[SI-LIST] Differential traces over a split in the ground plane

Hi,

Consider, first, the ideal case.

Considering edge coupled differential traces over a plane(I suppose 
that one could generalize to any kind of differential pair over a 
plane). If the legs of the pair, in the ideal case, were identical 
in length, width, etc, I would expect that there would be no 
differential to common mode conversion and there would be no return 
current through the ground plane. 

If there is no return current in the ground plane then I would 
expect that if the ground plane was "choked" with ferrites (these 
are ideal ferrites that take up no physical space) at some point in 
between the two ends of the differential pair, then the odd mode 
impedance would not be affected. I would expect that the even mode 
impedance would increase as a result of the ferrites.

Now consider the real world.

In a PCB, in order to "choke" the ground plane underneath the 
differential pair I would first have to physically cut the plane, 
bring vias to the surface and then use a ferrite chip to connect the 
two sections of plane. 

My question is, does that physical cut cause an increase in the odd 
mode impedance (and, hence, a discontinuity) of the differential 
pair, at the point of the cut in the plane? I would expect the 
answer to be yes.

Thank You
Tom
tom_cip_11551@xxxxxxxxxxx



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