John Woodgate’s Column

 

CISPR and TC 77 Plenary meetings

At the beginning of this month, CISPR and TC77 and their sub-committees and some Working Groups met on the resort island of Jeju, South Korea.  Not all of the reports are available yet, but it is clear that important developments took place.

 

Measurement Uncertainty

CISPR/A has already studied this difficult subject extensively, resulting in the publication of CISPR 16-4. Now TC 77 and its sub-committees need to address the application of the principles of that publication in specific cases.

 

It’s possible to look at the subject in two ways. The first, which might appear frivolous but isn’t, is that we know that many EMC measurement results are very uncertain, so that quantifying the uncertainty is likely to be difficult indeed and the outcome, very possibly, politically unacceptable.  The second way is to hold that we should quantify the uncertainty without considering ultimate acceptability, and use that study to eliminate, or at least reduce, those elements of the uncertainty budget that can be reduced.

 

The intention of TC77 is to produce a practical guide for standards writers and test houses, so as to encourage uniform treatment of the subject across the board. There is a proposed non-exclusive list of subjects to be considered, of which an edited version is:

 

·    Uncertainty due to the relevant standard not comprehensively and clearly describing the procedure;

 

·    Instrument uncertainty, as declared by its manufacturer;

 

·    Long-term time-dependence of instrument  uncertainty;

 

·    Calibration uncertainty (related to both  frequency of calibration and the time since last calibration);

 

·    Uncertainty caused by conditions local to the test site (such as local radio transmissions);

 

·    Uncertainty of second-order effects (variables not explicitly appearing in calculated results, such as temperature and humidity);

 

·    Human factors influencing uncertainty (tendency to believe expected results, but to re-test outliers, for example);

 

·    Mathematics of uncertainty (uncertainties don’t always add arithmetically);

 

·    Interpretation of results accompanied by uncertainty statements.

 

Radio-frequency emission measurements

It appears likely that traceably-calibrated bi-conical and log-periodic antennas will become preferred over the tuned dipole as reference antennas. The calibration of the absorbing clamp is still the subject of further applied research work. For radiated  measurements in the new 1 to 18 GHz range, the materials and construction of the site turntable and the equipment support table have large influences, which can be minimized if measurements are made with different antenna heights. More applied research is needed on this subject, as well.

The subject of calibration of common-mode absorption devices (ferrites) has been controversial for some time. Measurements with a vector network analyser can be shown to be reliable, but a simpler method, using a spectrum analyser is being studied.

 

The emission patterns of equipment in the frequency range 1 to 18 GHz can show very narrow lobes of high intensity. In order to investigate these, measurements of very fine angular resolution (such as 1 degree) might be required, involving very long test times.  At present, it seems that test houses will bear the responsibility for determining the angular resolution required in a given case, which may prove an unacceptable arrangement.

 

CISPR 16 (at present in four parts) will be restructured into sixteen Parts under an accelerated time-scale. This makes individual Parts more accessible and affordable, for those who don’t need the whole thing.

 

CISPR Product Standards

A new draft revision of CISPR 11 is being prepared, taking into account the numerous comments on the previous draft. It is hoped that the new draft will address the real disquiet expressed, that the first draft  could be ‘read on’ to almost any type of equipment.

 

An amendment to CISPR 15 is proposed, that extends emission measurements to 300 MHz, this upper bound being chosen on the evidence that no interference problems are reported at higher frequencies. This proposal has no doubt been influenced by the problem resulting from CISPR 14-1 specifying only a  limited frequency range for emission measurements. This latter subject is to be very intensively investigated. In addition, the extension of the scope of CISPR 14-1 to include battery-operated equipment  is to be included in a second draft for National Committee comment.

 

Yet another new edition of CISPR 20 has reached the final voting stage. The continuing differences between CISPR 20 and EN 55020 are being addressed in both IEC and CENELEC. First voting drafts for the extension of CISPR 22 requirements above 1 GHz are being prepared. The proposed limits have proved very controversial. Additions to CISPR 24 are being developed. One unusual test is for the sound pressure level due to demodulated disturbances affecting products with loudspeakers.  Immunity requirements above 1 GHz may also be added at a later date.

 

Some editorial problems in CISPR 13 may be dealt with in the short term by the publication of a clarifying Information Sheet. A Technical Report on objective assessment of the effects of disturbances on television picture quality is in preparation.  While intended to help with the progress of CISPR 20, this Report may also be significant for EN 55103-2.

 

EMC and functional safety

A team is being formed to revise and update IEC 61000-1-2.

 

Immunity tests

A new team is being formed to take over the maintenance of IEC 61000-4-2.

 

The proposed revision of IEC 61000-4-4 has run into the difficulty that the first voting stage was passed by a very narrow margin, but there is a very strong body of opinion that the draft is not acceptable.  It may be that the situation will be resolved by a greater number of negative votes at the second voting stage. It was suggested that  National Committees should resolve to make more comments at the comment stage(s), and not save them up for the first voting stage, because other National Committees need to be aware of negative comments before they vote, not afterwards.