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Quarter wavelength rule?

Question asked by bruce_qin on Aug 11, 2009
Latest reply on Sep 14, 2009 by Kenneth_Wood

      I know something about the mysterious quarter wavelength rule, it is usually to be said as: when routing high speed signal trace, the trace length must be avoided to become integral multiple of quarter wavelength (such as 1/4, 1/2 ,1, 3/4, 5/4 and etc.) so to avoid the standing wave, which cause seriouse EMI problem.

      Have I got the correct meaning? If not, what is it? Or if so, my questions:

     1. How to calculate the wavelength?

        considering a material with Dk=4, so I can got the velocity of the signal by 11.8/squar(4)=11.8/2=5.9in/ns, so for a  transmission line with 6Gbit/s signal on it, the wavelength is

          wavelength=velocity*period=velocity/frequency=5.9/6=inch?

     

     2.Even the material is constant, the signals travell in microstrip  and stripline have different velocities, the signal in microstrip will be faster. It means the wavelength for the same bit rate is not constant.  So should I use one wavelength for microstrip and another for microstrip? How about the same net changing from outer layer to inner layer?

 

     3. If there is a capacitor or a via or a connector in the net, should I add its length to the whole net length? Or just calculate each net segment?

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