Magic shield plates helps naked A20-OLinuXino-LIME2 board to pass CE EMC requirements.

Ax0-OLinuXino-LIME-1

A20-OLinuXino-LIME and A20-OLinuXino-LIME2 are choice for many industrial applications, when the board is inside metal box like BOX-LIME:

BOX-LIME-BLACK-1

it has no problems with shielding the radio emissions, but when just board is used as component, our customers has to shield it by their own in their designs. We had number of request to do official CE EMC certifications for A20-OLinuXino-LIME2 as this would make their life easier, and we decided to do this after we already get some experience with the certification of USB-ISO and ESP32-POE-ISO.

Initially the naked board failed on EMC measurements as seen here from the test results.

LIME2-naked

The major problem is the Gigabit Ethernet PHY. You can see spikes at 125 Mhz, 325Mhz etc. Disabling the PHY removes all noise and everything is in safe levels. So we knew who is ther trouble maker.

The major noise emitter are the Transmit and Receive clock lines. We put 33 ohm resistors in series R119 and R123 in A20-OLinuXino-LIME2 schematic , but obviously this was not enough. To further fade the line harmonics we replaced the resistors with 0402 Ferite Beads with 600 ohm impedance at 100 Mhz. This lowered the emissions, but still not well enough to be at safe levels.

So we had to make small Faraday cage on top:

LIME2-SHIELD-top

and bottom of the board:

LIME2-SHIELD-bot

this effectively removed all noises and the A20-OLinuXino-LIME2 pass CE requirements:

LIME2-1

LIME2-2

LIME2-3

LIME2-4

We are preparing now to implement the Ferite Beads on PHY Tx and Rx lines in all our other boards production assembly, and will offer the shield plates as kit they are good for A20-OLinuXino-LIME and A20-OLinuXino-LIME2 and we will prepare same for A20-OLinuXino-MICRO and A64-OLinuXino.