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Watching the Clock

A tag's clock frequency is critically important to tag performance. Here's why.
By Chris Diorio
Jan 09, 2006Anyone who has shopped for a PC in recent years is familiar with the "clock-rate wars" that have, until recently, dominated computer marketing. As we all know, clock rate is only one of the many factors that determine a PC's performance. However, it is an important metric—if you buy a PC that is too slow to run your children's favorite video game, you'll hear no end of their unhappiness. Clock rate is an equally important parameter for Gen 2 tags—if you choose wrongly, you will suffer an equally ignominious fate as buying a PC that's too slow. The following information will help you prevent this outcome.

The EPCglobal Gen 2 protocol defines a robust mechanism for communication between readers (interrogators) and tags. It does not, however, dictate how readers and tags are designed, nor how well they perform. In fact, performance is not even addressed in the EPCglobal certification process. Certification ensures that Gen 2 readers and tags operate correctly, but it does not dictate how well they must perform. As such, while certification is certainly important, caveat emptor is still very much advised. The fact is, certified tags may perform poorly if their designers make poor engineering choices.

The selection of a tag's clock frequency is one engineering choice that is critically important to tag performance: Aim too low, and the tag may miss interrogator commands or return data at the wrong rate; aim too high, and it will consume excessive power, shortening read and write ranges.

Fortunately, the minimum clock frequency for a Gen 2 tag—1.92 MHz—can be calculated from purely theoretical considerations. Unfortunately, some legacy RFID tags use a 1.28 MHz clock frequency. If a Gen 2 tag designer cuts corners and reuses one of his existing 1.28 MHz clock oscillators, tag performance will be compromised. Worse yet, a tag designer is actually incentivized to use a 1.28 MHz clock, because the resulting lower chip power translates into longer read range, which can be easily demonstrated to an end user, whereas the correspondingly degraded command decoding and incorrect response frequency can be blamed on the reader or on the "noisy environment."

Tag vendors generally don't publicize the clock frequency their chips use—after all, 1.92 MHz doesn't carry nearly the same bragging potential as a 3.2 GHz microprocessor—nor can end users easily test a tag to uncover that value. So what should you do? The answer is simple. Ask your vendor what clock frequency its tags use. If your vendor asks why it matters, here's what you need to know as a savvy buyer:

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