9-4 Option 100 Fault Location and SRL
Characteristics
SRL Measurement Uncertainty vs System Directivity
SRL Measurement Uncertainty vs
System Directivity
System directivity, system and test lead stability, and cable connector
mismatch all affect measurement uncertainty. Figure 9-1 shows a graph
of measurement uncertainty curves for a −49 dB directivity system
applied to various return loss values. The upper trace is return loss plus
error, the bottom trace is return loss minus error, and the middle trace is
the return loss with no error.
Figure 9-2 shows the same for a −40 dB directivity system.
Notice as the return loss gets larger (closer to 0 dB), the effect of the
error is smaller. One use for this graph is to determine the measurement
guard band needed to specify cable performance. For example, if a cable
must meet a −30 dB return loss specification, and the system directivity
is −49 dB, then the value of −32 dB must be measured to guarantee
−30 dB.
To determine what the measured value must be, draw a line
perpendicular to the x-axis at the specification value. Draw another line
from the point at which it intersects the lower uncertainty trace to the
y-axis. The y-axis value is the value that must be measured to guarantee
the specification.
Conversely, if a cable is shipped with a measured value of −35 dB for
structural return loss, the value that might be measured by a system
other than the one it was originally tested on can be determined by
drawing a vertical line through the x-axis at −35 dB. (Refer, still, to
Figure 9-1). The upper and lower traces show, on the y-axis, the limits
with which the cable SRL can be determined. For this example, the
−35 dB cable could measure as bad as −33.5 dB, or as good as −37 dB. But
there is more to consider. If the first measurement of the cable was
performed on a similar system, it will have a similar uncertainty. For
this example, a cable measured as −35 dB on the first system could be as
bad as −31.5 dB. This could be measured on a second system as good as
−35 dB (if the directivity error is exactly the same magnitude and phase
as the first system), or as bad as −30.5 dB (if the directivity magnitude is
the same, but the phase is opposite). Fortunately, as with impedance
measurements, the directivities are unlikely to be worst case in
magnitude at the same frequency and with opposite phase on two
different systems.
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