ST25R95 poor reception

[Jvan .23]

I have this board with a large (91x119mm) surrounding it. There is circuitry including the ST25R95 in the middle, but it has quite some clearance to the antenna. Matching network is calculated using the ST excel sheets.

Now I can get it to read NFC stickers, but only if I put them right on the matching network. Before I do a big investigation into the antenna and matching network, what are the parameters I can adjust in the ST25R95 that effect reception strength?

I see chapter 5.1.1 in the datasheet " Improving RF performance " mention Modulation index and receiver gain. Are those the two parameters I should be looking at? Are there more?

[Brian TIDAL]

Hi,

the ST25R95 has very little tunable parameters inside the Analog Register Configuration register (modulation index & receiver gain). 

The antenna on your board seems very big. Any reason or such a big antenna?

Rgds

BT

[Jvan .23]

As in: not much options (only the 2 mentioned), or they do not have much effect?

The antenna is so big because it is on the outside of the board. Do you think this is an issue?

[Brian TIDAL]

Hi,

my own feeling is that the 2 params will not have so much effect.

Most of the applications I've seen have an antenna located outside the ICs parts (see for example the X-NUCLEO-NFC03A1 PCB). I do not have much experience with antenna surrounding the ST25R95 IC and other components but my feeling tells me that the ground plane should be reduced as much as possible and only kept on critical components/bus.The ST25R3911B_DISCO design uses a surrounding antenna. I would suggest to have a look on the ground plane being used on it.

Rgds

BT

[Jvan .23]

We just cannot get this antenna construction to work. Would it be possible to connect the ST25R95 to something like a U.FL or IPEX connector so we can seperate the antenna from the board with the RF chip? It seems a balun is what's needed if I look online. Does ST have any appnotes on this, specifically for the ST25 or just in general?

[Henry Crane]

Hi,

no need for a balun because TX1 and TX2 are differential signals. you just need a wire pair to connect the output of the antenna tuning circuit present on the ST25R95 PCB and the antenna. Antenna can be another PCB or flex PCB.

U.FL-IREX cable is a perfect solution. 

Please note that the antenna impedance is impacted by the presence of the cable. So, to calculate the ST25R95 matching circuit components, you'll have to measure the antenna impedance with the cable attached.

best regards,

Henry Crane NFC/RFID technical support.

[Jvan .23]

So just twisted wire pair would work to connect the main PCB with the external antenna? We already have a JST-PH 2P connector on the board, then we'll use that. Is it possible to put the matching circuit on the antenna PCB as to keep the main PCB unchanged?

[Henry Crane · Accepted solution]

Hi,

sorry first for our very late answer.

the antenna matching circuit shall be close to the ST25R95. Else, the twisted pair transmission line impact won't be compensated by the  matching circuit and an impedance mismatch may occur at ST25R95 output. Doing that, RX1 and RX2 signals will taken from matching circuit with the shortest possible connection. If you place the matching circuit close to antenna, you'll need a twisted pair to connect TX1 and TX2 to the antenna matching circuit, plus an additional twisted pair to feed back antenna signal to RX1 and RX2 inputs of ST25R95 with possible disturbance on RX signal transmission because of this additional transmission line.

best regards,

Henry Crane.