To counter-react radiated noise, measures should be taken at the source of the noise.
If the PCB design (e.g. decoupling capacitor, grounding) and shielding are taken into consideration, most of the noise problems will be prevented.
Unfortunately the radiation levels frequently increase when an interface cable is connected even if radiation countermeasures are done at each device.
The EMC problems are usually caused when a common-mode noise is created sometimes by both unbalanced signal lines on the PCB and the inferior
quality of the cables being used, not to mention the cables magnify the problem themselves by acting like antenna.
The EMC problems are unexpected and serious.
The EMC solutions for USB2.0 devices can be limited since these device use a high speed data rate with sensitive signals.
Chip beads usually cannot be used since they will distort frequencies in the operating frequency range.
An effective EMC solution would be to use a 2 line CMF(common-mode filter) on the signal lines and a power bead used on VDD line. Also a multi-line CMF, which lumps the signal lines, the VDD line and GND, will work well.
A CMF is suitable on high speed differential lines which do not allow signal distortion if the CMF has a low differential-mode impedance.
This note will present some proven common-mode filters which can be used to filter USB2.0.
Notes:
This note is for designers who have EMC problems stemming from possible PCB design and for the person who wants to prevent noise influx from other peripherals.
We confirm that NECfs USB2.0 controller IC has high integrity signal waveforms and does not have any EMC problems.
If you design the PCB precisely, you will be able to get enough margin to FCC(or VCCI) standard in EMI test. |
