Coplanar waveguide sandwiched between GND and VCC

[raebrm]

Hi

A question about coplanar waveguides (CPW). A conventional CPW structure is a signal line sandwiched between two ground tracks. Can we get the same characteristic impedance if we sandwich a signal line with the same dimensions between a GND track and a VCC (some DC supply voltage, say +5V) track?

Thanks

 

[shlomo22]

Yes. The AC component of the voltage on VCC is zero. From an AC perspective, this trace functions identically to a ground trace.

(You can separate between AC and DC like this because electromagnetic fields are linear.)

 

[volker_muehlhaus]

For coplanar lines, it is good design practice to insert bridges between the grounds, to keep both grounds at the same potential and surpress other modes. This is not required for straight lines, of course, but as soon as you have bends, the path length of both grounds are different and this would create other modes. Now these bridges on both sides of the bend will allow the ground current to balance between both ground strips, keep the CPW mode and surpress other modes.

Now if one of the two "grounds" is VCC, then you can use capacitors instead of a metal bridges, so that RF current can flow between the GND and VCC.