR95HF demo board from M24LR-Discovery kit is 26turns and 424nH. the antenna from Nucleo-NFC03A1 has almoste the same size but an impedance of 1120nH. 

Becasue of its lower impedance firts antenna sink a higher current and allows to generate a higher field strength. However Q factor of the antenna circuit (antenna + matching circuit) is a bit higher causing a lower bandwidth. The consequence is taht this board is suitable for energy harvesting and communication in long range in ISO15693 protocol. I works also for ISO14443 but receiving bandwidth is not optimized causing less performances than reachable. 

With a higher impedance 4-Turn antenna of Nucleo-NFC03A1 shield leads to a higher bandwidth in the system with optimized performances in ISO14443. performances in ISO15693 remains very good but a bit lower because the generated field strength is a bit lowered. For an application supposed to work in both ISO1443, ISO15693 and felica, I recommand to use such antennas with impedance around 1µH. The incresed number of turns (2 compared to 4) also participate to the performance by improving the coupling factor between tag and reader.  

2) it seems to be a typo in section 2.1.1 of the document you are talking about: ther is a mix between the serial equivalent inductance La=420nH and the corresponding reactance which is 420nH*2*pi*13.56e6=36 Ohm. this document mainly deals with antenna tuning without use of EMI filter. Please note tthat there is a second document witha new assuciated calculation sheet called AN4327 Application note CR95HF transceiver antenna tuning circuit with EMI filter . It covers tuning circuit calculation with and without EMI filter.

3) as described in the application note , TX1 and TX2 RF outputs corresponds to two 13.56MHz square signal generator working in opposite phase. Each square signal generator has an output impedance of 13.5 Ohms. The total differential output impedance between TX1 and TX2 is then  13.5x2=27 Ohms. This is for the RF output circuit.

CR95HF receiver input pins (the one used to probe antenna voltage and decode tag response signal) are RX1 and RX2. Differential input input impedance between RX1 and RX2 is equivalenet to a a 22pF capacitor in parallel with 80 kOhm. This elements have te be taken ino account during antenna matching circuit calculation because it comes in parallel to the antenna.

TX1-TX2 RF outputs differential output impedance and RX1-RX2 differntial input impedance are intrinsic to CR95HF:

• TX1-TX2 output impedance gives a criteria for antenna tuning circuit calculation (AN4217 discusses this point)
•  RX1-RX2 input impedance takes part of the tuning circuit.

The 330 Ohm resistors put in series between respectively the RX1 pin and the antenna, and RX2 pin  and antenna are used to attenuate the antenna signal so that the voltage between respectively RX1 and GND and RX2 and GND never exceeds 8-9Vpeak (which corresponds to 16-18Vpeak between RX1 and RX2). These resistors are external components, not part of the RX1-RX2 inout impedance of CR95HF.

These resistors are commun to NUCLEO-NFC03A1 and M24LR-Discovery; all other external components (EMI filter components and antenna serial and parallel capacitors) are different because antenna impedance is different.

I strongly recommands you to read AN4327 and download associated excel sheet to calculate and view impedance curves.

feel free to tell us if you need more calrifications.

best regards,

Henry Crane