1. Field of the Invention
This invention relates to microwave system analysis, and in particular to a method of determining the power response of a microwave system.
2. Description of the Related Art
Microwave components and systems are specified in terms of their power or voltage responses. Since power is equal to voltage squared divided by impedance, the power response is related to the square of the voltage response. Insertion loss (IL), return loss (RL), and voltage standing wave ratio (VSWR) are useful indications of the power response of microwave components and systems.
The voltage standing wave ratio is a measure of the amount of power reflected from a device or load, relative to the amount of power incident on the device. It is therefore a purely dimensionless number. The return loss is the dimensionless ratio of reflected to incident power, usually expressed in decibels.
On the other hand, insertion loss is a dimensionless ratio, usually expressed in decibels, of the amount of power that can continue to the circuitry that follows the device, relative to the amount of power incident on the device. The insertion loss measures, in addition to the reflection loss effect represented by the voltage standing wave ratio, the effect of heat loss under normal conditions. Also included in the insertion loss are losses due to radiation, coupling, leakage, cross talk, propagation mode transformation (moding) and so forth; each of which is, however, totally controlled under normal circumstances.
Each microwave system has a unique insertion loss and voltage standing wave ratio, which is determined by both the microwave response characteristics of the individual components in the system and the manner in which these components are interconnected. However, for systems consisting of multiple diverse components, the voltage calculations conventionally used to predict the VSWR response rapidly become intractable, and the margins of error between predicted response and observed response increase greatly because the worst-case combination of voltages is not normally seen on the system level.
One such VSWR method is the graphical or mathematical implementation of the Smith Chart. This technique begins with a load impedance, and processes the complex impedance and electrical length of a microwave system's components back through the system to predict the worst-case voltage combination of reflections. There are several limitations to use of this technique. First, the system components are normally specified in terms of insertion loss and VSWR, not complex impedance and electrical length. Second, complex impedance and electrical length are not derivable from insertion loss and VSWR specifications. Third, complex impedance and electrical length, if known, vary tremendously with frequency; so this technique is a reasonable approach for narrowband systems, but tedious even for computers for bandwidths beyond a decade. Attenuation, if known, can be accounted for, but it also varies greatly with frequency and complicates the process even further.
An alternative VSWR response analysis technique begins with a load reflection, proceeds back through the system operating on voltage values independently of phase considerations, and results in the worst-case voltage combination of reflections. The equations necessary to implement this technique are found in Moreno, Microwave Transmission Design Data, Dover Publications Inc., N.Y., N.Y., 1958. Moreno's equations apply directly to the situation of two reflection points in intimate contact, and do not account for the existence of an attenuation element between reflection points. Furthermore, Moreno lists only the equations, and does not present a treatise on the technique required to implement them on a system level.
For transmitted power, the conventional method of analysis is to add the insertion loss values in decibels of the system components in order to find the insertion loss of the system. This known technique inherently ignores the possibility of a worst-case voltage combination of reflections because it operates on power values rather than voltage values.
The insertion loss addition technique assumes that the components will operate independently, without appreciable mutual interactions, when assembled into the system configuration. The value returned by this approach is not influenced by the direction of power flow, and predicts the system's most likely insertion loss. Under normal conditions, the technique of adding component insertion loss values is reasonably accurate because the worst case combination of voltages is not usually seen on a system level.
Because both VSWR techniques process the combination of voltages, the resultant value is not congruous with the technique used for insertion loss, which processes the combination of power. A set of system response parameters for insertion loss and VSWR that are calculated via the conventional techniques are therefore not consistent with each other, and do not present a basis from which to draw conclusions on the acceptability of measured data.
Additionally, both VSWR techniques start at the load, and proceed backward through the circuit with respect to the direction of power flow. Such a technique does not provide an intuitive feel for the design of system circuits because design normally proceeds in the same direction as the flow of power.
From the standpoint of system specifications, both VSWR techniques require knowledge of a load. This means that a system manufacturer must either have a priori knowledge of the user's setups in order to specify the system, or tie the system specification to a variety of load conditions, because the conventional calculations intrinsically depend on the load's characteristics. It is much more desirable to specify a system independently of any external equipment, and without knowledge of its intended use.
Each of the two conventional microwave system analysis methods for finding the worst case voltage combination of reflections is relatively difficult to implement. Both are inadequate in that neither method enables calculation of the worst-case power combination of reflections, which represents the VSWR most likely to be exhibited by a system. The technique of adding component insertion losses represents the power transmission characteristic most likely to be exhibited by a system. The VSWR values and insertion loss values arrived at via these conventional methods are therefore not mutually consistent.