microwave office coplanar waveguide
Hi zhipeng!
If the size of the design is not a corcern, then excitation of a higher-order mode limits your choice of W and G for a given characteristic impedance. Typically, cut-off frequency of that next higher-order mode is quantified with the relation
lambda_0/(a*sqrt(epsilon_r))=(2*G+W)
where
lambda_0: the free-space wavelength at the corresponding cut-off frequency,
a: A scaling factor, 8 or 10 is provided in the literature
epsilon_r: Relative permittivity of the substrate being employed
2*G+W: Pitch of the CPW line
When the next higher-order mode is excited, CPW line is said to radiate significantly into the substrate, that is why epsilon_r is provided in the formulation instead of epsilon_effective.
Then, what you would do is to select a cut-off frequency that is sufficiently away from your operating frequency and subsequently determine the maximum pitch dimension of your CPW. Next, determine W and G to satisfy a given characteristic impedance.
Hope this helps!