Digital Signal Processing Reference
In-Depth Information
18 Measuring Digital TV Signals in the Broadband
Cable
In contrast to the measuring techniques used on digital TV signals trans-
mitted via satellite, a wider range of measuring techniques is provided for
broadband cable testing and is also necessary. The influences acting on the
broadband cable signal, which can be modulated with up to 256QAM, are
more varied by far, and more critical than in the satellite domain. In this
section, the test instruments and measuring methods for measurements on
DVB-C and J83A, B, C signals will be discussed. A large amount of space
is reserved for the so-called constellation analysis of I/Q modulated signals
which is also encountered in DVB-T. The influences or parameters to be
considered in cable transmission are:
•
Signal level
•
C/N and S/N ratio
•
I/Q modulator errors
•
Interferers
•
Phase jitter
•
Echoes in the cable
•
Frequency response
•
Bit error ratio
•
Modulation error ratio and error vector magnitude
To be able to detect and evaluate these influences, the following test in-
struments are used:
•
An up-to-date spectrum analyzer
•
A test receiver with constellation analysis
•
A test transmitter with integrated noise generator and/or channel
simulator for stress testing DVB-C and J83A, B, C receivers
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