When driving the ethernet shield 2, I get rather high amplitudes / peaks at 25 MHz, 125 MHz and 150 MHz that radiate over a few meters (picking it up with an antenna). Looking at it with a localization poste, it appears to come out of the TWI connectors, at least there seems to be the highest intensity.
The shield is not connected to anything. It is driven with an Arduino Giga, also with no other connections than the power supply.
Since I didn’t find any similar issues, does anyone experience something similar? Could there be an option to deactivate the possible sources for radiation or shield / ground it in an effective manner?
25 MHz is the base crystal frequency used by many PHYs, 125 MHz corresponds to the 5th harmonic, and 150 MHz is the 6th harmonic. Even if the PHY is idle (no network connection), the oscillator continues running, producing these harmonics, and unconnected pins like TWI can indeed act as small antennas, radiating these frequencies into space. This is not uncommon for development boards running at high speeds.
I guess it’s time for mitigation, like ensuring the shield and Arduino ground are solidly connected. If you do not use I²C, you could try grounding the SDA and SCL pins to reduce radiated emissions. Enclosing the shield in a grounded metal case would likely contain most of the radiated energy.
One additional thing you may want to check is whether the Giga is running the PHY in RGMII or RMII mode. Some PHY configurations keep the reference clock and internal PLLs active even harder in RGMII, which can increase the amplitude of those harmonics. I’m not entirely sure on the Shield 2 implementation, but if the clock mode is fixed you can’t disable it you can only reduce the coupling paths.
Also, any high-impedance line (like the TWI pins floating) tends to behave like a small monopole antenna at those frequencies. Adding small RC terminations (for example 1–2 kΩ to ground or a 33–100 pF cap) on unused lines sometimes reduces the radiated peaks without touching the PHY itself. Not guaranteed, but it has worked for similar boards.