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Why are common mode chokes with 90 ohm @ 100 MHz recommended for USB 2.0 Hi-Speed data lines

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newbie_hs

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When one looks at common mode chokes for USB 2.0 applications choke manufacturers always recommend CMC with a common mode impedance of 90 ohms @ 100 MHz.

Example

How does this relate to

  • USB 2.0 differential impedance of 90 ohms (& 45 ohm single ended impedance)
  • the "Hi-Speed" data rate of 480 MBit/s (respectively a frequency of 240 MHz)
  • an approximate USB 2.0 bandwidth of ~1200 MHz (5 x 240 MHz)
Why aren't they made for, say 90 ohm @ 240 MHz? Why are they trimmed for 90 ohm @ 100MHz? Couldn't chokes with values that differ from this 90 ohm @ 100 MHz be used as well? What are the determining factors here?
 

Hi,

USB works in differential mode, this is the opposite to common mode.

So the values of impedance and frequency are independent for both modes.

The common mode chokes are made to have low influence of differential mode signals. They pass the USB signal with a series impedance (probably dominated by resistance) with way lower than 90 Ohms at 240MHz.

In common mode they represent an increasing impedance with increasing (common mode) frequency. So common mode noise above 240MHz will be suppressed, while the differential USB signals will pass.

Klaus
 

Several wrong assumptions.
These are common mode chokes, USB differntial signal doesn't "see" their common mode impedance, only the slow SE signalling of full speed USB could be affected by it. Only the differential mode impedance (leakage inductance + losses) has an effect on fast USB signal.

Chokes are inductors, their impedance at 100 MHz is purely reactive. EMI inductors are often specified with an impedance value to allow an estimation of their filter effect. The impedance is however frequency dependent and they are not trimmed.
 

My understanding about CM choke selection is that it's Differential mode attenuation will be 0dB or minimum at our frequency of interest. Common mode impedance will be maximum at it's operating frequency. Here they are specifying common mode impedance only at 100Mhz,but I am operating at 240Mhz
Please correct me if I am wrong
 

Hi,

did you read the datasheet of the device?
there I expect more detailed information.

Klaus
 

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