1. Field of the Invention
The present invention is directed to a vector network analyzer. More particularly, the present invention is directed to a vector network analyzer that operates in the microwave frequency range.
2. Description of Related Art
Measuring characteristics and parameters of microwave devices or using microwaves to measure characteristics of things or targets is difficult because at microwave frequencies circuit parameters that do not matter at lower frequencies become important. In addition to the conventional parameters of current and voltage, at the microwave frequencies the additional parameters of electrical and magnetic fields, phase and the physical position of a point of interest within a device become important. Furthermore, current and voltage cannot be directly measured in the microwave portion of the radio frequency spectrum. In addition, it is essential that phase information of the microwave signal be ascertainable. Therefore, special instruments are required to measure characteristics of microwave devices.
A linear electrical network is a number of impedances connected together to form a system that consists of a set of inter-related circuits and that performs specific functions. The behavior of the network depends on the network constants. Networks may be passive, i.e., those that contain no energy source or sink other than normal ohmic losses, or active, which contain an energy source or sink. Microwave circuits may be characterized by many other criteria, e.g., resistance, resistance-capacitance, inductance-capacitance, inductance, lattice structure, bridge structure, series or parallel structure, linear, nonlinear, bilateral, unilateral, etc.
Even more generally, for purposes of this specification, a network is an electrical black box with one or more inputs or outputs, called ports, and may include a network formed between the test device and a target device that may or may not be connected together by conductors. Measurements of microwave circuits and components involve the characterization of the circuit as a network, and measuring the reflection and transmission coefficients of the ports.
Network analyzers are useful for measuring the network characteristics of many electrical systems, such as cables, circuits, and so forth; for ranging, i.e., measuring the distance between the analyzer and objects that reflect microwaves, such as aircraft, buildings, discontinuities in cables, and for many other applications.
Network analyzers can measure the reflection coefficient for any device or target and the transmission coefficient for any electrical device. They must be vector analyzers to recover phase information. Reflection coefficients can be used to determine, among other things, range or distance, whether for guided or unguided propagation. For example, the reflection coefficient can be used to determine the distance from the point of connection to a coaxial cable to a break in the cable, or the distance to a target, such as an airplane in flight. The ability to work with vectors being assumed, the device will hereafter be referred to simply as a network analyzer.
Network analyzers of the prior art require at least one phase locked oscillator to generate the microwave signals they require. The oscillator must be very stable throughout the full range of operating conditions, e.g., temperature. They must produce cw signals that are stepped in frequency. Manufacturing precision oscillators is very tedious and expensive. Manufacturing pairs of precision phase locked oscillators, which are employed in some network analyzers, is even more expensive. In addition, many of the circuit elements of prior art network analyzers must be precision microwave frequency components, which are quite expensive relative to comparable components designed for operation at lower frequencies.
Network analyzers in the prior art are expensive, bulky, heavy, and not portable. These disadvantages have seriously limited their availability and uses.
Accordingly, there is a need for a network analyzer that is relatively inexpensive, compact, portable, does not require two phase locked oscillators, and that does not require precision microwave frequency components.