October 23, 2017
You cannot say that NFC is truly new as NFC modules are integrated into all modern smartphones, and the number of real-world applications is increasing. However, the real world is still a little cautious. Why is that? Well, the practical application is tricky: NFC is designed to make your life simple. If it doesn’t work, it won’t be accepted.
The Spec is just the Beginning
At first sight, it seems quite easy: Most NFC modules are developed, tested, and approved according to well-defined standards. Often the manufacturer proves the product compliance with a certificate. The integrator of the module simply mounts it to the assembly and adjusts the antenna to the geometry. You should think that recertification – based on the initial certificate – should be no problem and that the new NFC-compliant product would be ready immediately. Unfortunately, it’s not that simple, problems will occur, most likely in the field.
The integration of a lab-tested NFC module in a washing machine, ticket machine, or door lock may not necessarily lead to a good end product due to the surrounding material or other influencing factors. Also, in the real world, different conditions can’t be controlled by the manufacturer of the end product. For example, temperature and humidity may vary. Also, antennas are usually not positioned as ideally as in conformance tests. Very often, the user introduces the NFC device on an unpredictable course into the magnetic field. All these small differences may result in unreliable behavior.
To say it clearly: Conformance testing does not guarantee proper functioning of the final product in real life! It only verifies that an NFC device meets the parameters mandated by the specification – in a lab environment. No more, no less. Usage in the real world is another story.
(Un-)limited Interoperability
Needless to say that manufacturers facing these problems are searching for quality-improving measures, and interoperability testing is one commonly used method. The idea is testing your own device against as many other devices as possible in order to guarantee correct functionality. Basically, this is a good approach, but obviously it is impossible to cover all possible combinations. You can certainly have one hundred of the latest smartphones in your test lab, but sooner or later, the one-hundred-and-first smartphone will come up that doesn’t work properly.
Your test library would have to continue growing, increasing costs beyond measure in the process. However useful it is to test against as many other devices as possible, you should always be aware of your limits. It will just be a sample that proves interoperability between certain devices and provides a certain sense of security.
Close the Testing Gap
What to do instead? How can the quality of an NFC reader be determined at reasonable expense? What is the missing link between conformance testing and interoperability testing?
If you are asking these questions, take another look at NFC. Don’t just concentrate on the question “works / doesn’t work”, but include the whole system consisting of NFC reader and smartphone into your testing approach. Sometimes, the reader’s magnetic field is a decisive factor, especially when the reader is installed in a complex environment. Analyze the relevant data and derive an assessment of the magnetic field quality and of interferences.
Additionally, you could evaluate measures against the source of interference. This type of testing is neither conformance testing nor interoperability testing. You could call it “design validation testing” – proof for the design suitability being the focus. Here, the magnetic NFC field becomes visible and qualifiable.
Some Examples
In the simplest case, an NFC reference antenna is moved over an NFC reader in a freely definable space, and the field strength is measured in a defined grid. A possible result is shown in figure 1.
Figure 1
Here, the distance between reference antenna and reader is constant (Z = 20 mm). The image in the middle shows the measurements of a reader in an unobstructed environment. The green isoline marks the 1.5 A/m field strength, which is specified as minimum field strength by ISO/IEC 14443-2. The image on the left shows the course of the isolines of the same reader. For demonstration purposes, a metal plate has been positioned below the reader for a second series of measurements that simulates interferences that can result from integration in an assembly.
You can see that the 1.5 A/m range is only minimal. Even half the field strength (0.8 A/m) is only achieved in an area that is smaller than the original area. The deterioration of the field strength due to external influences is unmistakable. In the image on the right, the situation is noticeably improved by a ferrite foil that is tuned to 13.56 MHz – even though the ideal state is not recovered completely. It is often useful to measure not only on the Z-plane, but to repeat the measurements on different other planes. In figure 2, the measurements are compiled to form a single 3D model visualizing the reader’s operation volume.
Figure 2
The measurement setup described above identifies weakness of the magnetic field that may be caused by other assembly modules or by the antenna geometry. Furthermore, the effects of possible corrective measures can be visualized and evaluated.
Measuring the reader field with a simple antenna – as described above – already yields very precise information about the field’s condition and form. However, it has the disadvantage of a relatively coarse measuring grid. Another drawback being that this method is restricted to only one component of the magnetic field. Our new Vector Field Probe, however, allows further measurements and analyses. The Vector Field Probe has dedicated coils for measuring each single component (Hx, Hy, and Hz) of the magnetic field.
Furthermore, it is much smaller than a reference antenna which results in a finer measuring grid and consequently higher resolution. This allows for a profound analysis of the reader field and the influence of other modules.
Figure 3
Figure 3 shows the measurement results of the single components at Z = 0 mm. This measurement produces a very detailed image of the magnetic field showing possible weaknesses and interferences. With the help of this data, the reader field can be scanned, visualized, and subsequently optimized in a complex environment thus creating a user-friendly operation volume.
In the processes described above, the magnetic field strength is measured. This is certainly a critical parameter, but not the only one. The reader should be tested for its sensitivity as well. The minimal load modulation that the reader needs for communicating with the counterpart is measured at the single measurement points. These measurements can be visualized in a similar way. Based on the examination of field strength and reader sensitivity, the quality of the reader in the specific environment can be assessed.
It’s the Combination
Conformance testing is not useless. On the contrary: It is important as a basis. Interoperability testing is also an important element of development-driven quality assurance. Here, the costs are definitely the limiting factor. To close the resulting gap and to make sure that the NFC reader’s field meets the requirements, it is important to go one step further. And have another look at NFC.
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