Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 100

One vital VNA component is the directional device used at the test port to separate the reflected wave from the incident wave. This is most often a directional‐coupler or directional bridge, although simpler structures have been proposed as well. These devices are characterized by their main‐line loss (the attenuation of the a1 signal), the coupled‐arm loss (the attenuation of the b1 signal), and their directivity (the ability to separate the b1 signal from the a1 signal). In addition, any mismatch in the directional device will contribute to the port match and source‐match. If there is mismatch before the directional device, it will have no effect on its directional characteristics (directivity or isolation). However, any mismatch after the directional device, such as in a test port cable or fixture, will contribute equally to mismatch and degradation of directivity, as described in Section 1.10.