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

[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 · Accepted solution]

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

[GU.1]

Dear Henry Crane,

Thanks a lot for your review.

I will update the changes you suggested in my design and get these PCB's fabricated.

I will share the results after testing.

Thanks and Regards

Gokulan