How to increase the range of CR95HF receiver on a custom PCB?

[GU.1]

Hi,

I am using the following configuration.

protocol :- ISO/IEC 14443 Type A tags

TimerW value :- 0x58

Modulation Index and Receiver Gain :- 0xD0

Reader Antenna Dimensions

L=28 mm

W =28 mm

No of turns=3 turns

Find attached diagram for designator reference

C11=180pF

C12=180pF

C2=180pF

I am also using an Low pass EMI filter

L01=L02="Part number=742792042"

C01=C02=180pf

I have tried fine tuning with various capacitor combinations.

I got slightly better range of 1.3 cm when replacing C11 and C12 with 1nF capacitor.

otherwise the range is around 0.9 cm.

For Tag i am using Keyfobs tokken type tag.

Please help me understand how to obtain greater range?

Is it by changing the modulation and gain configuration?/tag used?/capacitance values? or should i redesign the antenna.?

I designed a small antenna as per edesign suite to fit CR95hF and antenna in my 30mm x 30mm PCB such that the circuits and components are in the centre of the PCB while Antenna is on the outer circling the circuit.

Kindly help me out.

Regards

Gokulan

[Brian TIDAL]

H Gokulan,

many parameters impact the read range:

• tag antenna
• reader antenna and matching circuit
• etc.

On my X-NUCLEO-NFC03A1 with the X-CUBE-NFC3 firmware, depending on the tag, I can achieve the following read range: 3.2 cm for ISO14443 tags and up to 5.2 cm (for some tags with very good antennas). The X-CUBE-NFC3 firmware uses Modulation Index and Receiver Gain = 0xD3. Most of the tags being used for those read range tests are ID-1 size tag (aka credit card size).

As a first step, I would suggest to measure the read range with the keyfob token on a X-NUCLEO-NFC03A1 as a reference.

See also https://community.st.com/s/question/0D50X00009XkWUk/cr95hf-antenna-design-query for some tips about the antenna design.

Rgds

BT

[GU.1]

I measured the same on a ST25R3911B-DISCO board and got about 4 cm read range with the keyfob token RFID tag.

I don't have the X-NUCLEO-NFC03A1 Nucleo to test the same. Hope the above reading gives you an idea.

[GU.1]

I also tried using Modulation Index and Receiver Gain = 0xD3 on my PCB, but the range was further reduced only.

[Ulysses HERNIOSUS]

Hi Gokulan,

your antenna is not really big. Not sure if you can achieve major improvements. Best would be if you could provide data for some known tag to have a common reference. I could think of: Banking cards (typically ID1 or half of ID1 size) or CLOUD-ST25TA which may have a similar form factor as your keyfob. What is in your possession what we could use as reference?

Regards, Ulysses

[GU.1]

Hi Ulysses,

The reason my antenna is not big is because my client want it in a very small foam factor, this is for an wardrobe lock application. they gave me a maximum PCB size of 30*30 mm. Kindly let me know if i can re-route the PCB with a different antenna to get much higher reception with the same 30 x 30 mm size.

Regards

Gokulan

[Ulysses HERNIOSUS]

Hi Gokulan,

to me (being a software engineers) increasing from 28x28 to 30x30 I would not expect a major improvement. As Brian has performed also some tests below on X-NUCLEO-NFC03 which give 30mm as opposed to your 13mm we think that improvement could be possible. He is discussing internally with people being more experienced with antennas on possible reasons and what you could try.

In general there are always two possible reasons why the read range is limited:

1. The output power is not sufficient to power the tag -> lower impedance / change antenna
2. The tag is powered and answering but the reader does not receive/decode the tag signal. -> Inspect noise and the receive path

To differentiate 1. and 2. you can use an oscilloscope probe where you connect the ground wire to the tip (building a loop) and put it in free air close to the tag antenna. You should be able to see/trigger on the reader modulation (WUPA in OOK) and some 90us later the tag modulation (subcarrier of 848kHz imposed on the 13.56MHz).

Regards, Ulysses

P.S. I assume you are currently using your antenna in free air. Please beware in a wardrobe lock you may have metal parts close by which may have a strong negative effect on your antenna.

[GU.1]

I have uploaded a 3D PDF of my PCB file for you reference, Also i have tested with CLOUD-ST25TA it gave slightly better range of about 13-14 mm compared to the keyfob's less than 10mm.

Also find attached a link of RFID fob's i am using

https://www.digikey.com/en/products/detail/mikroelektronika/MIKROE-2256/8564410

[Ulysses HERNIOSUS]

Hi Gokulan,

side note: I tried to check the mentioned MIKROE-2256 fob. The available data sheet from disties is just a marketing flyer. And on mikroe.com I did not even find this product. Do you have more information on this product?

Regards, Ulysses

[Brian TIDAL]

Hi,

on my X-NUCLEO-NFC03A1 (34x47mm/4 turns/0.8ohm~1120nH antenna), I reach a 30-31mm read range with the CLOUD-ST25TA02KB.

Rgds

BT

[Henry Crane]

Hello Gokulan,

1) antenna tuning consideration

Based on the 3D file you sent to us, tracks and spacing of antenna turns are in a range of 0.6mm which leads to a free space antenna inductance of 500-600nH.

Unfortunately, the ground plane copper area on top and bottom component areas, block the magnetic filed with the result that the antenna inductance is much lower than calculated value.

An electromagnetic simulation with ground plane in the range of your design show a reduction of inductance down to 400-420nH. A proposal for circuit matching component is:

C12=C11= 70pF

C2=300pF.

Please note that the ferrite bead is equivalent to L0=653nH. I kept C0=180pF.

Please proceed to circuit matching input impedance measurement as documented in AN5248 or AN4327 using a network analyzer at 13.56MHz and post the result.

Did you measure the impedance of antenna (disconnecting it from PCB) on your PCB?

2) antenna size/design consideration:

Because of the ground plane at the center of the antenna, magnetic field does not exist at center of the antenna and is concentrated in the space between the ground plane and inner turn. It spreads normally in the outer area of the antenna. The consequence of this field reduction at the center of the antenna is a performance reduction for small tags. Performance should be better with tags which antenna is larger than reader size (getting field from outer area of reader). I guess keyfob in in the range of reader antenna size, so a good candidate for explanation.

An improvement should be to reduce the size of the ground planes inside the antenna to the minimum, particularly in corners and areas where there no components or components less susceptible to 13.56 MHz noise.

3) For CR95HF parameter, 100% modulation depth is mandatory for ISO14443A and I suggest to start evaluation by setting the receiver gain to minimum, and increase it slightly

keep us posted of your progress,

best regards,

Henry Crane, RFID/NFC support team.

[GU.1]

Hi, Henry,

Thanks a lot for your detailed feedback.

3) I have already done the test you have mentioned with old capacitance values I have mentioned earlier and got the best result with D0 i.e., 34 dB gain (9 to11 mm) and worst with DF i.e., 8dB gain when the tag was not even detected.

I will continue my test with the capacitance values you suggested and will share result ASAP.

C12=C11= 70pF

C2=300pF.

Regards

Gokulan

[GU.1]

Hi All,

Once again i thank all off you for your valuable time spend reviewing and answering my queries.

We have decided to re-spin our RFID PCB, as we also suspect as Mr, Henry suggested the ground planes are blocking the RFID signal.

I have designed 4 different boards, attaching a 3D PDF of the same here.

Kindly review let me know if i am making any further mistakes in the routing.

Thanks

Gokulan

[Henry Crane]

Hi Gokula,

thanks for sharing your new designs.

Keeping copper ground plane as shield around components is generally recommended, particularly arounbf cristal oscillator and tuning circuit. Don't know you design rules like clearance around tracks, but it is possible without living too much dead copper, you may try to restore a bit grounf plan on design with components out of the antenna. you may start ground plan on a line B1-R1 (to leave 2mm minimum between groung plan and antenna, up to the PCB border close to the crystal.

For designs with components inside the antenna, you may just put some ground plan to shield the crystal oscillator and CR95HF.

see pictures below, ground plan suggestion in yellow.

best regards,

Henry Crane