Agilent Technologies HP 8719D Service Manual page 257

Isolate the Fault in the RF Path
Since the calibration devices have been verified, the problem exists in the test
port connector, the coupler, or elsewhere in the internal RF path. lhble 9-l
shows which assemblies affect each error term. If more than one error term is
bad, note which assemblies are common to each of the bad terms. These are the
suspects.
The method of fault isolation that must be used is assembly substitution.
Assembly Substitution Fault Isolation
At this point, the error term problem has been isolated to a specific port and
you should have a list of suspected assemblies.
1. Swap identical assemblies between the port 1 and port 2 signal paths and
then regenerate the error terms.
2. If the problem moves from one port to another, you have found the offending
assembly.
Note
Before trying this, be sure to inspect the front panel test port
connector for obvious damage. Tighten ail semi-rigid cable
connectors inside the instrument.
Cable Test
The load match error term is a good indicator of cable problems. You can
further verify a faulty cable by measuring the reflection of the cable. Perform
an S11 l-port calibration directly at port 1 (no cables). Then connect the suspect
cable to port 1 and terminate the open end in 50 ohms.
Figure 9-l shows the return loss trace of a good (left side) and faulty cable.
Note that the important characteristic of a cable trace is its level (the good cable
trace is much lower) not its regularity. Refer to the cable manual for return loss
specifications.
94 Accessories Troubleshooting