1-18 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
Measurement Uncertainties
In any comparison of cable impedance or structural return loss data, it is
important to understand the measurement uncertainty involved in each
type of measurement. This is critical for manufacturers, who often use
the most sophisticated techniques to reduce manufacturing guard bands.
It is also important in field measurements that users choose the proper
equipment for their needs, and understand the differences that can occur
between manufacturers’ data and field data. Also, note that
measurement uncertainty is usually quoted as the worst-case result if
the sources of error are at some maximum value. This is not the same as
error in the measurement, but rather a way to determine measurement
guard band, and to understand how closely to expect measurements to
compare on objects measured on different systems.
The errors that can occur in a reflection measurement are reflection
tracking (or frequency response), T, source match, Γ
M
, and directivity, D.
The total error in a measurement can be shown to be
Equation 14
where Γ
DUT
is the reflection response of the DUT.
Error correction techniques can effectively remove the effects of tracking.
Also, source match effects are small if Γ
DUT
is small. This leaves
directivity as the largest error term in the reflection measurement. The
causes and effects of these error terms will be described for each of the
measurement methodologies.
For variable bridge measurements, the directivity of the bridge is the
major error term. One-port vector error correction reduces the effects of
tracking and source match, and improves directivity. The directivity
after error correction is set by the return loss of the precision load,
specified to be better than 49 dB at 1 GHz. However, the directivity is
only well known at the nominal impedance of the system, and the
directivity at other impedances should be assumed to be that specified by
the manufacturer. For best performance, the bridge should be connected
directly to the cable connector, with no intervening cable in between.
The directivity of the bridge could be determined at impedances other
than 75 ohms, by changing the impedance and measuring the resulting
values. This can be done by changing the reference impedance to the new
Γ
MEAS
TD
Γ
DUT
()
1 Γ
M
Γ
DUT
–()
-----------------------------------+⋅=
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