Chapter 8
Measure Mode – Basic Tab
Constellation
174
DSAM Help User Guide
Rev. 001
3
Press
ENTER
.
4
To adjust the reference level by (1) dB increments, use the
up
and down arrow keys
.
The reference level range is -40 to +60 dBmV.
5
Press
ENTER
.
Constellation
The Constellation measurement displays a constellation graph of the
demodulated digital Quadrature Amplitude Modulation (QAM) type (64
QAM, 128 QAM, or 256 QAM) signal prior to error correction. The
channel of interest must be defined as either digital or DOCSIS type,
64 QAM, 128 QAM, or 256 QAM, within the active channel plan. The
Constellation measurement also displays:
– Channel number
– Channel frequency level information
– Modulation Error Ratio (MER)
– Bit Error Ratio (BER) Pre and Post data
Identifying a specific constellation pattern on the grid and associating
this pattern with a specific type of impairment quickly leads to trouble-
shooting options that minimize or eliminate the impairment.
Running a
Constellation Test
To run a Constellation test
1
Press
Measure
.
2
Press the
Basic
softkey.
NOTE
A good QAM signal displays a tight grouping of dots in the center of
each square on the grid.
The current zoom level is displayed in the lower right corner of
screen. “Zoom:1” indicates that the entire constellation is displayed
with no zoom applied.
21128026 R001 DSAM Help User Guide.book Page 174 Thursday, February 5, 2009 2:02 PM
