Controlled impedance PCBs are usually produced using microstrip and/or stripline transmission lines in single-ended (unbalanced) or differential (balanced) configurations.
A micro strip line consists of controlled width conductive traces on a low-loss dielectric (in practice the dielectric may be constructed from a single dielectric or multiple dielectric layers) mounted on a conducting ground plane. The dielectric is usually made of glass-reinforced epoxy such as FR-4. For very high frequencies PTFE may be used. Other reinforcement/resin systems are also available.
For close spaced differences on woven glass reinforced dielectrics, refer to application note AP139 on the Polar Instruments web site, www.polarinstruments.com.
There are several configurations of PCB transmission line:
Exposed, or surface, microstrip
Coated microstrip (coating usually solder mask)
Buried, or embedded, microstrip
Centred stripline
Dual (offset) stripline