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40 The PCB Design Magazine • March 2017 MICROSTRIP COPLANAR WAVEGUIDES Figure 4 illustrates a dual (or differential) coplanar waveguide, analyzed in the iCD CPW Planner. This structure has equal gaps and ground pours on each side of the strips. The strips are also referenced to a solid ground plane below. Note the fence of stitching vias that are placed at less than a quarter wave length apart, close to the edge of the GND pours. This structure is used to improve isolation between components that would otherwise be coupled by electromagnetic fields. It consists of a row of plated through via holes that, if spaced close enough together, form a barrier to electromag- netic wave propagation. Planar strips readily couple to each other when in close proximity, an effect called parasitic coupling. The coupling is due to fringing fields spreading from the edg- es of the strip and intersecting adjacent lines or components. Even if you are not involved in RF or micro- wave design, the use of CPWs is extremely use- ful when dealing with isolated differential strips without a ground reference plane as in Figure 5. In this case, the ground plane is cleared around a gigabit Ethernet connector to provide isola- tion from the outside world. So, a plane can- not exist in this area although the differential pair must maintain 100 ohms impedance to match the line. Good design practice for Ether- net ports requires over-voltage and over-current protection devices in addition to proper com- ponent creepage distances and electrical trace clearances for both sides of the Ethernet I/O connections, i.e., the line connector and driver (physical layer, or PHY) sides. Loosely and tightly coupled grounded co- planar waveguides (CPWG) circuits respond dif- ferently to the application of conductors with and without a plating–such as electroless nickel immersion gold (ENIG) finish. A tightly coupled CPWG circuit, with an ENIG finish, will suffer greater conductor loss than a loosely coupled CPWG circuit with the same ENIG finish. At approximately 2.7GHz, the resonant Figure 4: Dual strip coplanar waveguide grounded (source: iCD Design Integrity).