VeEX VePal CX350, Руководство пользователя

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Spectrum analysis is not overly complicated, but if you are not careful it is very easy to make mistakes. The following overview provides simple guidelines to accurately
measure the amplitude of analog and digitally modulated carriers.
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7.4.2 Amplitude Measurements
Overview:
Analog and digital carriers are very different in terms of the signal content and distribution of power over the channel and therefore need to be measured differently. Signal
level meters that are designed to measure only analog carriers will not always measure digital carriers accurately.
Power is measured either as an absolute level or relative to another power level. Carrier levels are absolute measurements and are measured in power units, such as dBmV
or dBuV. Relative power measurements are C/N, delta audio to Video, CTB and CSO and are always measured in dB.
Analog TV channel amplitude measurement
The DOCSIS RF Specification recommends that the average power of a downstream digitally modulated carrier be set 6 dB to 10 dB below what the amplitude of an
analog TV channel on the same frequency would be. Before measuring or setting the amplitude of a digitally modulated carrier, first measure the amplitude of the
cable network's analog TV channels.
Digital Carrier amplitude measurement
On a digital carrier, the power is spread out more evenly over time and frequency compared to an analog carrier. To obtain an accurate representation of the
power, you must measure the total power across the frequency band, not just at one narrow frequency the way an analog SLM measures level.
The 6/8 MHz downstream QAM carrier is often referred to as the haystack because it resembles a pile of hay you would see on a farm. The haystack is a
continuous MPEG bit stream. The idea is not to just measure the amplitude of the QAM signal, but to measure the total power contained within the 6 or 8 MHz
carrier. This is similar to needing to measure the area within the signal (haystack) instead of its height only.
Using this example, the power has to be measured within one channel or within a specific frequency range. Use caution when viewing digital modulated signals
because the signal level is dependent on the selected measurement bandwidth (resolution bandwidth).
Spectrum Analyzers typically do not have an IF or resolution bandwidth wide enough to measure a whole digital signal at once. To overcome this problem, the
CX350 makes multiple measurements across the frequency range of the carrier. The powers of each of these measurements is summed and the average
power of the whole channel is calculated.
Note: Amplitude Measurements
Analog TV channel visual carrier amplitude and digitally modulated carrier average power are commonly
measured in decibel millivolt (dBmV), a unit of power expressed in terms of voltage.
Mathematically, dBmV = 20log (signal amplitude in millivolts/1 millivolt).
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7.4.3 Ingress Measurements
7.4.3.1 Forward Path Ingress
Overview:
Forward Path Ingress can be analyzed between 54 to 1000 MHz (6 MHz channel plan) or 108 to 1000 MHz (8 MHz channel plan).
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 is 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.
7.4.3.2 Reverse Path Ingress
CX350 e-Manual D07-00-037 RevC01 Page 33 of 81