Bamboozled by Rx voltage - NFC antenna resonance (in a Samsung Li-ion battery) - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: June 02, 2019, 04:48:56 pm
content: The tale:Found two old Samsung Li-ion batteries in a recycling jar for batteries. They are two different models (B600BE/2600mAh/9.88Wh/3.8V and EB-BG900BBE/2800mAh/10.78Wh/3.85V) yet both of them have an internal antenna sticker for NFC (Near Field Communication). The NFC sticker is destined to be the NFC antenna for the whole phone, in both NFC Rx and Tx modes, so its terminals are available at the battery connector. The sticker is not just a passive NFC tag.Googled a little about NFC battery antenna and seen a couple of videos with dudes peeling off the antenna:- it is not just a flexible PCB sticker with a simple coil, it's a fully tuned antenna with some capacitors near the battery connector (no continuity when the antenna terminal is measured at the battery connector)- 13.56 MHz/50 ohms- BW about 2 MHz (sometimes AM modulated with another 850KHz carrier for digital modulation schemes, that's why the 2 MHz bandwidth)- expected power for Tx mode 100...200mWThe goal:- curious about RF in general, fiddle with such an antenna mostly just to learn more- first experiment, to measure the frequency response of such an antennaI don't have a VNA or a spectrum analyzer, but I have a dual channel 100MHz DDS generator (Rigol DG4102) and a 4 channel 100MHz digital oscilloscope (Rigol DS1054Z).The setup:- generator CH1 (50 ohm output) set on 13.56 MHz, sinus, 2.5Vpp (on a 50 ohms load)- a 75 ohms cable (not 50 ohms cable, it's a BNC video terminated in 2 aligator clips) with shorted aligators to form a Tx coil- Tx coil created by the shorted aligators is about the same size as the battery, and sitting around the battery sticker Rx antenna- oscilloscope probe with a DYI 50 ohms feed-through terminator on the oscilloscope side- oscilloscope probe connected to the NFC Rx antenna (a DYI battery connector made with 2 pogo pins)The results:After sweeping the frequency back and forth, found out one battery antenna has a maximum RF Rx voltage at about 15MHz. I guess this is close enough for such a sloppy setup, but the other battery antenna has a maximum RF voltage at about 32MHz!!The questions:1.TBH this is my first antenna ever, so I have zero practical experience, only some theoretical knowledge. Is it plausible to see the resonance at 15 MHz instead of 13.56 MHz for a NFC antenna (considering the amateurish measuring setup)?2.Why the second battery antenna has a maximum RF Rx at about32 MHzinstead of the expected 13.56 MHz? Is it possible such a big difference in the resonant frequency just because of my sloppy setup? Could that be a defective antenna, or am I doing something completely wrong with the measuring setup?

author: erikka
author_rank: Regular Contributor
timestamp: June 02, 2019, 05:34:38 pm
content: Can you post a picture of the setup?

author: RoGeorge
author_rank: Super Contributor
timestamp: June 04, 2019, 04:50:47 pm
content: In the first pic, the red+black loop made by the shorted alligators is the Tx antenna, and the Tx signal is coming from a 50ohms DDS generator, through a 75ohms BNC, then to the alligators cable loop.Underneath the red+black Tx loop is the phone battery. Inside the phone battery is the NFC antenna under test, in this setup used for Rx only.The Rx voltage is going through the x1 oscilloscope probe, and it is terminated on the oscilloscope side with a feed-through DYI 50ohms terminator.Note there are two batteries in the first pic, the square-ish B600BE, and the long-ish one, EB-BG900BBE.I was expecting to see a maximum voltage induced in the Rx antenna at 13.56MHz, because 13.56 is the central frequency for the NFC specification, and the NFC antenna inside the battery is suppose to be a fully tuned 50+j0 ohms antenna. Not sure about the 50+j0 tuning expectation, but this was my understanding so far.Instead of being at 13.56MHz, the peak is at 31.70MHz for the square-ish battery's antenna, and a 16.25MHz for the long-ish one.See attachments 2 and 3 for a frequency sweep from 0 to 100MHz, so 10MHz/horisontal division. The yellow trace rising edge marks 0Hz, and the yellow falling edge marks the peak. The blue trace is the Rx voltage.Square-ish battery's antenna peaks at 31.70MHz.Long-ish battery's antenna peaks at 16.25MHz.I've tried today again, this time using for the oscilloscope's probe the short spring pin instead of the GND alligator wire, and the resonant frequency doesn't changed much.A slightly bigger difference happens if I replace the Tx side with a 50 ohms BNC cable and a wire loop.Since this Tx/Rx antennas ensemble is in fact a pair of strongly coupled coils, I want to get rid of the uncontrolled Tx antenna.Next plan is to make a directional coupler and feed the Tx signal to the battery's NFC antenna through the directional coupler, in order to properly characterize the NFC antenna (this time in Tx mode).

author: erikka
author_rank: Regular Contributor
timestamp: June 06, 2019, 11:40:01 am
content: The application part in the phone may contain some impedance matching causing your 50 ohm drive to be imperfectIts easy to lower the resonance by having some added C. Not sure what happens in the driver/receiver part in the phoneAnd the difference is only a couple of dB so it would not matter in a near field application I guess

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2019, 12:10:50 pm
content: Right, the response of a network is only correct when the loading is correct. With something other than 50 ohms (or more specifically, other than what it was tuned for), it can be... whatever!Tim

author: ThomasDK
author_rank: Regular Contributor
timestamp: June 06, 2019, 06:07:16 pm
content: It's not an antenna, so don't expect 50 ohms.The NFC coil and the capacitor creates an LC resonator. Impedance at resonance will be infinity, not 50 ohms.The resonance frequency will be lower with the phone case near the coil, so they are usually tuned a bit high.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2019, 01:00:55 am
content: If you can show me a tank with infinite impedance at resonance, I have a bridge I would like to trade for it!Tim

author: ThomasDK
author_rank: Regular Contributor
timestamp: June 07, 2019, 08:48:39 am
content: Quote from: T3sl4co1l on June 07, 2019, 01:00:55 amWell, okay... In the real world inductors and capacitors are lossy.Maybe with superconducting inductors and capacitors in a perfect vacuum... And nothing to couple with it

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2019, 08:23:54 pm
content: SC resonators (used in particle accelerators) have a Q in the 10^7 range, quite large but still not infinite! The loss mechanism is quite enigmatic actually.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: June 07, 2019, 09:03:16 pm
content: If we are to go to that slope, then there is no infinity in the real physical Universe as we know it so far. Same with the number zero. We treat the physical absence of something as the mathematical zero, yet the nature seems to distinguish between absence and zero, so no zeroes in the real world, just absence.But if we talk about an ideal LC parallel tank, then we can have our infinite impedance at resonance. If this is not the case, I will like to learn more about it.Yes, infinite impedance will never happen in real life, but still, it's a valuable concept in grasping the mess of reality.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2019, 09:40:40 pm
content: Indeed, one can never measure zero nor infinity, only error bars around them.This is the deeper truth to the universe: we can never be certain about anything, but we can get close enough to feel confident about something. One should not be afraid of the unknown, but rather freed by it!Incidentally, infinity is bounded above and below, because it wraps around to negative values. Technically, the complex numbers are a closed ring, where infinity (any infinity, positive or negative, in any direction) is a single connected point. This is easily demonstrated on the projective plane z <-- 1/z, where inputs near zero give outputs around infinity.This in turn is measurable with the connections of resistors for example, which can be made in arbitrary values, positive and negative (within certain limitations). If we connect R and -R in series, we get near zero; if we connect R and -R in parallel, we get near infinity. If we adjust one of those resistors, continuously, over some small range, we can trim the result around zero or infinity, respectively.Tim