What is the recommended strategy to reduce range / gain ?

[Marcelo Barros]

We have a ST25 project running and able to read tags from 8cm distance. However, we would like to reduce this distance. Our idea is to reconfigure only "Receiver configuration register 3". Is this the best way ? Should I change "Receiver configuration register 4" as well ?

Thanks in advance.

[Ulysses HERNIOSUS]

Hi Marcelo,

there are multiple strategies which you can apply:

1) Reduce gain ins receiver configuration register 3 + 4 - I would probably reduce only a bit in receiver config 4 (to give AGC still a chance to operate) and also change the gain in first stage (receiver config 4).

2) filtering after reception by looking at RSSI + gain reduction status values is another option.

3) reduce the output power by increasing the driver resistance (d_res)

In general the simplest approach is probably to reduce the output power. Please beware that this method compared to methods 1+2 has a fundamentally different concept and may behave differently on different tags/tag populations. Reducing the output power will mainly cause tags from being powered in farther distance. With Gain/RSSI based methods they will still answer but the receiver will not pick up their signal.

So depending on your tags (antennas, matchings, power requirements, load modulation strength) the amount of range reduction will vary.

Which device are you using? ST25R3911B or ST25R3916? ST25R3911B gives you many possible steps where ST25R3916 is bit more coarse in number of power reduction steps.

Regards, Ulysses

[Marcelo Barros]

Thanks a lot, Ulysses. Our device is the ST25R3911B. We will test the alternatives before sticking to one solution.