RFID (13.56MHz) tag properties depending on read distance

Question

In a reader design i experiencing some problems with RFID tags. I've evaluated the different tags, and they all are made equally - the tag size, tag chip type, antenna and antenna geometry are perfectly identically, so no problem of dis-alignment or obvious visually identifiable problem with the tags. In a bag of 1000 Tags, the quality is very mixed, ranging from not working at all to medium performance and exceptional performance

The following test fixture was made:

• NXP CLRC663 reference design board (to exclude problems with the custom reader)
• printed holder for the reference board with alignment posts for a slid-able tag holder (to ensure same position over reader)
• printed distance plates to rise tag holder from reader antenna
• testing a large number of finished tags (key fob like), as well as bare antenna coils with tags

All tags reading 8mm or more were accepted as "working". There are around 60% working. The other 40% are reading below 8mm down to zero. The tags deviate in readability distance, for example tags that read >8mm range from some that read to 8.5mm, and some that read to 16mm. Same goes for the below 8mm ones.

Since the reader has designated pins for analog debug, i checked them with a scope, and it clearly can be seen that the less readable tags have a lower voltage signal on the receiver path of the reader. For example a very bad signal is 250mV, and a very strong one is 550mV. Attached is a screenshot of a oscilloscope. The yellow trace is used as a trigger, and is "transmit active" from the reader. The green trace is the correlation result. The dotted cursor line, marks the approx. level of a "bad" tag. The green trace shows the result of a very strong tag (16mm reading distance)

My question now is, what factors, else than physical changes (antenna shape or size, tag coating material etc), can influence the readability of RFID tags. Second i would like to know if there is any reliable option for checking the tag quality before they are coated into a key fob like item.

Answer 1

The problem could be in your IC or with the card construction, and it looks like the tuning. The return signal you are seeing is dependent on two components: quality factor and the resonant frequency, both resulting from your card's antenna and the input capacitance of the the device. A higher "Q" will give you a better range, but only if the resonant frequency is close to the transmit frequency. A lower Q will allow a wider variance of the resonant frequency at the cost of signal strength. Since the IC's input capacitance varies from chip to chip, a trade-off is made to allow for some variance in the resonant frequency.

Although two coils may appear identical, they also may vary because the encapsulant is a dielectric and adds a capacitance component to the resonant frequency equation. So consistency of manufacture is at a premium if no testing is done on a card-by-card basis. If the antennas are indeed identical, then the Q factor must be too high for the variance in input capacitance (and resonant frequency) of the device with that coil design. This is not too likely, since there is so much published material on how to shape the antennas.

One other idea - there is an Android app available from NXP that can detect whether you are using an NXP device or a clone. The clones are somewhat notorious for variation.

Good luck!