Watching the Clock

By Chris Diorio

The bottom line is that tags operating with a 1.28 MHz clock cannot meet Gen 2's error-rate requirements, whereas tags with a 1.92 MHz clock meet the Gen 2 requirements with margin.

To compound matters, EPCglobal's certification process does not actually test whether a vendor's tag meets the Gen 2 symbol-measurement and backscatter-accuracy requirements. Rather, it allows a vendor to self-certify (guarantee by design) that its tag meets the requirements. If a tag vendor performs an overly simplified analysis and blindly picks a low clock frequency, the tag may pass certification, but RFID system performance will suffer.

So why do we believe a tag should use a 1.92 MHz clock, for both reader-to-tag symbol decoding and tag-to-reader backscatter? The answer is simple: The team that wrote the Gen 2 protocol performed an exacting analysis of possible clock rates before finalizing the protocol.

The more pertinent question, though, is this: Does the choice of clock frequency really matter? The answer is a resounding yes. Tags with poor decoding margin, or that respond at the wrong data rate (because they're out of tolerance), simply will not perform as well in the real world as tags that get it right. More insidiously, their performance will "appear" to be better in laboratory testing, because their potentially longer range, achieved by choosing too low a clock frequency, will make them seem superior. And in the field, it's easy to blame the reader rather than the tag for readability problems.

Now, just as a clock rate doesn't tell the whole story of a PC, the same can also be said of an RFID tag. Parameters such as read sensitivity and interference rejection also figure prominently in establishing a tag's ultimate performance capability. If the fundamentals of the design aren't right, however, it's a bit like building a house on sand.

So what should an end user do? Well, for the same reasons no one would ever buy a computer without understanding its most critical attributes, you shouldn't procure tags without appreciating their important characteristics. Simply ask your tag vendor this vital question: What clock frequency does your tag use?

Chris Diorio is cofounder and chairman of Impinj, a fabless semiconductor company in Seattle that makes Gen 2-based RFID chips, inlays and readers. He is also one of the chief architects of the EPCglobal Gen 2 specification. John Schroeter, responsible for technical and product marketing communications at Impinj, assisted in the writing of this article. A detailed analysis and report of the tag clock rate issues summarized in this article is available for download from Impinj's Web site.