FreeLists / si-list / [SI-LIST] USB 2.0 and common mode chokes

["SEOW,ERWIN-SP (HP-Singapore,ex6)" <erwin-sp.seow@xxxxxx>]

Hi All!
 
I have a question regading the use of common mode chokes and USB 2.0. I have
read through the USB 2.0 Platform design guideline by Intel  (ver 0.92)and
it states that the use of Common Mode choke (On the D+ and D-) was possible
if the impedence was less than 270 Ohms (At 100MHz). 
 
My question is why would a Common Mode choke affect the waveform of a
DIFFERENTIAL signal? My understanding of Common Mode Choke is that no matter
what Impedence it is, it should not have any effect on the Differential
Signal (forward and backward current cancels out the resulting Magnetic
field). But from reading this document, it seems like the Common Mode
Impedence DOES affect the signal.
 
Anyone care to explain?
 
Thanks!
Erwin Seow
email:erwin-sp.seow@xxxxxx
  
 

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