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Live Fixes and Recording Findings

A pre-scan ends with fixes tried on the bench and findings written down. A finding names a frequency, a source and a fix, and it carries the evidence that the fix works.

The known-emitter list​

Before the scan, a list of the board's known emitters is built from the schematic, the design briefs and the bring-up records. Every peak is labelled against it. The list takes every source of a periodic signal on the board:

  • each switching converter, at its switching frequency from its datasheet or design brief;
  • each PWM output, such as backlight dimming, at the frequency the firmware actually sets;
  • a class-D audio amplifier, at its switching frequency and the width of its spread spectrum, noting that it switches whenever it is enabled;
  • each high-speed link, such as a MIPI DSI display link, at the clock that its lane rate implies;
  • each clock, such as a pixel clock or a processor crystal, and the memory interface; and
  • each radio, at its band.

Each entry also generates its harmonics, and each pair of switching converters generates beat frequencies at the sums and differences of their frequencies. A peak that matches nothing on the list is labelled unknown and located with the near-field probe.

Live fixes​

The two or three worst offenders each get a quick fix on the bench, where one exists, followed by a re-scan with nothing else changed. Typical fixes are:

  • a ferrite on a cable;
  • a clip-on shield over a part or a flex; and
  • a lower setting on the load, such as a lower backlight brightness, to confirm the source.

A fix that lowers the emission by 3 dB or more is a finding worth keeping. Below 3 dB the change is inside the repeatability of an uncalibrated bench and is not counted.

Recording​

  • Each trace is saved as a screenshot with the analyser settings visible, the mode, the supply, the distance and the cable layout recorded beside it.
  • Each finding is raised against the board's next revision as a change, giving the frequency, the source, the evidence and the fix, as Recording results and closing out describes.
  • The session record holds the setup, the settings, every trace and the benchmark comparison, and states that no reading is a compliance result.