EMC Problems with Arduino Nano

[luky315]

I have a product that failed EMC precompliance testing.
It uses an Arduino Nano clone (chinese version with the CH340 and with as bad schematic and layout as the "original") and not much else appart from a bunch of LEDs (with PWM dimming, 5mA each), switches and connectors. We saw a noticeable emission around 120MHz but strangely no significant harmonics, just this line. Any ideas what this can be? It is for sure from the Arduino because it disappears when the microcontroller is in reset.

[Niklas]

What type of measurement, radiated or conducted?
What do you have connected to your test object? Is the Arduino board connected as a CPU module to some kind of kind or motherboard, possibly with several cables attached?
What is used to power the test object?

[Shonky]

Conducted is typically only up to 30MHz so would be radiated.

First thought was USB related as 480/120 is neat and there's usually some multiple of 20MHz around but CH340 is only Full Speed USB so should only have a 12MHz clock.

What other clocks are there including on your board?

Got some near field probes?

[luky315]

The USB is not connected, the Arduino Nano is used as a cheap "CPU module" and there is not much else on the main board except of a few 5mA leds which are driven by a PWM (directly from the AVR), no long wires or anything. This could make some noise, but then I would expect to see a lot of harmonics and not a single line at 120MHz. It looks like some kind of oscillation, but there are no DC/DC regulators (just the LDO on the Arduino) and no Opamps or something similar that could oscillate.
I didn't expect any problems with this product and was very surprised.

[moffy]

My guess would be the CH340 chip and its 12MHz crystal, even though the USB is not connected the CH340 could still be signaling the D+ and D- wires as well as its own 12MHz square wave clock. Can you disable the chip and its oscillator as a test since it is not in use?

[luky315]

I think that I have to desolder it since the CH340 has no RESET Pin but it is definitely worth a try. Interestingly it is the CH340C variant with the internal clock, so there is no 12MHz oscillator.