Test fixture for permittivity measurements of dielectric films at microwave frequencies.
Original Publication Date: 2001-Mar-21
Included in the Prior Art Database: 2005-Jun-09
National Institute of Standards and Technology
Ryusuke Nozaki: INVENTOR [+2]
A microwave test fixture for broadband permittivity measurements of dielectric films comprises a coaxial input section and an adjustable coaxial termination section. Both sections have the same diameter of the outer conductor and a suitable matching assembly mount. A circular dielectric film or disk specimen is electrically coupled to the center conductor of the coaxial input section. The adjacent side of the specimen is electrically coupled to the adjustable coaxial waveguide termination section. In the adjustable coaxial termination section, the center conductor fits precisely with the diameter of the outer conductor. The position of the center conductor can be adjusted along the main axis. This allows a low inductance coupling to the test specimen_ The arrangement can also form an adjustable reference plane, a standard air-gap dielectric specimen or it can be used to achieve a flat and parallel contact with the inner conductor of the input section, which then configures the fixture into an electrical short termination. The fixture is electrically equivalent to a network in which the dielectric film represents a transmission line with capacitance that is inserted between two matched transmission lines. The wave propagation takes place along the diameter of the specimen rather than across its thickness. By using a vector network analyzing instrumentation having a characteristic impedance matching that of the coaxial input section, the complex scattering coefficient of the network can be measured and transformed by an appropriate computational algorithm into permittivity of the specimen. The test fixture allows accurate broadband permittivity measurements at frequencies of 100 MHz to over 40 GHz for film specimens with high and low values of the dielectric constant.