Re: PCB trace with controlled impedance

Zhiheng Cao wrote: 
Is there any reference on how to calculate Zo
when the top plate is surrounded (horizontally)
by ground plate.


  This structure is sometimes called a "Coplanar
Waveguide with Ground". Eq 3.4.3.1 from Waddell
says

Zo = 188.5ohm / [ sqrt(er,eff) ( K(k)/K(k') + K(k1)/K(k1') ) ]

where

          1 + er K(k') K(k1) / (K(k) K(k1'))
er,eff = ------------------------------------
          1 + K(k') K(k1) / (K(k) K(k1'))

k = a/b, where a is the trace width and b is the width
of the groove (trace plus the two insulating gaps)

k1 = tanh( Pi a /(4 h)) / tanh( Pi b /(4 h))
where h is the substrate thickness

k' = sqrt(1 - k^2)
k1' = sqrt(1 - k1^2)

The function K(x) is the elliptic integral.

Regards,
         Mikko
.