![]() ![]() The variable capacitor is shown at top centre, and the chip is sitting in its pick-up coil on the left. The Simple But Clever Tech In Your Bank Card Here’s what’s going on inside your bank card. In practice most readers provide more than enough energy so the tag can still be energised despite the resulting inefficiency, but for any NFC tag system to work at maximum efficiency it should have both reader and tag adjusted to resonate at the 13.56MHz frequency of communication. ![]() In writing this article I found that both cards and readers appear to resonate anywhere between 13.5 and 15 MHz, with the majority being measured at about 14 MHz. Immediately, the VNA pinpoints one of the problems inherent to mass-produced NFCs, that the resonant frequency is rarely exactly on 13.56 MHz. The VNA shows a clear SWR dip for an NFC tag This works well with most reader coils and with lower power NFC tags that simply contain a memory chip, but my VNA doesn’t provide enough energy to measure those tags with higher power integrated circuits such as bank cards, a public transport card, or my passport. When I place the pickup coil over an NFC tag, I’m rewarded with a sharp peak on the VNA from infinity down to near 1:1 SWR. I’m loosely coupling it to the NFC antennas I’m testing by means of an RF pickup coil, one turn of wire about 10mm diameter soldered to a coaxial connector and secured with a bit of glue. For the purposes of this article I’m using a NanoVNA because of its extreme convenience, and I’ve set it to measure SWR on port 1 with a sweep between 10 MHz and 20 MHz. The RF properties of the antenna can be explored with instruments as simple as a signal generator and an oscilloscope, or if you’re a radio amateur old enough to have picked one up, a dip meter. There’s nothing complex about the antennas, indeed it’s easy enough to make one yourself by winding a suitable coil and tuning it with a small variable capacitor. Very Few NFC Tags And Readers Are On The Same Frequencyįor the majority of tags likely to be experimented by Hackaday readers the RF frequency is 13.56 MHz, and the RF emissions are supposed to be in the magnetic field plane rather than the electric field. The reader sends out pulses of RF which is maintained once an answer is received from a card, and thus communication can be established until the card is out of the reader’s range. ![]() Both reader and tag achieve this through an antenna, which takes the form of a flat coil and a capacitor that together make a resonant tuned circuit. “NFC” stands for “Near Field Communication”, in which data can be exchanged between physically proximate devices without their being physically connected. The tags contain chips that are energised through the RF field that provides enough power for them to start up, at which point they can communicate with a host computer for whatever their purpose is. This article will attempt to demystify what is probably the most likely avenue for an NFC project to have poor performance, the pickup coil antenna in the reader itself. Yet they’re not always easy to get right, and can often give disappointing results. NFC tags are a frequent target for experimentation, whether simply by using an app on a mobile phone to interrogate or write to tags, by incorporating them in projects by means of an off-the-shelf module, or by designing a project using them from scratch. ![]()
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