1-20 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
greater effect on the resulting impedance measurement. The uncertainty
caused by the connector is difficult to predict, but large errors could occur
if the low frequency return loss is compromised to achieve better high
frequency structural return loss.
Finally, note that since both methods average, in some way, the
measurement over the entire frequency range, it is probable that the
worst case error will never occur at all frequencies, and with the same
phase. In fact, it is more likely that the errors will cancel to some extent
in cable impedance measurements. Also, the loads that are used will
invariably be somewhat better than specified, especially over the low
frequency range. From this, it is reasonable to assume that the errors in
impedance measurements are at least 50% less than listed in Table 9-1
on page 9-5.
Measurement Uncertainty for Structural Return Loss
The same factors that affect cable impedance — directivity, system and
test lead stability, and cable connector mismatch — also affect structural
return loss. However, since structural return loss is measured at all
frequencies, it is much more likely that a worst case condition can occur
at any one frequency. For that reason, the measurement uncertainty
must include the full effect of the above listed errors.
Refer to Chapter 9 for further discussion on this subject.
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