Application Notes: Return Path Troubleshooting
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The preamp is used to raise the signal above the noise floor of the test
equipment. This is especially a problem on the return signals that are read
from high loss test points.
The SDA-5000 has a preamp built-in and compensates all measurements
accordingly.
If the problem is at the output of a tap when testing at the seizure screw,
continue on.
Some new probes from SignalVision and Gilbert create a good ground and
quick connect.
NOTE: One caveat to this is a probe will always be bi-
directional and will cause an impedance mismatch itself.
This is something to keep in mind when troubleshooting.
Sometimes an in-line pad can be attached to decrease the
amount of energy be tested, which is turn, may create a
better match. Be careful when probing seizure screws,
though. The AC present will harm in-line pads and certain
test equipment. The Acterna equipment is AC blocked for ~
100 Vac.
If the problem of a tap is at the input and not the output, then the problem is
from one of the drops.
Disconnect one drop at a time to determine the point of entry.
The Noise Mode
The ability to switch between a headend mode and a remote analyzer mode
has many advantages. You can successfully use the “divide and conquer”
technique to quickly find the source of the problem so that you will not have
to rely on another person’s interpretation. This also eliminates inefficient use
of resources and employee time.
NOTE: Be careful with spectrum analyzer, noise level
readings. 2 dB/div is a good scale for sweeping and 10 dB/
div is good for the spectrum mode. The level displayed is
based on the RBW setting and will be very different from
one setting to another. A -20 dBmV noise floor with 30 kHz
RBW is really 1.2 dBmV in a 4 MHz bandwidth and there’s
usually a correction factor associated with it. That’s why
measurements with no point of reference are very
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