External Amplifier for ST25R3911b

[Ak.2]

I designed an amplifier to increase power up to 5 w at the output to achieve larger reading distance,however once it operates at its full power i get no readings and when i reduce the output power to 200-400 mW i get readings however the performance is limited.

The output power is adjusted by changing VDD voltage to power amplifier.

Please help,how to work it out?

[Travis Palmer]

​Hello AK,

The first step would be to check at ~400mW if you can see the tags backscatter on a scope.

Therefore please place a ISO pick-up coil ( you can also do a 2 turns wire wound antenna and tape it to the tag). close to the tags antenna put both in some distance to the antenna. The pick-up coil should then be connected to the high impedance probe of the scope.

I would recommend ~70% of the maximum read range at this power. Which means if you can read the tag up to 10cm @ 400mW, place the tag at 7cm.

Then slowly increase the output power of your amp. Try to observe the difference between read and no-read condition. e.g. The backscatter gets smaller or the backscatter disapear. Please don't hesistate to share scope shots.

Also you can have a look at single sine wave. Does the Amplifier limit the the Amplitude?

Where have you placed your RFI connection? Before or after the Amp?

Please let me know, if you have any questions.

BR Travis

[Ak.2]

Thanks for prompt reply

I placed RFI connection across a coil right before the output ,and this perhaps is the wrong as per ST25 IC perspective.

I used a reference design from TI which constructed around TRF7960 RFID chip.

See pictures below.

Will upload response of tag tomorrow as described.....

[Travis Palmer]

Hello AK,

I think one problem could be, that the two RFI's should pick up a differential signal, but there is only an inductor in between.

I would recommend to check the two signals on J1 (RF1 vs. RFI2) using a differential or two high impedance scope probes.

One (quick) solution could be to simply remove R3 and keep RFI1 open.

Also you should check the RFI input signal. It should not exceed 3Vpp on any RFI.

BR Travis

[Travis Palmer]

Hello A.

I briefly looked through the app note. There are some test points mentioned before and after the Filtering. In the Appnote also scope shots are mentioned. Have you compared your signals with the given scope shots? it would be good to see, if there is a difference.

The ST25R3911B offers a feature, to look at internal receiver signals on CSI and CSO. You can for example output Analog and Digital AM signal as described in the datasheet chapter "Test Access".

You could then see, if something is happening to the back scatter. The app note also mentions another PI filter in the receiver path (C119, L118, C118). Have you tried to populate it?

br Travis