Understanding Phase Versus Temperature Behavior "The Teflon Knee"
Understanding complex microwave transmission concepts can be difficult. Understanding complex material properties can also be difficult. Understanding phenomena that combine both of these can be mind boggling. Phase stability versus temperature is one of these phenomena. This application note will provide the reader with a basic grasp of the situation.
Before we can discuss phase stability we need a basic understanding of phase length and phase delay. Phase length, also known as electrical length, is the number of wavelengths in the cable assembly at a given frequency. All cable assemblies have a mechanical length and an electrical length. The electrical length is usually specified in degrees.
Phase delay is the time that it takes a signal to travel from one end of the cable to the other relative to the time it would take in an equivalent air line. This is also sometimes referred to as the signal delay or propagation delay, and is usually measured in nano-seconds.
A microwave signal propagates in a coaxial cable at a speed that is primarily dependent on the dielectric constant of the insulating material between the center conductor and the outer conductor. Dielectric constant relates the ability of a material to store charge, relative to air. A material with a high dielectric constant can store a lot of charge. The function of a cable assembly, however, is to conduct the charge, not store it. Storing charge is the equivalent of slowing down the signal. Thus, microwave cable assemblies use insulating materials with a low dielectric constant, such as Teflon™ , to minimize signal delay.