1. Field of the Invention
This invention pertains to transformers and specifically to coupled transmission line impedance transformers for very low impedances.
2. Description of the Prior Art
There are many prior art patents detailing the many applications where it is desired to transform electrical impedances from one value to another. A very common application is the matching of input and output impedances between the standard 50 ohm transmission line and low impedance level electronic devices such as diodes and transistors. Previously, microstrip impedance matching was one of several methods which could be used. As electronics has gone to lower and lower impedances, the microstrip method was found less applicable because conventional microstrip techniques could not match low impedances.
Among the known techniques for providing impedance transformation is U.S. Pat. No. 3,965,445 to Ou titled MICROSTRIP OR STRIP LINE COUPLED TRANSMISSION LINE IMPEDANCE TRANSFORMER. The Ou patent is based on two articles, one by E. M. T. Jones and J. T. Bolljohn, "Coupled Strip-Transmission-Line Filters and Directional Couplers" from IRE Transactions on Microwave Theory and Techniques (April 1956), pages 75-81 and a second article "Parameters of Microstrip Transmission Lines and Coupled Pairs of Microstrip Lines", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-16, No. 12 (December 1968) by T. G. Bryant and J. A. Weiss. These articles show some of the basic fundamentals used in coupled transmission transformers.
The technique shown in the Ou patent relies on coplanar coupled transmission line conductors on a dielectric substrate which are tuned by attached capacitors. While the Ou patent specifically shows using coupled transmission lines of a quarter wavelength electrical length, .theta., Ou states that .theta. is not critical. This is because the capacitors permit tuning for any irregularities caused by poor match of electrical length. The use of capacitors for tuning the imaginary component results in the use of such things as gold wire, epoxy and other electrical conductors and insulators which will fatique and break down with repeated use. Due to the nature of such miniaturized electronic devices component breakdown frequently results. Repair requires the complete unit to be disassembled or the appropriate components replaced in their entirety.
Thus, an advancement of the art clearly called for is the use of coupled transmission lines resonant transformers which are not subject to tuning conditions and can be used for low impedances. This requires that the resonant element transformers or impedance matching devices be specifically designed to have a natural oscillation or resonance at the desired frequencies.