Resolving RE issues in High Speed Designs

Radiated emission tests are conducted as a part of certification tests to make sure that the device under test doesn’t radiate un-intentional radiations above a specified limit. The levels are specified by the test standard & vary depending on the device application.

One of the main reasons for radiation is impedance mismatch of the high frequency signal lines in the PCB. At high frequency, a signal trace present in the PCB, acts like a transmission line. Thus all the properties related to transmission line such as reflection coefficients, impedance mismatch can be applied to the PCB signal trace.

Ideally, if the input impedance, trace impedance & load impedance of a trace are perfectly matched, then there will not be any reflection of the incident signal. Any mismatch in the impedance results in reflection of the incident wave & multiple reflections in the signal path results in ringing of the signal. Usually, reflection of the signal & subsequent ringing result in the radiation being emitted from the device at the harmonics of the signal frequency.

Adhering to the generic EMI/EMC design guidelines such as matching trace characteristic impedance of the trace to the source impedance, routing high frequency signals in internal layers, having proper ground planes, providing termination resistor options for tuning etc can help to reduce the RE issues during testing.

If there are any issues during RE testing, below approaches can be considered to resolve the same.

  • Resolve the impedance mismatch issues on the signal traces
  • Place EMI/EMC shields on the high frequency circuits

In the first approach, the emission will be reduced by matching the impedance of the signal traces thereby reducing the ringing. It is very seldom that a circuit will have perfect impedance matching from source to load. One of the methods to correct the impedance mismatch is by using termination techniques. Different termination techniques such as series, parallel, thevenin can be used depending on the circuit design. In addition to this, excessive ringing can also be reduced by reducing the drive strength of the signal at the source.

Magnitude of the Radiation emitted from the unit can also be reduced by placing the EMI/EMC shields on the circuits emitting the radiation. The shields attenuates the radiations emitted by the unit, thus helping to reduce the emissions within the limits. Emission levels will also be reduced if the device is placed inside a metal enclosure.

With the above design & tuning techniques, required certification tests can be passed without multiple iterations of the hardware design thus reducing the cost & time impact on the product design.

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