Secret Sauce - Passive RFID Tags
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Secret Sauce Part 1

One question that I get quite often is “How is it that you can make a tag that can withstand crazy amounts of radiation while no one else can even come close? How is that possible?”

It’s that last part that gives away what people are really thinking. “How is that possible?” It’s a reasonable question given that there has been serious demand for radiation-resistant RFID tags for as many years as there’s been RFID, but no one has managed to pull it off. Most people I meet think it’s impossible. So when someone asks how is it possible, what they’re really saying is, they don’t believe it.

But once you understand what’s going on inside the tag, it’s really quite simple. And perfectly believable. So I’m going to explain here just how the Tego tags pull off this technological feat that was once thought impossible.

Before I get to the workings of the internal circuitry and explain why it’s so indestructible, let’s take a step back and talk about what we mean by radiation resistant. Or temperature resistant or long-life for that matter. And before we can do that, we need to talk about chips and memory.

All RFID tags include two critical pieces: an electronic chip and an antenna. There’s no battery or other power source because we’re strictly talking about passive tags here. The antenna gathers the RF signal so that the chip can communicate with the reader. The chip contains all the electronics that run the communications protocol and basically do all the things that the reader tells it to. And the chip also contains memory, which is where data is stored.

Whenever we talk about memory, it’s important to realize that there are two fundamental types. The most common type of memory needs to be powered to hold on to the data it’s currently storing, so it is sometimes referred to volatile. The standard random access memory or RAM in your computer is of the volatile type. When you go to buy a new computer and see it has 4 Gig of internal memory, they’re referring to the volatile internal RAM. When you turn off your computer, any data stored in RAM is lost.

In contrast, there is also non-volatile memory, which can retain its stored data even after power is removed. The hard disk in your computer is a form of non-volatile storage, although it is magnetic storage instead of the electronic variety like RAM. Still, when you look at the new computer and see it has 400 Gig of disk space, that’s the non-volatile memory you have available. When you hit save, your document gets moved to the hard disk so that it’s still there after you power down.

In addition to hard drives, we also have electronic versions of non-volatile storage. Almost all of them use a memory technology known as flash memory, the structure of which we’ll get to in a moment. For now understand that flash memory is an electronic memory circuit just like RAM, except that it is non-volatile. The compact flash card or SD card you used to put in your digital camera before you bought a smart phone is made using flash memory. And these days we even have solid-state disk drives, which replace the magnetic spinning disk with flash memory. As you can see, non-volatile storage is pretty important to have around, and electronic versions can be pretty useful.

Passive RFID tags have a particularly critical need for non-volatile memory. The tag’s only source of power is from the over-the-air RF energy provided by the reader. When the reader shuts off or leaves the field, the tag is left unpowered. Any data that is needed in the future, like the all-important ID number or that history record you just wrote to User Memory, had better be stored in non-volatile memory or it won’t be there when the next reader comes along.

Why all this talk about memory when we’re supposed to be discussing radiation resistance? Because the memory is the hard part. When it comes to large doses of radiation, or other harsh insults like high temperature or long lifetimes, there are three levels of resistance we can talk about. The first and easiest level to attain is simply that the electronics remain functional. You have some piece of electronics equipment, let’s say a wristwatch. You turn it off, expose it to radiation, and when you turn it back on, it’s fully functional. You may have to reset it to the correct time, but it still works.

The next level of radiation resistance is that the item is still fully functional after being irradiated, and any data stored in the device remains intact. If you exposed your computer to radiation and afterwards it still worked but your hard drive and boot ROM had been erased, you wouldn’t be so happy. So, in many cases, this can be a pretty meaningful level of radiation resistance. Functionality often doesn’t amount to a hill of beans without stored data also surviving. Which is exactly the case with passive RFID. For a long time, this was an unsolved problem in electronic technology without resorting to heavy duty shielding. It’s pretty exciting to think that we now have a solution that works for passive RFID, and understandable why so many people find it unbelievable.

The last level of radiation resistance is for the electronics to remain fully functional in the presence of radiation. This is the holy grail for rad-hard electronics. Thankfully this is not something we need for sterilization applications as it’s not really a solved problem yet. We don’t need to use the tags during sterilization, we just need the tags and their data to be usable afterwards. If you need this level of radiation resistance, contact me and I’ll put you in touch with some smart people who are working on the problem (and barely making any progress).

Ok, we’ve covered a lot of ground here. We’ve covered the meaning of non-volatile memory and why it’s so important for passive RFID tags. And we’ve talked about what we mean when we say radiation-resistant; that it’s not enough for the electronics to survive but the stored data needs to survive as well. Let’s call that Part One. In Part Two, I’ll explain how flash memory works and why it’s a poor choice for these harsh environments. Then in Part Three, I’ll explain the magic inside the Tego Tags and how they manage to do the impossible.

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