How to get rid of noise around 200MHz caused by USB?

[uoficowboy]

Hi - I have a device that I'm doing EMC testing on. I'm seeing narrow band noise around 200MHz at harmonics of 12MHz. Any suggestions as to how to get rid of it? I tried putting a large capacitor (0603 300pf) from the D+ line to GND and from the D- line to GND, and that fixed it, but puts me out of USB spec (the spec only allows 75pf). I tried a 75pf capacitor instead and it was not enough - the noise was still there, just a little smaller.

The device is a small PCB with a USB microcontroller on it. It has about 6" of cable with a USB-A connector on the other end. The cable is shielded and the shield is tied to the connector's shield but not to anything on the PCB. The device is connected to a laptop during this testing. The laptop, without this device connected, is perfectly quiet at the frequencies of interest.

Any ideas? Thanks!

 

[hans512]

Try using balun (ferrite bead) at USB cable.

 

[uoficowboy]

Try using balun (ferrite bead) at USB cable.

I'm not sure what you mean by balun. My guess is that that is not the word you meant.

There already is a ferrite bead in line with the +5V line. I've been thinking about trying to put ferrites in series with the data lines as well, but I have not had a chance to play with that just yet.