Option 100 Fault Location and SRL 1-19
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
value, say 76 ohms, changing the bridge to that value, and measuring the
impedance on a Smith chart display. The difference from exactly 75 ohms
represents the directivity at that impedance.
For fixed bridge methods, the reflection port is often connected to the
cable connector through a length of test lead. A one-port calibration is
performed at the end of the test lead. The directivity will again be set by
the load, but any change in return loss of the test lead due to flexing will
degrade the directivity of the measurement system. In both fixed and
variable bridge measurements, the repeatability and noise floor of the
analyzer may limit the system measurement. A convenient way to
determine the limitation of the measurement system is to perform a
calibration, make the desired measurement, then re-connect the load to
check the effective directivity. A very good result will be better than
−80 dB return loss of the load. Typically, flexure in the test leads,
connector repeatability, or noise floor in the network analyzer will limit
the result to between −60 to −40 dB. If the result is better than −49 dB,
then the system repeats better than the load specification for the best
available 75 ohm loads. Thus, the effective directivity should be taken to
be the load spec of −49 dB. It is possible to reduce this limitation by
having loads certified for better return loss.
Measurement Uncertainty for Impedance
Measurements
The fixed bridge method calculates the cable impedance by averaging the
impedance of the cable over frequency. The variable bridge uses a
reading of the impedance from the dial on the bridge. The directivity at
any impedance can be determined, as stated earlier, but only to the limit
of the return loss of the load, and the system repeatability. Table 9-1 on
page 9-5 shows the effect of directivity on cable impedance measurement
uncertainty.
Any connectors and adapters used to connect the test-lead cable to the
cable under test can have a significant effect on the impedance
measurement. With the variable bridge method, the operator determines
the appropriate setting, taking into account the capacitive tuning
adjustment. With the fixed bridge method, it is also possible to
compensate somewhat for the connector. However, it is often the case
that the cable impedance is determined by the low frequency response,
up to perhaps 200 MHz to 500 MHz, where the connector mismatch
effects are still small. The choice of frequency span to measure cable
impedance can itself affect the value obtained for cable impedance. In
general, as the connector return loss becomes worse, it will have a
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