EMC Pre-Compliance Testing Before the Accredited Lab

The accredited test house reports whether the product passes. The pre-compliance work in the company’s own facility is what determines whether it passes on the first visit, and it costs a fraction of a repeat test.

What Pre-Compliance Achieves

It finds the large problems before the formal test, when the design can still be changed cheaply. A product that fails by twenty decibels at the accredited laboratory has a circuit problem, and one that fails by two decibels has a margin problem.

It also establishes the repeatability of the measurement, so that a change can be attributed to the change rather than to the setup.

The pre-compliance facility does not need to be accurate in absolute terms. It needs to be stable enough to compare two configurations, which is a much weaker requirement. Our EMI immunity notes describe the design measures that reduce the problems in the first place.

Facilities and What They Cost

A screened room with a spectrum analyzer and a set of antennas covers radiated emissions up to a few gigahertz. The room’s performance at the frequencies of interest determines how much the results can be trusted.

A line impedance stabilisation network and a current probe cover conducted emissions on the cables, which is where most products fail first.

A near field probe set is the cheapest useful tool. It does not predict the radiated field, and it locates the source unambiguously when used with a spectrum analyzer. Our ferrite bead notes describe how the source is suppressed once found.

Conducted Emissions First

Conducted emissions on the power and signal cables are usually the easiest to measure and the easiest to fix. A product that fails them will usually fail the radiated test as well, since the cable radiates.

The measurement is made with the product in its normal configuration, with the cables of the specified length and arrangement. A short cable on a bench does not represent the product.

The fix is a filter at the connector, whose effectiveness depends on the return path as much as on the components. Our design release checklist notes where the filter requirement is recorded.

Radiated Emissions and the Setup

The radiated measurement depends on the antenna distance, the height, the polarisation and the orientation of the product. The standard defines all of these, and a pre-compliance setup that differs will not predict the result.

The product must be in its final enclosure with its final cables, because both affect the result more than most design changes. A bare board on a bench gives a number that cannot be extrapolated.

The measurement should be repeated with the product rotated, since the maximum emission is at one orientation and it is the maximum that the standard applies.

Immunity Pre-Compliance

Immunity testing requires field generation equipment, which is more expensive than the emission side. A basic setup with an ESD gun and a transient generator covers the most common failures.

The ESD test should be applied at the points the standard specifies, including the enclosure and any accessible connector, with the product observed for a functional change rather than only for damage.

The product’s behaviour during the test should be classified: does it recover by itself, does it require a restart, or does it need intervention. The distinction is what the standard asks for. Our quality notes describe how the behaviour is recorded.

Margin and the Decision

A product that passes pre-compliance with six decibels of margin is likely to pass the formal test. One that passes with one decibel is likely to fail on a different day or in a different configuration.

The margin must be established at the worst case configuration, which includes the maximum load, the worst cable arrangement and the temperature extremes where the clock frequency shifts.

Where the margin is small, the decision is whether to make a design change or to accept the risk. The cost of the change is known, and the cost of a failed formal test is a re-test, a schedule slip and sometimes a redesign.

Documentation

The pre-compliance record should include the configuration, the cable arrangement, the measurement distance and the results, so that a later comparison is meaningful.

Where the product is tested again after a change, the two records show whether the change helped and by how much. Without them the comparison is a memory.

The record also supports the technical file that the formal test requires, and it is the evidence that the design was evaluated rather than merely submitted.

Process Control and Verification

On a design of this kind, cable is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Process Control and Verification

On a design of this kind, cable is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, cable is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

FAQ

Can a pre-compliance setup replace the accredited test? No. It is a screening tool that improves the chance of passing, and it does not produce the report that a market requires.

What is the cheapest useful measurement? A current probe on the cables with a spectrum analyzer, which finds the conducted emission and points at the source.

What does gopcb provide for EMC readiness? We provide layout and filtering measures that reduce the emission at source, filter and return path design at the connectors, guidance on the test configuration, and review of the measurement results so that the design change is aimed at the mechanism rather than at the symptom.