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Transmission lines and impedance control PCB design

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  • Transmission lines and impedance control PCB design

    Hello everyone,

    I have wondered about the rule of thumb about transmission lines that says - if trace length is over 10% of wavelength of the signal then transmission line effects should be taken into account.

    Let's say that I route a USB 2.0 signals with 480 Mbps the frequency is equivalent to ~200MHz square wave (according to google )
    So if I make the signals length bellow
    150mm(10% of the wavelength) can I route the signals without impedance control, length matching or termination resistors?

    Is my theory right? or am I missing something? Please tell me.

    If my theory is correct, why does engineers consider impedance control ,length matching or termination resistors when the signals length are bellow 10% of wavelength?

    Thanks alot!

  • #2
    Your "square wave", will contain harmonics of higher frequencies.


    • #3
      Thank you very much Robert for the quick response.

      Ok, I got it.

      I have another example. lets say that I have routed DDR2 signals and the signal's length from CPU to RAM is 25mm.
      in this case it would mean that frequency should be roughly above 1 GHz .RAM clock frequency is only 133 MHz and even the first few harmonics are under the 1 GHz.
      is it should work fine? or I need to consider impedance control, termination resistors and length matching.