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Jul 29, 2016

Battery Assisted Passive (BAP) Tags – Do You Know Your Reader Receiver Sensitivity?

中文版 Chinese version

One of my customers in Taiwan is developing battery-assisted passive (BAP) tags. He called me recently and asked why the read range that they reach with their RFID reader is only a quarter (1/4) of the distance that they measure with their Tagformance RFID measurement system. I answered him with another question: “Do you know your reader receiver sensitivity…?”

What Is a BAP Tag?

A BAP tag has an on-board battery to power its IC, but like a passive tag, it does not have an active transmitter. BAP tags are generally used to reach longer read ranges than what passive tags can provide, or for logging some physical quantity when a reader is not present. As known, the typical limiting factor for the read range of a passive tag is the forward link. In other words, the read range of a normal passive tag is determined by how far the passive tag can be powered or activated, i.e., the tag sensitivity is the limiting factor. Therefore, by default designing the passive tag to receive power from an on-board battery as a BAP tag, read range could be increased.

However, since the on-board battery is only used to power-on the RFID IC or to increase the BAP tag sensitivity, the battery does not really increase the tag backscatter power. As a result, the return link will become the limiting factor for the read range of a BAP tag. In order to fully realize the maximum read range of a BAP tag, the reader receiver sensitivity becomes crucial.

BAP – Battery Assisted Passive – Tag

The Performance of a BAP Tag

When evaluating the performance of an RFID tag, the starting point is usually measuring the sensitivity of the tag as a function of frequency. The graph below shows the Tagformance Pro’s Threshold Sweep measurement results of one BAP tag. As can be seen, the theoretical read range for this BAP tag is close to 37 meters at 930 MHz. That is a lot; the read range of a good passive tag is around 10 meters.

Sensitivity of a BAP RFID Tag Measured with Tagformance Pro

But the forward link read range above is only the theoretical upper limit of the read range that can be reached. Below we use the Tagformance Pro’s Read Range measurement functionality to test the BAP tag with different reader parameter settings. The radiated power is set at 2W ERP. The yellow curve below shows that the read range is about 19 meters at 930 MHz if the reader receiver (RX) sensitivity is -85 dBm.

Read Range of a BAP Tag Measured with Tagformance Pro; 2W ERP & -85 dBm Sensitivity

By changing the reader sensitivity in the Tagformance software, we can see what reader sensitivity would be needed to reach the theoretical read range maximum. This situation is shown in the yellow curve below. The reader sensitivity required to reach the 37-meter read range is -97 dBm.

Read Range of a BAP Tag Measured with Tagformance Pro; 2W ERP & -97 dBm Sensitivity

From my experiences, it may not be easy nowadays to find a reader with the RX sensitivity of -97 dBm. Therefore, in order to reach the read range of 37 meters, I have suggested my customer redesign the BAP tag’s antenna to make the tag having stronger backscatter power and use a higher sensitivity reader if possible.

Reader receiver sensitivity is getting more attention in the RFID market after the increase of tag sensitivity both in BAP tags and normal passive tags. This means that the limiting factor for the read range is the return link. Receiver sensitivity is the key to optimizing the read range. Interestingly, most system integrators and even UHF reader suppliers do not know how to measure the receiver sensitivity.

Jul 08, 2016

Downgrading Your Spec Does Not Make a Quality Tag – Thoughts About RFID Quality

中文版 Chinese version

I work as the CEO of Voyantic, a company that specializes in RFID test and measurement equipment. Since our systems are used by hundreds of companies around the world, we often come across cases where a customer needs our help to verify that their tags work as they should. This is a story about a case where things went wrong.

Tagging Athletes in Cross-country Skiing

We got involved with a very interesting case a couple of years ago. A small RFID integrator that we have known for some time wanted to tag athletes in a cross-country skiing competition. The idea was to use RFID for timing the race. And you can guess that if someone spends several hours on the ski track, being left without a race time is definitely not an option.

Cross-country skiing is a healthy hobby and also a good application for RFID timing – Photo by Sorbis / Shutterstock.com

There are many different ways to tag athletes in timing solutions. Common approaches in running competitions include integrating the RFID tags to the race bibs or attaching them to the shoes of the runners. However, in this case, the chosen approach was to buy wrist-band tags and to attach them to the ankles of the skiers. The tags were read by fixed readers that were set up on the side of the tracks.

The integrator went through the specifications of several different wrist-band tags and finally contacted a fairly well known maker of specialty tags for some samples. The samples arrived the following week, and the integrator ran some field tests. Everything worked well. He was able to detect every skier that passed the reader antenna.

So the integrator decided to order the tags that he needed for his application. Once again, there was a timely delivery of correct amount of tags. However, when he started to build the application, he noticed that not all of the tags were working. A typical discussion between an unsatisfied customer and a worried supplier followed.

The conclusion was that all the supplied tags worked according to the specification.

The tag manufacturer tested both working and non-working tags attached to the wrist of whoever they considered to be their standard-human. All tags were readable from a distance of 2 m which was the specification. But they didn’t all work in the end application. How is this possible?

Analysis

We got involved with the case at this point. We were asked to take a look at the tag samples to try to understand what was going on. So we ran a performance test for the tags using the Tagformance measurement system. The results from the Threshold Sweep measurement are shown below.

Performance variations of wrist-band UHF RFID tags measured with the Tagformance system

We tested the tags in free air, so they were not attached to a wrist or an ankle. But even in this setup it is clearly visible that there are significant variations in the tuning and performance of the tags. So this is what we think happened:

  1. The integrator decided to use the tags differently from what the manufacturer had specified
  2. The first samples sent by the tag manufacturer performed clearly above the spec
  3. The integrator concluded that this tag will work in the application
  4. The next tag batch – even though still within the original spec – performed differently
  5. A part of the tags didn’t work.

So, it is very hard to point fingers in this case. But there is something that the RFID industry needs to improve in. The industry is already doing a decent job in reporting the performance of their tags, e.g. in expected read range. But I think there is a lot to improve in how performance variations are reported, because that is a key factor in building reliable RFID systems.

And what happened with the skiing competition? Did the integrator get the system to work? He did. But he had to manually test through the tags and hand-pick the ones that worked well enough. Hardly a perfect outcome but every skier got their time.

My company Voyantic specializes in test and measurement solutions for the RFID industry. Are you interested to learn more about RFID quality control? Download our sample quality test report and contact us.

Download the Sample Quality Test Report!

See what performance testing can tell you about a tag – and how this data can be used to improve your production quality