A Challenge of Portable Radio Transmitters Used in Close Proximity

Intentional RF transmitting devices seem to be everywhere. Smart phones, tablets and similar devices provide the ability for users to be connected to the internet any time, from any location using nearly any device. Other than the Boundary Waters Canoe Area Wilderness and the inner canyon of the Grand Canyon, it may be difficult to find any location without WiFi available.

RFID tags and transponders are used for inventory in retail stores, monitoring the location of equipment of all kinds and tracking patients in medical settings. We even see active RFID tags imbedded in electronic equipment undergoing EMC testing. (The experienced EMC professional can probably imagine the challenge this practice creates during an RF emission test!)

A sampling of transmission systems is shown in Table 1.

Table 1: A sampling of transmission systems

No doubt, the great expansion of this technology has improved society in many ways. The benefits of these devices are quite significant. An unintended side effect of the proliferation of transmitting devices, however, is the increased potential for malfunctions of electronic equipment in operation close to where the transmitters are used. Not only are more transmitting devices in use in all environments, the separation between any given transmitter and equipment that may be affected is generally decreasing. The separation distance is often uncontrolled with separations of a few centimeters not being uncommon. Contrast this proximity with the several meters or more of separation typical in the days before the use of portable devices with transmitters became so prevalent.

The types of equipment that may be adversely affected is nearly endless, including desk-top computers, point-of-sale terminals, gas pumps, vehicle control systems, computer systems and other portable electronics, to name just a very few.

This new-world reality creates some interesting challenges and opportunities for EMC professionals. What are the devices we must consider as sources of interference? What devices need to be hardened against new or changing interferences? How do we determine adequate immunity levels? Are existing test methods and standards sufficient? If not, are wholesale modifications required, or can existing standards be used with some (minor?) modifications? Which characteristics of the transmitted signals are important to the evaluation of disturbance potential?

These questions, and more, are being considered in multiple segments of industry, including standards developing organizations and various user segments. This article will explore some of the aspects of this situation, including the possibility of developing a new international test standard focused on close proximity immunity. The challenges that will need to be addressed to provide repeatable, meaningful test results will be explored.

Another Standard?

One may ask why do we need a new test standard for this phenomenon. IEC 61000-4-3 covers immunity of electronic equipment to radiated RF electromagnetic energy, establishing both test levels and test procedures. The current edition of this standard even states “Particular considerations are devoted to the protection against radio-frequency emissions from digital radiotelephones and other RF emitting devices.” [1]. IEC 61000-4-21 includes a detailed description for the test setup, chamber validation procedure and test procedures required to perform radiated immunity testing in a reverberation chamber [2]. IEC 61000-4-20 provides details for performing immunity tests on in-scope equipment in transverse electromagnetic (TEM) devices. [3]

These standards are excellent documents for their intended purposes and certainly can be used to simulate disturbances created by portable transmitters used at distance from equipment potentially suffering interference. They may not always produce a satisfactory characterization of equipment immunity to portable transmission sources used within a very short distance, say 20 cm or less. Test limits in the range of 3 to 10 volts/meter are typical when the disturbance source is a fair distance away. However, field intensities in close proximity to smart phones can be 100 volts/meter or more. Some equipment manufacturers and users reduce the risk of interference by specifying minimum separation distances that must be maintained between their equipment and portable transmitters. A typical specified separation distance is in the range of 1 to 3 meters. At the same time, we are seeing a move toward having service personnel use their smart phones very close to installed equipment while performing service. A practice gaining popularity is to place QR codes on equipment covers for service personnel to scan for accessing service information related to the equipment. Doing so while keeping smart phones 3 meters from the equipment would be, shall we say, a challenge.

Multiple industry segments have highlighted the problems of trying to use these existing standards to evaluate immunity of equipment to cell/smart phones used in close proximity. Notably, the automotive industry and the medical device industry have raised concerns with the suitability of existing test methods that could be used for this purpose. Groups within these industry segments reached the conclusion that the existing RF immunity test standards do not represent the close-proximity electric and magnetic field characteristics accurately enough and could produce results that are not fully in line with malfunctions created by interference sources used in close proximity in real-world situations.

The concerns raised by these groups helped initiate a new project in IEC to develop a new basic standard for immunity to devices used in close proximity. This project is in its early stages in Working Group 10 (WG10) of IEC SC77B.

WG10 is considering all aspects of interference caused by portable transmitting devices in close proximity and comparing them with characteristics of existing standards to determine where those standards are a good match and where they are not appropriate. The characteristics that need closer scrutiny include:

• Field strengths very close to cell/smart phone versus common test levels
• Input power levels required for achieve those very high field strengths
• The significance of using near field sources as opposed to far field sources
• The significance of the source type, such as electric field or magnetic field and
• Modulation schemes.

One of the first things we considered was whether the existing IEC standards could be used for this purpose, either wholly or in part.

Existing Standards

The practice of using a linearly polarized antenna to create a uniform field area (UFA) in which the equipment being evaluated is immersed is described in IEC 61000-4-3. The standard states its test methods can be applied up to 6 GHz and that disturbances from portable transmitting devices such as cell phones have been given consideration. The method of independent test windows facilitates testing at frequencies greater than 1 GHz, the frequency typical for many types of portable transmitters. These factors certainly seem to indicate this standard could be used to test for immunity to disturbances from portable RF transmitting devices. Some test labs have had good experience in doing just that. However, the input power levels required to establish field strengths on the order of 100 volts/meter can be quite large. They are possible to achieve, but large. For the independent windows method, the test distance between the transmitting antenna and EUT is 1 meter. Consequently, this method does not reproduce the near-field effects that exist in real-world close proximity situations. In some cases, not reproducing the near-field effects may not be an issue, particularly for equipment where the intensity of the disturbances is the predominant effect. In such cases, IEC 61000-4-3 could be applied. Where this is not so, a different test method and standard would be needed.

Reverberation chambers can be used to immerse the equipment under test (EUT) in a field that is statically isotropic, homogeneous, unpolarized and uncorrelated. As described in IEC 61000-4-21, the entire EUT is exposed to simulated disturbances without the need to rotate the EUT or to move the transmitting antenna to multiple, discrete positions. Fairly high field strengths can be generated using moderate input power levels, thereby avoiding input power level concern when testing according to IEC 61000-4-3. Similar to the practice of using a linear antenna to generate a uniform field area, the near-field effects that happen when the transmitting device is very close to the equipment experiencing interference are not reproduced in a reverberation chamber.

Based on the analysis that is summarized briefly here, the current position is that these standards certainly can be used to evaluate the immunity of equipment to interference from portable transmitting devices, including cell phones. However, they are best suited to evaluate situations when the transmitting device is far enough away that it would not be considered as being used in “close proximity.” Therefore, an independent standard defining a test method that more fully replicates the particular characteristics of disturbances from transmitting devices used in close proximity to the equipment suffering interference and can be used when the test methods in the existing standards is not appropriate, adequate or sufficient should be developed.

Test Methodology and Challenges

One of the challenges to be worked through is how to define what it means for the transmitting device to be in close proximity to the equipment experience the disturbance. We could consider the transition from near field to far field, the intensity of the disturbance signal, an arbitrary physical separation or some other characteristic. However it is defined, this characteristic is important to establishing all the technical details in the standard.

An international standard must meet certain formal and informal criteria before it can be published and put into use. This requirement is especially true for a basic standard that is likely to be applied to a wide variety of equipment types. Test methods that are perfectly acceptable for a small, hand-held device may be totally impractical and produce questionable results for large industrial equipment. The people tasked with writing the standard must always keep in mind the bigger picture, considering how the standard may be used, the types of equipment that are likely to be evaluated against it and the state of the art in test equipment and the disturbance sources the standard intends to simulate.

The future standard is in the early stages of development. The work so far has identified some possible test methodologies as well as a number of issues that must be resolved before publication.

The test method being considered is based on the concept of a small RF coupler or antenna being scanned across the surface of the EUT. The coupler would be located some small distance away from the EUT surface, perhaps on the order of a few centimeters. To aid in repeatability of test results, the surface to be tested would be divided into a rectangular grid pattern and the coupler moved in discrete steps according to the size and shape of cells in that grid. See Figure 1 for an example of how the EUT may be partitioned into test grids. The RF coupler shown is intended to be of generic design and not an indication of what an actual coupler would be.

The test is conceptually simple, but some specific details are not quite so simple to develop. The details that need to be resolved before a useful basic test standard can be published include the following.

Defining the RF coupler
The coupler could be defined in terms of its electrical or mechanical parameters. It needs to be defined in a manner that allows commercial production by multiple suppliers. Facilitating construction by individual test laboratories could be considered as well. It must be able to withstand the input power needed to meet expected test levels. Some degree of uniformity of the field generated is also a must. Given the wide frequency range that must be considered, which could include approximately 800 MHz to 6 GHz, it is likely that multiple couplers would be needed. The definition would need to support this practical reality.

Calibration or verification of the RF coupler
Verifying that the RF coupler is functioning is not likely to be a major challenge. Defining a calibration procedure that will satisfy the rigors of laboratory accreditation requirements will probably be more difficult, not to mention essential to the reproducibility of test results.

Establishing a level-setting procedure
Given that the RF coupler will be placed very close to reflecting surfaces that may be very large relative to the size of the coupler, the effects of reflections from those reflecting surfaces must be considered. Can test levels be established in an environment
with no reflecting surfaces nearby? Can forward power to the coupler be used as the test level without regard to effects from the reflecting surfaces under test?

Test time
Stepping the RF coupler across the surfaces to be tested will take some time. The amount of time, of course, depends on the size of the cells in the rectangular grid and the total size of the surfaces to be tested. Larger cells will reduce test time but must be balanced against the uniformity of the field radiated by the coupler. Add in a number of discrete frequencies or multiple frequency ranges, and the time required for the test can be very long, especially for large equipment being tested. One estimate for a full rack of computer or telecommunication equipment pegged test time in terms of days not hours.

Modulation schemes
Traditionally, amplitude modulation (AM) with a 1 kHz tone has been used for RF immunity testing. Evaluations and experiments have shown that AM sufficiently predicts performance for many other modulation signals. Is this still true given the large number of different modulation schemes being employed in RF transmitting devices today? If additional modulation schemes will be required, which ones need to be used and how do we decide how many difference schemes are necessary and sufficient?

Conclusion

Technology – isn’t it grand? As technology evolves at a pace that seems only to get quicker, society reaps many benefits and improvements to daily life. For new technologies and applications to continue providing benefits, the unintended consequences must be considered. The test methods and associated standards for quantifying the effects of unintended consequences must also be examined and, in some cases, evolve along with the technology.

The proliferation of portable intentional RF transmitting devices is one of those shifts providing significant benefits and the potential for undesired consequences. The standards community recognizes these consequences and the need for test standards to evolve to address them. The future standard for close proximity immunity testing will be one more tool in the EMC professional’s toolkit to facilitate a seamless transition and enable progress well into the 21st century and beyond.

References

1. IEC 61000-4-3:2006, Electromagnetic compatibility (EMC) – Part 4-3: Testing and measurement techniques – Radiated radio-frequency, electromagnetic field immunity test, Amendment 1:2007, Amendment 2:2010
2. IEC 61000-4-21:2011, Electromagnetic compatibility (EMC) – Part 4-21: Testing and measurement techniques – Reverberation chamber test methods
3. IEC 61000-4-20: 2010, Electromagnetic compatibility (EMC) – Part 4-20: Testing and measurement techniques – Emission and immunity testing in transverse electromagnetic (TEM) waveguides

John Maas is a Senior Technical Staff Member and Corporate Program Manager for EMC at IBM Corporation, where he has responsibility for IBM’s worldwide EMC regulatory compliance programs. John has more than 30 years of EMC experience including hardware design and test. He is a senior member of the IEEE and has been involved in international standardization for much of his career, with his contributions to EMC standardization being recognized by the IEC when he received the IEC 1906 Award. John is currently convenor of IEC SC77B/WG10, Technical Advisor of the US technical advisory group (TAG) for IEC SC77A and a member of the US TAGs for IEC TC77, SC77B and CISPR/I. Mr. Maas can be reached at johnmaas@us.ibm.com.