Recommended antenna impedance for ST25R39XX family : Specifically for ST25TV02KAD6H3

[vaibh_p]

Hi , We want to design the Antenna for NFC Reader IC ST25TV02K , so any reference design available for Antenna Designing ?

What is the recommended Impedance or more specifically inductance for the antenna ?

[vaibh_p]

Any ST25R39XX Family product specialist there ???Please take it into consideration as early as possible ,

Thanks

[Travis Palmer]

Hello Vaibh_p,

There are some general questions to be answered before.

What is the antenna size of the ST25TV02KAD6H3 and what is the reader antenna size? Are there any space constraints for the reader antenna?

What is your power budget? (e.g. the average current the NFC IC is allowed to draw during constant wave output).

What is the read range you are targeting and what read range are you experiencing now?

BR Travis

[Travis Palmer]

Hello Vaibh_p,

how much does the read range reduce when metal is put close to the reader coil?

Since the metal stick is close to both, two things can happen:

1.) The reader antenna gets de-tuned by the metal.

1a. Measure the matching impedance with and without metal in the field. Either change the matching statically or use AAT.

1b. You can also use a calibration coil and S11 measurement of to the VNA to measure the Q-Factor of the reader system. Does the Q-Factor reduce dramatically?

2.) the metal is de-tuning the tag and shifting its resonance frequency.

2a. Use the same measurement as 1b to observe the tags q-factor and resonance frequency.

3.) Observe the communication, when moving in and out the metal. Who stops first to communicate, reader or tag? If the reader stops, then your system is sensitivity limited, if the tag stops, your system is power limited.

In my opinion, it is very likely that both, tag and reader gets affected by the metal. Therefore both should be designed and matched when the metal is in its final position.

BR Travis

[vaibh_p]

Hello Travis ,

Is there any way to measure the frequency while reading the tag ?

Our ST25R3911B reader and ST25TV tag communicate at 13.56MHz , i.e. at this frequency they give maximum reading range.

But with metal in proximity , the range decreases and in most cases it goes to almost zero.

So we come to know that , due to metal interference the frequency gets distorted , so just want to know what is the distorted frequency.

So the question is , across which two points we have to measure the frequency ?

[Travis Palmer · Accepted solution]

Hello Vaibh_p,

There is no procedure to measure the tags resonance while reading the tag using the NFC reader.

Generally we are using a ISO calibration coil and S11 measurement. Also any other coil - like wire wound antennas can be taken.

You then have to setup the VNA to do a S11 measurement and do a short calibration of the attached antenna. Afterwards you can put it close to the tag and measure its resonance frequency and Q-factor.

Changing the output power, you can simulate a different field strength. Typically the VNA can be changed between 10dBm and -30dBm output power.

You can then also bring the unpowered reader into the field and see how reader and tag are impacting each other or which influence the metal has.

If the metal is always there when reading the tag, then the reader (or tags resonance) can also be tuned with this metal in proximity.

BF Travis