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7.1.3 Constellation Measurements
In contrast to the measuring techniques used on digital TV signals transmitted via satellite, a wider and necessary range of measuring techniques is provided for broadband
cable testing. The influences acting on the broadband cable signal, which can be modulated with up to 256QAM, are more varied and critical than in the satellite domain.
Constellation measurements are an example of such advanced measurements and are an ideal tool to identify QAM modulator problems.
Note: QAM lock
Ensure QAM lock is achieved on the digital carrier prior to making any Constellation measurement.
Constellation button:
The following measurements are reported:
QAM Power Level (numeric in dBmV)
MER (numeric in dB) is an indication of the operating margin of the system
Pre BER & Pre Error Seconds (numeric) is the error rate of the incoming signal prior to being corrected FEC circuitry
Post BER & Post Error Seconds (numeric) is the error rate of the signal after the FEC has corrected all the possible errors
Severely Error Seconds (numeric)
Constellation (graphic)
Max Adjacent Channel Delta (numeric in dB)
The constellation display supports a zoom function. For QAM64 and QAM256 modulation, the constellation is split into 4 quadrants for simplified viewing. Tap on any of the
four quadrants, which will zoom into this specific quadrant.
Note: Bit Error Rate (BER)
Bit Error Rate (BER) measures how often an error occurs in a given amount of data.
The more errors, the more difficult it is for the FEC circuitry to correct those errors before picture
problems occur.
BER is useful for measuring long term system performance and periodic transient impairments that
can occasionally effect system performance.
BER is usually displayed in scientific notation e.g.:
Where 1E-3 means one error in every 1000 bits
Where 1E-6 is one error in every 1,000,000 bits
Ideally, the signal should have the highest exponent value - the higher the number, the lower the
errors.
Constellation diagrams
Constellation diagrams are used to graphically represent the quality and distortion of a digital signal.
CX350 e-Manual D07-00-037 RevC01 Page 26 of 81