Reference design to connect ST25R3911B to external antenna

[WThal]

Hi,

I was wondering if any of you have tried connecting the ST25R3911B to an external 50 ohm antenna via coaxial cable. My understanding is that this chip has a 50 ohm output and connecting an external antenna is possible although you lose the automatic tuning feature. I'm struggling to work out how the circuit would work in differential mode if you need to use 4 pins on the chip but the coaxial cable has only 2 connections. Are there any schematics somewhere that we could use as a reference when a connection to an external antenna is needed? Thanks

[Travis Palmer]

Hi Will,

The ST25R3911B is capable of driving one differential antenna (using RFO1 and RFO2) or switching between two single ended antennas (one connected between RFO1 and GND the other to RFO2 and GND). Since the ST25R3911B can adjust its driver output resistance (Reg. 27h) it could be adjusted to match a 50 ohm output.

Which RFO/RFI is used by the chip can then be selected by flipping the "rfo2" register.

These antennas could either be normal antennas (like the differential one) or 50Ohm antennas.

50Ohm antennas are already foreseen with damping resistor, series and parallel capacitor (e.g. C_ANT_para) to be tuned to 50Ohm at 13.56MHz. They can then be plugged using a 50Ohm coax cable to the NFC reader unit. In addition the NFC Reader unit requires a matching circuit to match the IC's load impedance seen on RFO1/2 to 50Ohm.

The minimum load impedance for differential use is Zload = 8Ohm. (Datasheet Table 100) Therefore the minimum load impedance for single ended use would be Zload = 4Ohm.

Since the target (50Ohm) is fixed, the matching values do not have to be changed when changing the antenna:

For example:

4Ohm to 50Ohm matching network:

LEMC = 240nH

CEMC = 330pF

Cseries = 680pF

Cpara = 270pF

8Ohm to 50Ohm matching network:

LEMC = 330nH

CEMC = 220pF

Cseries = 470pF

Cpara = 180pF

AAT can be used in single ended Antenna configuration only if no cable is used. The intention is to adjust the antenna resonance when being detuned, which is not the case if a cable is placed between trim circuit and antenna.

BR Travis

[WThal]

Hi Travis,

Thanks for your reply. Does that mean that if cable is needed (external antenna) the only way to connect to the antenna is through a single ended circuit?

[Travis Palmer · Accepted solution]

Hi Will,

Well it depends.

1.) Usually if someone talks about an external 50Ohm antenna, he thinks about an normal antenna with parallel, series capacitor and damping resistor, tuned in a way to show 50 + j0 Ohm impedance at 13.56Mhz (system described above - C_ANT_para, C_ANT_series, R_dmp). Such an antenna can then be plugged with any Z0 = 50 Ohm cable of any length to a reader which is also matched to 50 Ohm. Then the load presented to the reader IC should always be the same. Those systems are also used for higher frequency systems ( ISM, UHF-RFID, ...).

2.) If you have a closed system where always the same cable with the same length and Z0 will be used, someone could just follow the normal design flow as described in AN4974 and use the external antenna + cable instead of a PCB antenna. Any change in the above mentioned parameters (Z0, length, cable manufacturer) could result in heavy de-tuning and malfunction of the reader. Small changes might be corrected by using AAT.

Approach 1. can be used in single ended configuration only.

Approach 2 can be used in either single ended or differential configuration by simply connecting the cable + antenna instead of the PCB antenna. You can already try this using the ST25R3911B-DISCO.

BR Travis

[WThal]

Thanks for the clarification Travis.

[ESayr.1]

When a chip has differential outputs, the usual and customary way to interconnect the signal from driver to receiver is to use 100 ohm twisted pair or controlled impedance differential trace pairs. At the frequencies being discussed, differential twisted pair cable would provide more than adequate low loss transmission losses (insertion loss). If there is a concern with EMI, use shielded twisted pair. The twisted pair approach is very much more efficient that two coaxial cables or for that matter a differential to single ended balun on driver end.

Ed Sayre