One method to transition controlled impedance lines from layer to layer is to form a coaxial line where the center via is your transmission line which is then surrounded by a ring of ground vias. Ground via diameter is dictated by the dielectric constant of your substrate material. \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \_ Mike McNulty \_ \_ Lockheed Martin GES \_ \_ Moorestown NJ \_ \_ Phone:(609)722-7323 \_ \_ Fax:(609)722-2207 \_ \_ [log in to unmask] \_ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ ***************************************************************************** does anyone have any data concerning 50 ohm transmission lines starting on the first layer and using vias to transport it to an inner layer. I have looked at the IPC specs concerning microstrip etc. good info. unfortunately it just doesn't mention anything about how to get a impedance controlled line from one layer to the next. I know that someone out there has done the numbers or knows where to go to get them. Please let me know thanks in advance.