Spectrum Analysis - Digital Signal
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 6dB to 10dB
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 6/8MHz 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,
Summary of Contents for VEPAL CX350S-D3.1
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