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.