Radiated Emission Troubleshooting: Finding the Source

A product that fails a radiated emission limit has one or more current loops that are acting as antennas. Finding which loop is the work, and the tools for it are cheap.

Start With the Cables

In most products that fail, the emission comes from the cables rather than from the board. The cable is long compared with any board feature and it is an efficient radiator at the frequencies of interest.

A current probe placed around the cable with a spectrum analyzer shows the common mode current directly. If the current is high, the board is driving the cable and the problem is at the connector.

Disconnecting the cable and measuring the board alone distinguishes the two cases in a few minutes, and it is the first measurement to make.

Locating the Source on the Board

A near field probe held close to the board and connected to the analyzer shows the field above each area. Moving the probe across the board while watching the amplitude locates the strongest source.

The probe should be moved slowly and in a grid, and the frequency of interest should be set on the analyzer. The probe does not measure the far field, so the amplitude is not a prediction; it is a relative indication.

Once the area is identified, the mechanism is usually a loop formed by a switching current and its return, or an untermianted trace acting as a stub. Our high speed design rules notes describe the routing measures that prevent both.

Identifying the Frequency’s Origin

The frequency of the failure identifies the source. A narrowband emission at a clock harmonic points at the clock, and a broadband emission points at a switching supply or a data bus.

Changing the clock frequency slightly and observing whether the emission moves confirms the source. The test is quick and it is conclusive.

Where the emission appears at a frequency that is not a clock harmonic, the source may be a resonance of the cable or the enclosure, excited by a broadband source. Our EMI immunity notes describe the related coupling mechanisms.

Structural Changes That Move the Frequency

A cable that is a quarter wavelength long at the frequency of interest resonates and radiates efficiently. Changing its length moves the resonance away, which sometimes appears to fix the problem and does not.

A slot in an enclosure or in a plane resonates at a frequency set by its length. Splitting the slot into smaller openings moves the resonance up and reduces the radiation at the original frequency.

These changes should be made deliberately rather than by trial, since a change that moves a resonance can move it into a band that is measured elsewhere.

Suppressing the Source

Reducing the loop area of the switching current is the most effective measure. The loop is formed by the switch, the inductor or transformer and the capacitor, and its area determines the radiated field.

Adding a series element such as a ferrite bead or a small resistor slows the edge and reduces the high frequency content. It must be placed so that the current cannot bypass it.

Filtering at the connector reduces what reaches the cable, which is often the difference between passing and failing. Our ferrite bead notes describe the selection.

Grounding and Shielding

A shield that is connected at only one end does not carry the current that would cancel the field and may make the emission worse by acting as an antenna.

Where the shield is bonded at both ends, the current flows in the shield and the field is confined. The bonding must be continuous around the perimeter for the high frequencies involved.

A chassis that is connected to the board at a single point leaves the rest of the perimeter as a slot. Our industrial assembly notes describe how such enclosures are bonded.

Verification

Each change should be measured with the same configuration as the original failure, so that the improvement is attributable.

The measurement should be repeated at the worst case orientation, because an improvement in one orientation can be accompanied by a worsening in another.

The final configuration should be measured with a margin that allows for production variation, since a component tolerance or a supplier change can move the emission.

Additional Considerations for This Build

Practical attention to shield bonding pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating shield bonding explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, radiated emission is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. 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, radiated emission is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

Process Control and Verification

On a design of this kind, radiated emission is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

FAQ

Can an emission be fixed by software? Sometimes. Reducing the drive strength of an output or spreading the spectrum of a clock reduces the emission, and both are legitimate measures.

Does a ferrite clamp on the cable always work? It reduces the common mode current where the impedance it adds is significant compared with the circuit impedance, which is often the case.

What does gopcb provide for emission problems? We provide switching loop layout that minimises the area, connector filtering with short returns, edge rate control on the fast nets, and measurement guidance that identifies the mechanism before the change is made.