I am using an industrial 5.25" SBC and am having some issues with EMC. All the problem frequencies are harmonics of the 12.288Mhz bit clock on the audio chip.
Since high speed digital is not an area I have any experience with I am hoping someone here will be able to advise me if what I am seeing is normal or not.
The reference design for the audio chip (ALC655) shows a 22 ohm series resistor and 22pF capacitor to ground on the bit clock line but the SBC manufacturer has just used a 33 ohm series resistor. This resistor is about 50mm from the audio chip.
There is no room for me to fit a capacitor and even if I could I would want to move the resistor so it was close to the pin of the audio chip. This is also very difficult since the track disappears into the inner layers of the PCB right at the chip.
Here are some pictures of what I see on the bit clock line. It seems to me that the ringing is excessive.
The supplier responded to my request for help by supplying their CE and FCC reports which don't show any of my problem frequencies at all. If I were a cynical type, I might think they had switched the audio chip off for the test. ;-)
The only options I can see open to me at the moment is the either switch to another SBC or add an extra shield to the SBC inside my metal case.
Any suggestions will be appreciated.