Applications of Waveguide and Circuit Theory to the Development of Accurate Microwave Measurement Methods and Standards Page: 5

                                
An interesting variation of the tuned reflectometer is described in 5.6. It
consists of constructing a tuned reflectometer using rectangular waveguide components,
except for the output waveguide, which is coaxial. One then slides loads in the
coaxial section and adjusts the tuners belonging to the rectangular waveguide instru-
ments. Once adjusted, it is used to measure reflection coefficients of coaxial
terminations and devices.
The tuned reflectometer is also used to measure the reflections and losses of
waveguide joints or coaxial connectors by a sensitive technique described in 5.7.
There are many applications of microwave circuit theory to attenuation measure-
ments as discussed in Chapter 6. The subject of attenuation definitions is discussed
at some length in 6.2. Circuit theory is used to clearly show different results from
different definitions. Precise definitions suitable for highly accurate measurement
applications are formulated and the conditions of measurement are tightly specified.
The error due to mismatch often causes the greatest uncertainty in attenuation
measurements. Mismatch effects in cascade-connected attenuators are analyzed in 6.3
and mismatch errors in measuring fixed and variable attenuators are treated in 6.4.
Usually, only the magnitudes of the reflection coefficients of the circuits and the
attenuators are measured or estimated when evaluating mismatch errors. Of course,
the phases are also involved, but it is assumed that they can take on any value,
and the limits of error are calculated, assuming the most unfavorable conditions.
Actually, the realizability conditions limit the range over which reflection coef-
ficient phases can vary. Thus the limits of error calculated by the above method
may be too conservative in some cases. In 6.5, it is shown that the effect of
realizability on error limit calculations is practically unimportant except for
low-loss attenuators, below, say 1 decibel.
In most analytical techniques, an attenuator is represented by a simple 2-port
network. However, this model gives no information regarding the effect of imper-
fections of connectors or adapters. A more complicated model is required, and this
is discussed fully in 6.6.
In sections 6.7 and 6.8, techniques for measuring attenuation are described.
The first makes use of the theory of linear fractional transformation of reflection
coefficient. The second technique is a simple measurement of power ratio, but is
refined to give unprecedented accuracy. Circuit theory is applied to evaluate
small mismatch errors which contribute to the uncertainty of the measurement.