Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 55

One difficulty with microstrip transmission lines is that the ground and signal conductors are on different physical planes. Coplanar waveguide (CPW), as the name implies, provides a coplanar structure of ground‐signal‐ground, as shown in Figure 1.30b. An alternative is grounded coplanar where the backside is a conductor as well, and in practice, all coplanar lines have associated package ground, but the ground may be ignored if there is a substantial air‐gap between the substrate and the package ground. The references provide some computations of coplanar waveguide impedance for various configurations (Wen 1969; Simons 2001). In microwave measurements, CPW is used extensively as a contacting means for on‐wafer measurements and is used to provide extremely low ground inductance for measuring microwave transistors and circuits, as shown in Figure 1.31, with either topside grounds (left) or backside grounds (right). Note that since the impedance depends only upon the scale of width to space, this allows contacts of large scale (such as probes) to be transitioned to small scale such as IC devices.

Figure 1.31 CPW‐mounted IC.

CPW has some inherent problems due to the ground being on a surface plane or sheet. In many instances the CPW line is mounted in a metal package, and the ground plane is grounded at the package wall. If the distance from the package wall to the ground plane edge approaches a quarter‐wavelength at the frequency of interest, or multiples thereof, then a transmission line mode can form such that the ground of the CPW appears as an open relative to the package ground. This concept of “hot grounds” for CPW has been observed in many situations and is sometimes avoided by periodically grounding one side to the other through a small cross connection on the backside of the CPW. Another method is to provide a lossy connection to the sidewall ground through absorptive material or thin film resist material to suppress energy in the unwanted mode. Another alternative is treating the CPW as a suspended substrate only under the gap between ground and conductor and “stitching” the CPW ground to the backside ground through a series of conductive vias. The impedance of these structures is lowered by the added ground, so an adjustment of the center line width is usually made to accommodate the additional ground paths.