AN2972 (chapter 4.1): build and connect excactly the self-made single turn loop?

[AWart.1]

A question about the application note AN2972 (chapter 4.1): How do I build and connect excactly the self-made single turn loop?

I want to do the Antenna tuning measurements with a network analyzer as described in chapter 4.1 (see application note in the appendix). I will use an ZVL-6 Rohde und Schwarz and a coaxial cable. I plug this cable to port 1 in order to do a S11-Measurement as descirbed in the AN2972. But I'm not sure what to do with the other end of the cable.

[Henry Crane]

hello,

the other end of the cable should be connected to a single turn coil that will be used as a probe to measure the tag under test tuning frequency.

It can be a self made coil done with a simple wire soldered on a connector you can adapt or connect directly to the VNA cable.

if you make your own probe coil, give it a size and shape close tot he tag antenna you want to measure the tuning frequency.

The probe coil has a low self inductance value. As a consequence its impedance is almost constant other the frequency sweep range you will use. the role of the NVA is to measure the impedance or reflection coefficient of the loop probe (S11). in free pace the loop probe reflection coefficient is close to -1 (the probe is almost a short) and the 20 log (S11) curve is flat @ 0dB.

When a tag is very close to the loop probe, the VNA measures the impedance of the loop probe magnetically coupled to the tag. tag proximity changes the loop probe impedance, particularly at tag resonant frequency where the loop probe real part of impedance increases while imaginary part remains as in free space. This causes the loop probe S11 to decrease and the 20 log (S11) curve shows a drop as shown on fig 20 from the application note.

Instead of using the S11 measurement, it is possible to display the loop impedance in R+jX format and look for the frequency where the loop impedance real part shows a peak.

feel free to tell us if you need more details,

Best regards,

Henry Crane

NFC/RFID technical support.

[AWart.1]

Hello Henry,

thanks for your detailed answer and good explainations. With this measurement I will get the natural frequency of my oscillation circuit. Will it also be possible to determe the quality factor for example? I ask because of this unknown impedance of the loop probe, which is not calibratet.

Kind regards

Aaron Wartenberg

[Henry Crane]

Hello Aaron,

measurement of tag Q factor is done measuring the real part of probe coil impedance real part over a sufficient frequency range to get:

• the frequency Fres at which Re(Zprobe) is maximum (the tag resonant has discussed previously)
• the two side frequencies F1 and F2 where Re(Zprobe) is half the peak value.

the tag Q factor derives from formula Q=Fc /(f2-F1)

you may have a look to the AN5443 which deals with tag performance comparison and particularly inappropriate Q factor comparison. the measurement is presented.

As stated in AN5443, tag Q factor varies with received power. As a consequence, Q factor as to be measured at a well known received power and ST recommends to do it at the minimum voltage operating level. this value is given in the datasheet of most of our products and is measured btween AC0 and AC1 pins of the tag IC.

feel free to tell us if you need to go further in the tag Q factor measurement.

best regards,

Henry.

[AWart.1]

Hi Heny,

thank you for your good answer. Just in order to make it really clear. With "the real part of probe coil impedance" you talked about the coil of my device, not of the selfmade single turn loop probe, right? So the measurement of the real part of probe coil impedance is already this measurement, which is described in AN2972 (chapter 4.1). And with my results below, I can easely determ the quality factor for this specific received power (and reading distance). Am I right or is there anything else to do?

I did this measurement without any external capacitor in my oscillating circuit. The results were as expected (see the picture in the end):

- the resonant frequency is higher than 13,56 Mhz due to parasitic impedances of the board

- in order to reach the optimium natural frequency, I have to add capacitors with 84,4 pF

Kind regards

Aaron

[Fhgfj.1]

I gets the herbal frequency of my oscillation circuit on the page. Will it additionally be feasible to deter me the first-rate factor as an example? I ask because of this unknown impedance of the loop probe, which is not calibrated.

[Henry Crane]

Hello Aaron,

"the real part of probe coil impedance" is the impedance of the self made coil connected to the VNA, not the tag. The two frequencies where the self made coil equivalent serial resistance (Re(Zself_made_coi)) is half the equivalent resistance at center frequency correspond to the frequencies where the current inside the tag coil is the center frequency current devided by sqrt(2). In other word, the tag Q factor measurement is done looking for the particular values reached by the VNA probe coil impedance when the power inside the tag is devided by two compared to resonant frequency.

Variation of tag Q factor with RF power is visible on S11 curve because S11 curve is directly linked to the probe coil impedance which is also dependant on tag impedance and coupling. However, measuring the tag Q factor based on S11 curve with -3dB bandwidth criteria is not right. Indeed, S11 shows the mismatch to 50Ohm reference impedance and not directly the VNA probe coil impedance. As a result, S11 measurement is the right measurement to do with VNA, but you have to go a step further and setup the VNA to display the measured impedance, particularly the real part of the measured impedance) and apply the R/2 method described above (R being the real part of VNA coil impedance at peak). The peak frequency norammy corresponds to the resonant frequency observed on S11 curve at same power condition.

Sorry for the long explanation :-), feel free to ask for more infomation about that.

About your tag tuning frequency: just to confirm, it has been obtained with the the tag IC soldered on the tag antenna right?

calculation of the additional tuning capacitance Cadd to solder in parallel with the tag IC is the following:

Cadd= ((f1/F2)²-1)* Ctag where:

• f1 is the actual tag tuning frequency (25.3MHz in your case)
• F2 is the desired tuning frequency
• Ctag is the ST25XXX Tag IC internal tuning capacitance (called Ctun in the product datasheet).

What is the exact RFID tag IC part number (ST25XXX or M24LRXX)?

Care must be taken when using external tuning capacitance: smd ceramic capacitance generally have a very high Q factor which means that they are close to ideal caps. the more the external tuning capacitance is high the more the tag Q factor will also increase. for instance, using an external tuning cap whith same value as tag IC internal tuning cap multiplies the tag Q factor by 2 because the total tag IC capacitance becomes twice its tuning capacitance value.

tag Q factor impacts the received voltage but also the shape of the RF signal modulation received on the tag IC inputs. to make it simple, a square modulation send by the RFID reader in air will become trapezoidal with a rising and falling time of modulation proportionate to the Q factor. Our tag ICs are designed to meet standards requirement's with antennas designed to offer a tuning frequency around 13.56MHz without external caps. Antennas using an external tuning capacitance in a range o 1, 2, or more time the tag internal tuning capacitance may have a Q factor much larger than "natural" antenna and encounter performance limitation due to the slew rate of received signal.

in such condition, we strongly recommend you to validate the performance of the retuned tag, monitoring the read range in front of the reader.

In case you tag will be reader by various readers , we recommend to redesign the tag antenna to limit the external capacitance to less than 20% of tag IC tuning capacitance.

best regards,

HC.

[AWart.1]

Hi Heny,

Sorry for this long delay. Your advices where really helfpul. We got good results in our measurements. Thank you very much!

Kind regards

Aaron