VeEX CX380C, Руководство пользователя

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7.3.3 Ingress Measurements
Ingress noise is a term assigned by cable operators to describe any interference that is coupled into the forward or return path cable
plant via an external source, and can be broadly categorized as follows:
Narrow band ingress: The predominant coupling mechanism for ingress noise is a poorly shielded drop coaxial cable that
acts more like an antenna than a drop cable. AM modulated carriers, amateur band and maritime radio transmission generate
unwanted signal frequencies at varying amplitudes according to the propagation conditions.
Location specific ingress: Electronic equipment in the subscriber premises can pass strong signal carriers back into the
cable system, and these can interfere with reverse signals, such as cable modems.
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Return Path Outages
Return Path Noise and Ingress can be a result of:
Stationary impairments
Thermal noise
Intermodulation Distortion (IMD)
Frequency response problems
Transient impairments
RF Ingress
Impulse noise
Signal clipping
Multiplicative impairments
Transient hum modulation
Intermittent connections
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Troubleshooting Techniques
1. The "divide and conquer" method normally works best. Establish the most distant point from the headend at which the signal
is known to be good quality. Start midway between this point and the affected subscribers to locate the problem amplifier
2. Systematically check the signal level at the return path receiver test points in the headend or hub until the problem node is
found
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Ingress Measurement Notes:
The spectrum analyzer’s ability to measure low-level signals is limited by the noise generated inside the spectrum analyzer. The
sensitivity required to measure low-level signals is determined by the measurement setup. The spectrum analyzer
Input Attenuator
and
Resolution Bandwidth
settings are the key factors that determine how small of a signal the spectrum analyzer can measure.
Input attenuator - When enabled, reduces the level of the signal at the input of the mixer to avoid compression, which can produce
unwanted frequency products. An amplifier at the mixer’s output will re-amplify the attenuated signal to keep the signal peak at the
same point on the analyzer’s display. However, in addition to amplifying the attenuated input signal, the noise present in the signal
and analyzer is amplified as well. This has the undesired effect of raising the displayed noise level of the analyzer.
Resolution bandwidth - Affects how closely a small signal can be seen in the presence of a large one. By increasing the width of
this filter, more noise energy is allowed to hit the envelope detector of the analyzer. This also has the effect of raising the displayed
noise level of the analyzer.
Maximum sensitivity - For maximum sensitivity, both the input attenuator and resolution bandwidth settings must be minimized. If,
after adjusting the attenuation and resolution bandwidth, an ingress signal is still near the noise, stability of the displayed trace can
be improved by video filtering the display using the Video Bandwidth (VBW).
Common Path Distortion (CPD)
Produced by poor contacts in the cable distribution network, these contacts create a diode or rectifier effect which produces
potentially harmful 2nd and 3rd order Intermodulation Distortion (IMD) products or beats. These beats will occur every 6, 7
or 8MHz in the reverse path, depending on the channel plan used. Although the magnitude of these beats is small, they
increase at a node when several reverse paths are combined.
CX380C User Manual RevA00 Page 38 of 105