[SI-LIST] Re: Question on varying the coupling ratio on a differential pair

Hassan-

It has always been my understanding that the instantaneous 
characteristic impedance (at any point) of a transmission has only a 
slight frequency dependency.

As we all know, Zo =3D sqrt(L/C).

Inductance has a frequency dependence only at low frequencies, where 
there is an internal inductance caused by the fields internal to the 
conductor.  But as frequencies increase, skin effect causes all the 
fields to travel on the exterior.  At this point, the instantaneous 
inductance of the transmission line is for the most part constant.

As for capacitance, I've always thought that it was a function of the 
size of the conductors and distance between the two conductors.  Is 
there any frequency dependency to that?

Now, return loss is measured from input to output of any given 
structure.  Therefore we don't view return loss as "instantaneous".  
So when I think of impedance and return loss I see two different 
(although very related) values.

Am I right?  Is there something I'm missing?

Regards,
Ryan Satrom
Everett Charles Technologies

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