TechArchive

Original article date: May 1998

Have you access to a true Open Area Test Site (OATS)? If you are one of the vast majority that do not how much are your measurements worth? David Mawdesley of Lapalce Instruments explains the principles of self-testing for EMC emissions.

Any radiated emissions measurements you make without calibrating the site are almost certain to contain significant errors. This simple truth has been largely ignored by many users and EMC test kit manufacturers alike. Probably because there has been until now little that could be realistically done about it other than investing a considerable sum in a true OATS which is what the test houses have to do.

Laplace has now developed a solution to this very common and intractable problem. Accurate measurements can be taken even in very poor non-compliant sites with strong background and adverse reflections. The system uses an ERS (Emissions Reference Source) and a SA1000 EMC analyser using the latest version of the EMCEngineer software. The ERS is an accurately calibrated source of emissions as measured at 3m at the NPL reference site. It is supplied on disk with the calibration data.

When the test site is ready for use the ERS is placed at the intended location of the Equipment Under Test (EUT) and the resultant emissions are measured. By comparing these readings with calibration data and automatically adjusting for ambient signals the software can compute the correction factors for the site. Any deviation from an ideal site will be reflected in the correction data which is them automatically applied to any subsequent emissions measurements from the product.

The process takes only a few minutes and the software prompts the user as required so no training is required.

Other features of the software include

• background cancellation
• limit line display
• single frequency mode with auto peak tracking
• QP and average detectors
• antenna factor correction.

Can you test for EMC compliance in-house?

Absolutely yes. There is no restriction in law to restrict you testing and self certifying your own products. Once you are sure that you product complies with the relevant directives you can create and sign the Certificate of Conformity apply the CE labels and sell the product.

A range of appropriate test equipment is available to test for emissions and susceptibility to enable you to perform the relevant tests. Note that the tests that you actually perform are at your discretion. You do not have to perform any tests but for most products if you do not test how can you be sure that the product complies?

Appropriate tests depend on the nature of the product.

An all-analogue audio amplifier with a linear (as opposed to switched mode) power supply should be ‘benign’ as regards emissions and it could be argued that emissions testing is not required. On the other hand something as apparently ‘innocent’ as an electric toothbrush could be an emissions disaster area whereas its susceptibility is quite irrelevant.

Careful thought about your product can save you unnecessary time and expense provided you justify and document any decisions you make.

Is self-testing the right strategy?

Sometimes self-testing is not appropriate. The alternatives are:

• Test house….. around (UK pounds)2000 per typical product. The danger is that the product may fail. If so you have the cost of retesting. Also how are you going to ‘fix’ the problem? You will need some equipment in-house so that you can ‘see’ the emissions and try the usual trial and error modifications until you find the one(s) that work.

• Consultant….. cost depends on how ‘difficult’ the product is to make compliant. Consultants charge by the day and are not cheap.

• Equipment hire…. Needs careful planning so that the equipment is on-hand when you need it. If used regularly may be more expensive in the long run.

• But purchasing you own kit will generally be the most cost and timescale efficient option if more than three or four products per annum need CE certification.

The advantages are:

• Convenience… test when you need on your own premises.
• As many tests as required.. no extra cost
• Kit enables you to see the problem emissions and if you can see them you can fix them. In using your own kit you gradually acquire EMC expertise in-house invaluable for future developments.

Disadvantages:

• Cost of the kit
• Need to learn a new discipline
• Difficulties of measurement
• Unknown measurement uncertainty

May 1998