Technical information

Application Note: Return Path Troubleshooting
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A single source of interference is easy to track down. If it’s
constant, just use the “divide and conquer” theory to dissect
the system. Observing how it reacts and changes could indi-
cate different sources such as a trucker or home user. A CB
level changing quickly or slowly could indicate this source
quickly.
Multiple ingress sources, bursty noise, and electrical tran-
sient noise are a totally different story and are very difficult
to pinpoint. Remember that the lower value taps contrib-
ute more noise and ingress than the higher value taps. The
lower attenuation from tap values of 14 and below coupled
with the low attenuation in the cable at lower frequencies
creates an easy path for noise to funnel back.
Return Path Power Addition
Many people don’t fully understand power addition and
become discouraged when trying to perform noise miti-
gation. A little decrease could be more than you think.
Understanding power loading for return path ingress is
essential to help aid in troubleshooting.
For example: I observe a CB signal at 27 MHz in the
headend at 20 dBmV total power. I disconnect one leg
by removing a reverse input pad and the level drops to
18.8 dBmV. I disconnect the second leg and the level drops
to 17 dBmV. After disconnecting the third leg, the total
power drops to 14 dBmV. After disconnecting the last leg,
the ingress at 27 MHz is eliminated. So the question that
remains is, which leg has the largest amount of ingress?
The answer is none. All four legs of the node are funneling
equal amounts of noise to the headend of 14 dBmV! Two
14s equal 17. Three 14s equal approximately 18.8 and four
14s equal 20 dBmV. Remember, every doubling of power is
3 dB.
Test Location Considerations
• Because the return path signals are low in level, it may
be warranted to use a preamp.
• e preamp is used to raise the signal above the noise
floor of the test equipment. is is especially a problem
on the return signals that are read from high loss test
points.
• e newer units have a preamp built-in and compen-
sate all measurements accordingly.
• If a problem is observed at the output seizure screw of a
tap, 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-direc-
tional and will cause an impedance mismatch itself.
This is something to keep in mind when troubleshoot-
ing. Sometimes an in-line pad can be attached to
decrease the amount of energy tested, which in 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 equipment
is AC blocked for ~ 100 Vac.
• Start with 14 dB taps and lower. If the problem is at the
input of the tap and not the output, then the problem
is from one of the drops or farther upstream possibly
from a cracked cable before the next amplifier.
• Look at one drop at a time to determine the biggest
contributor.
Noise Readings
• Be careful with spectrum analyzer, noise level readings.
2 dB/div is a good scale for sweeping and 5 or 10 dB/
div is best for the spectrum mode.
• e 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.
Note: The “Spectrum” mode is not the same as a true spec-
trum analyzer. The RBW is set at 280 kHz and a VBW >
1 MHz. This is optimized for analog carriers and burst
noise measurements. It has a peak noise detector so
the noise reading may be significantly higher than a
normal spectrum analyzer with the same RBW set-
ting.
• A pad on the analyzer will lower the level as well. At-
tenuation and gain affect noise and carriers equally.
• Measurements with no point of reference are very
misleading. If there’s a reference carrier present, you
can make a relative measurement, such as desired-to-
undesired ratio (D/U). One fault with this, though, is
RBW settings affect noise and continuous wave (CW)
carriers differently. A CW carrier is theoretically 1 Hz
wide and the level won’t change with different RBW
settings while the noise level will, thus giving a differ-
ent D/U ratio. A CW carrier will change shape on the
analyzer display because of the RBW filter width.
The “Noise” Mode
• e ability to switch between a headend mode and a
remote analyzer mode has many advantages. One can
successfully use the “divide and conquer” technique to
quickly find the source of the problem and not have
to rely on another person’s interpretation. is also
eliminates inefficient use of resources and employee
time.