Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 40

Microwave components differ from other electrical devices in a few respects. The principal discerning attribute is the fact that the components' size cannot be ignored. In fact, the size of many components is a significant portion of a wavelength at the frequency of interest. This size causes the phase of the signals incident on the device to vary across the device, implying that microwave devices must be treated as distributed devices. A second, related attribute is that the reference ground for the device is not defined by a point but is distributed as well. Indeed, in many cases the ground is not well defined. In some situations, grounds for a device are isolated by sufficient distance that signal propagation can occur from one device ground to another. Further, even if devices are defined as series only (with no ground contact), one must realize that there is always an earth ground available so there can always be some impedance to this ground. In practice, the earth ground is actually the chassis or package of the device, or a power or other ground plane on a printed circuit board (PCB).

Finally, only in microwave components can one find the concept of wave propagation. In waveguide components, there is no “signal” and no “ground.” Rather, a wave of electric‐magnetic (EM) field is guided into and out of the device without regard to a specific ground plane. For these devices, even the transmission structures, waveguide for example, are a large percentage of a signal wavelength. Common concepts such as impedance become ambiguous in the realm of waveguide measurements and must be treated with special care.