A look into the standards under development by the many C63 subcommittees helps predict how the EMC industry could be affected.
The C63 Electromagnetic Compatibility Committee reports headway in a wide range of topics, from the use of time-domain reflectometry (TDR) in evaluating test sites above 1 GHz to working with the FCC to rescue a stalled standard. To gain some first-hand knowledge of the progress, I attended the C63 series of meetings held at Northwest EMC�s laboratories in Irvine, CA. This allowed me to witness the standards development process in action and gather information on some of the engineering aspects involved.
What Is C63?
In a nutshell, C63 is an American National Standards Institute (ANSI) accredited standards committee comprising interested parties from industry, government, trade organizations, and academia working together in a neutral environment to develop voluntary standards relating to EMC in the United States. With the IEEE as its secretariat, C63�s charter is to create and maintain voluntary U.S. standards pertaining to EMC and, where opportune, minimize duplication of effort by harmonizing stand-ards development.
To maximize harmonization opportunities, the SC3 subcommittee of C63 is dedicated to monitoring standards development in international standards organizations. Also, many committee members belong to standards development committees in the Society of Automotive Engineers (SAE), the IEEE, the International Special Committee on Radio Interference (CISPR), and the International Electrotechnical Commission (IEC).
The secretariat ensures ANSI procedures are followed to maintain balance of input through the makeup of the membership and that openness on activities is realized via public announcement on the ANSI website.
The milestones in developing a standard are the following:
� Registration of projects with ANSI.
� Pass C63 membership ballot.
� Submission to ANSI for public scrutiny.
� Finalize standard using public comments as input.
� Publication by IEEE.
As shown in Figure 1, the parent committee creates and disbands subcommittees as required, and the subcommittees themselves establish working groups to deal with specific technical issues. The working groups are made up of members with expertise in the specific engineering disciplines at hand.
Figure 1. C63 Internal Reporting Structure
Before launching into some of the standards currently under review, there are two interesting histories to relate. The first was initiated by an EMC crisis during a teleconference at Goldman Sachs.
The story goes that an all-important board meeting had trouble getting input on year-end figures due to noise on a teleconference connection. It turned out that many of the electronic devices carried by board members were of the continuous-polling type, and the constantly emitted RF signals were being converted to audio noise inside the teleconference equipment. A board member was assigned to investigate the circumstances, which resulted in the initiation and development of an ANSI standard for the RF immunity of office equipment.
The second interesting history belongs to ANSI C63.18.1 This standard describes a recommended method of testing medical equipment for immunity against common emitters such as cellphones and walkie-talkies. In this case, the standard actually was initiated after an electric wheelchair took off on its own when a police officer in a nearby cruiser keyed the radio set.
Much of the work underway in the committee is leading to the revision of standard ANSI C63.4-2003.2 The revision is a major undertaking, and many C63.4 projects have been registered with ANSI and currently are progressing in parallel.
Regarding U.S. technical innovation, a very topical project is 1-13.2 Site Acceptability Above 1 GHz. This project proposes the use of TDR in a novel way to characterize test chambers above 1 GHz. Once the basic research is complete and work on the project has been validated, it will be the source of any changes to clause 8.3.1 of ANSI C63.4 Measurements on a Test Site.
The TDR Technique Explained
Based on the same principle as RADAR, TDR fires a short-duration pulse into the system to be tested and monitors any echoes bounced back by system imperfections. Its most common use today is locating the position of cable faults. By firing pulses through an antenna, a regular TDR instrument could be used to find imperfections in a test chamber by examining the echoes, but this would not provide full characterization of performance with frequency.
A modern vector network analyzer (VNA) uses Fourier transform algorithms to display information interchangeably in the frequency domain (frequency along the X axis, amplitude along the Y axis) or in the time domain (time along the X axis, amplitude along the Y axis). Using software, a TDR type function now can be accomplished with VNAs by capturing data from a frequency sweep and then manipulating it so the chamber performance is isolated.
The proposed chamber characterization method differs from conventional TDR. Instead of a common transmit/receive channel, there are separate transmit and receive channels.
Figures 2a and 2b show the basic theory of operation. An RF signal is transmitted from the antenna. The direct path (blue) has the least distance to travel so it reaches the receive antenna first. The energy delivered via this path holds no information so it is blanked out in software by the use of the VNA gating function.
The amplitude of any reflected signal (red path) arriving after the gate has timed out is stored, and then the next test frequency in the test sweep is transmitted. Even with fairly high resolution, the entire sweep completes in a matter of seconds.
The data is displayed in the frequency domain allowing chamber performance to be plotted vs. frequency at one particular test angle. The transmit antenna then is rotated to the next test angle and the whole process repeated. Even though at some test angles the transmit antenna may be facing away from the receive antenna, the filtering out of the direct path pulse still is required to blank out any direct signal traversing via antenna side or back lobes.
Figure 2c shows two identical test sweeps at one specific antenna angle conducted one week apart and using different VNAs. Both sweeps are for the same transmit antenna angle. The traces show a repeatability of less than 1 dB. The time to complete the entire test is about half a day.
2b. Pictorial of Characterization Method
2c. Two Identical Test Sweeps Taken One Week Apart Using Different VNAs
Figure 2. Basic Theory of TDR Characterization Method
During a pause between committee meetings, I witnessed a TDR test conducted in a 10-meter chamber at Northwest EMC. Careful scrutiny of the time-domain trace allowed the distance to the chamber back wall to be ascertained, and as a sanity check, a sheet of copper screening material was held away from the direct path but parallel to a chamber wall. The trace height in the frequency domain rose by 10 to 20 dB.
Receiver or Spectrum Analyzer?
Werner Schaefer, a member of the CISPR/A subcommittee, updated the parent committee on the use of spectrum analyzers for emissions compliance measurements. CISPR emissions standards currently enable the use of spectrum analyzers above 1 GHz but specify receivers below 1 GHz.
ANSI C63.4, on the other hand, allows the use of spectrum analyzers for compliance measurements in the complete frequency range from 9 kHz to 40 GHz. The advantages and disadvantages with both test instruments were explained.
The key difference is that a receiver has preselection. Designed to meet the pulse response requirements of low repetition rate pulses, preselection also provides protection against front-end overload caused by broadband RF signals, but care should be taken here.
Digital devices under test can emit clock pulses, and these produce numerous discrete narrowband signals�the clock fundamental frequency and the associated harmonics. If a discrete frequency happens to fall within the band of the preselection filter, RF front-end overload still can occur. Regardless of which test instrument is used, the operator must know the particular strengths and weaknesses of the instrument and how to check for and rectify front-end overload.
C63.4 maintenance and revision are only parts of the work currently underway in the C63 subcommittees. Many standards under development also are industry or product specific.
Ad Hoc Medical Testing
Jeffrey Silberberg, chairman of Subcommittee 8, reported on the progress of the revision of C63.18-1997.1 This standard is a recommended-practice document used by health-care organizations when evaluating the radiated immunity of their medical devices to commonly available RF transmitters.
Not all hospitals ban the use of cellphones, and hospital staffs commonly use walkie-talkies to keep in touch with each other. Also, there has been a recent proliferation of hospital equipment fitted with wireless links.
This standard is intended to aid hospitals and clinics when developing their EMC policies, particularly defining the keep-out distances for various types of medical equipment. The testing takes place within the actual clinical environment and uses real emitters such as cellphones.
Even with a low-level emitter, the field strength can be high at very close proximity. So to avoid damage to sensitive medical equipment, the current procedure starts at a distance and then closes the distance until interference is observed. This is repeated on all three axes.
There is one complicating factor: Regarding pass and fail criteria, this is not a regular EMC test as recognized by most people involved in EMC. The prime concern is patient safety, so for instance, depending on the function of the equipment, slight jitter on the display may not be construed as hazardous.
The revision includes a refined test method that measures the test field strength. Subcommittee 8 is working with a field-probe manufacturer on this, particularly regarding measuring the field strength of complex waveforms with high peak to average power ratios. The new maximum field strength will be 20 V/m.
Cellphone Compatibility With Hearing Aids
Stephen Berger, chair of Working Group 1, reported on the progress of redrafted C63.19-20012 currently at the ballot stage. The redrafted standard failed to pass a second ballot.
The redraft is a simple maintenance exercise. The differences between the 2001 version and the proposed version are fairly minor, and more importantly, the technical criteria defining compliance did not actually change.
There was considerable discussion on the best way to proceed to a third ballot. The FCC uses C63.19 as a reference document3 and would like the maintenance process to be successful. With hundreds of thousands of hearing aids and hundreds of millions of cellphone handsets sold every year, some stalling may be because stakeholders, including both hearing-aid and handset manufacturers and cellphone service providers, have such divergent interests.
The FCC, always concerned about unintended consequences, counseled a measured approach where, prior to further balloting, a special meeting of interested parties would take place to discuss any changes. Changes agreed upon could be written into the draft standard, and then the third ballot could take place with a better chance of a successful outcome. The committee agreed to schedule the special meeting within10 days of the conclusion of the C63 series of meetings.
One of the technical aspects to come out of the discussions was the use of GTEM cells to test hearing aids for immunity compliance. The hearing aid is placed inside the test cell, and a cellphone communications signal is fed into the RF input of the cell to produce the specified field strength.
Two audio transducers are required, one to provide a reference audio signal to the hearing aid and one to monitor the audio output from the hearing aid. The transducers are connected to the outside world via a screened/filtered connection.
Effect of Various Standards on EMC Industry
VNAs are commonplace test instruments in the wireless industry and may well become standard equipment in the EMC sphere, if only on a shared-stock basis. These instruments are ideal for displaying wireless telecom waveforms, and telecom waveforms may need to be generated by test laboratories as test signals.
Regarding measuring the power of these complex wireless waveforms, we may see the return of the bolometric power head. Popular in years gone by, this is a thermistor that heats up when exposed to RF and, irrespective of complexity, only captures the average power of the waveform.
Most of today�s test chambers were designed to work up to 1 GHz. The cladding on semi-anechoic chambers may well become fully reflective at some microwave frequencies, creating a serious issue for test laboratories wishing to be accredited for above 1-GHz RF immunity testing. We also may see the demise of the fully reflective floor and the arrival of fully anechoic chambers.
References
1. ANSI C63.18-1997 American National Standard Recommended Practice for an On-Site, Ad Hoc Test Method for Estimating Radiated Electromagnetic Immunity of Medical Devices to Specific Radio-Frequency Transmitters
2. ANSI C63.4-2003 American National Standard for Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range 9 kHz to 40 GHz
3. FCC Public Notice DA 05-1134
Acknowledgments
The list of C63 Committee and subcommittee members is available on the C63 website. I would like to thank all of the members for their hospitality and their forthrightness in providing information for this article.
I would like to give particular thanks to Dr. Ralph M. Showers, University of Pennsylvania, chairman of the C63 Committee for 45 years; Don Heirman, Don Heirman Consultants, LLC, vice chairman; Dan Hoolihan, Hoolihan EMC Consulting, chair of SC6 and SC8; Mike Windler, Underwriters Laboratories, chair of the working group on site validation above 1 GHz; Werner Schaefer, Cisco Systems; Jeff Silberberg, FDA, chair of SC8 WG3; Stephen Berger, TEM Consulting, chairman of SC7; and Clark Vitek, Extreme Networks.
About the Author
Thomas Mullineaux, the chief writer at HighTechWriter, is an RF engineer with 15 years experience in leading design teams. HighTechWriter, 3332 Florista St., Los Alamitos, CA 90720, 562-400-4501, e-mail: [email protected]
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December 2005