Passive UHF RFID Tags

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Passive UHF RFID Tags
RF to DC
Getting Started, Getting Data
Tag IC Overall Design Challenges
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Written By Dr. Dan Dobkin with an Introduction by Louis Sirico


This article is from one of the most in-depth technical references I've ever read. It is chapter 5 from “The RF in RFID: Passive UHF RFID in Practice” written by Dr. Daniel M Dobkin printed with permission from Newnes, a division of Elsevier. Copyright 2008.

Louis Sirico Dan Dobkin

I had the pleasure of working with Dan for over a year in contributing content for the RFID Essentials suite of e-learning courses developed by RFID Revolution and he created the YouTag Virtual workshops, which you can find more information about here. I also had the pleasure of interviewing Dr. Dobkin for the premier episode of The RFID Network Cable TV Series where he explained how improbable it is to track a passive RFID tag from satellite. The video is below and is very funny regardless of your technical level. Anyone that spends more than a few minutes with Dan will quickly realize he knows a lot about the RF in RFID. Dan also has a PhD in Physics from Stanford. After reading this chapter, you will understand why we consider Dan a subject matter expert and trusted adviser in our community.

This article includes Chapter 5. You can order a copy of the book here: The RF in RFID: Passive UHF RFID in Practice.

Satellite Tracking

Can You Be Tracked By Satellite Using Passive RFID Tags?

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Power and Powerlessness

Tags identify objects. When the objects are very expensive, the cost of the tags is of little consequence, but their endurance is of great import, since expensive objects, and our interest in them, are usually also long lived. When the objects are cheap, the tags must be cheaper. These are the fundamental dynamics of tag design.

As a consequence, tags that are intended to label long-lived, expensive objects (typically viewed as assets on someone’s books) are usually active tags, with their own radio transmitter and receiver, powered by a local battery. Since battery technology has progressed very slowly relative to semiconductor technology, the key issues in designing a active tag are to minimize the duty cycle—the proportion of time during which the tag is doing something other than sleeping—and to minimize the power required both to support the active state and the sleep or idle state. While these are not trivial design challenges, the technology used to fabricate an active tag is substantially similar to that used in other radios including the reader radio: discrete components and ICs are soldered to a printed circuit board, with the whole attached to a compact antenna and placed within a plastic housing.

Passive tags are mostly meant to identify inexpensive objects, and must thus, submit to an economic asceticism that eschews such luxuries. Conventional batteries are far too bulky and expensive to be considered. A conventional radio transmitter or receiver, with the complex and expensive oscillators, mixers, and synthesizers is out of the question. Only inexpensive, low-speed circuitry and simple logic are permitted to us if the tag is to be powered by the pittance of microwatts available at several meters distance from a reader. Instead of a proper transmitter, a switch to change the impedance presented to an antenna must suffice. A single IC is usually the only electrical &rar component to be placed on the tag, and thus, this circuit must be a custom design solely for its specialized application. The expense of creating such an application-specific integrated circuit (ASIC) implies that only large volume usage can provide an economic return to the company responsible for it.

Between these extremes lie semipassive tags, possessed of a battery but bereft of a radio. To date, such tags have typically been constructed for specialized applications with moderate volumes, such as auto tolling, and use fairly conventional fabrication and design approaches, with special attention to duty cycle just as for active tags.

A greatly simplified diagram of the electrical constituents of a passive tag is depicted in Figure 5.1. The radio signal at around 900 MHz from the reader is converted by the antenna into an alternating current, from which the tag must extract both power and information. The tag must then interpret the resulting data, possibly requiring writing data to nonvolatile memory, and modulate the load presented to the antenna in order to change the backscattered signal returning to the reader.

In what follows, we shall examine a few of the special challenges of designing and manufacturing a passive UHF tag:

  • How is power to be extracted from the high-frequency radio signal?
  • How can we simultaneously acquire whatever data the reader has sent?
  • How do we send back information to the reader?
  • How is the resulting chip designed and fabricated?
  • How is a completed tag assembled from the chip and other parts?

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