The dots representing QAM symbols on the constellation display should be well within the grid lines (decision boundaries). When the dots approach or exceed these boundaries, significant errors are occurring in the transmission of the signal.

COMMON IMPAIRMENT TYPES

Three common impairments - thermal (system) noise, phase noise, and coherent interference. Gain compression and I/Q imbalance are examples of two other impairment types with specific graphic signatures.

Thermal noise

Thermal noise impairments cause the dots on the graph to spread out toward the grid lines (decision boundaries), indicating significant errors. The zoom feature would show more detail about the position of these dots in relation to the grid lines.

Phase noise

Phase noise impairments cause the dots on the graph to form a circular pattern. Phase noise is best seen on the grid without the use of the zoom feature.

Coherent interference

Coherent interference impairments such as CTB, CSO, and spurs -- cause the dots on the graph to cluster with an empty space in the middle.

CHANNEL INFORMATION

At the top of the screen in the right corner under the selected channel plan the QAM type (64 or 256) and channel number is displayed. Directly below the channel number is the channel frequency and channel level information.

Digital Performance

- Modulation error ratio (MER)

- Bit error rate (BER) before and after (pre/post) Forward Error Correction (FEC)

Interleaver Depth
(Annex B only)
When viewing a locked digital signal the interleaver depth is displayed in the BER area. The DSAM supports the following interleaver depth values:

- 1,128
- 2,64
- 4,32
- 8,16
- 16,8
- 32,4
- 64,2
- 128,1
- 128,4 (Optional Purchase Required)

NOTE:
If the displayed interleaver depth is greater than the interleaver depth supported, the BER values are displayed as dashes (-----). The frequency, level, MER and constellation display will remain accurate.

VIEWING OPTIONS

Several viewing options are available to assist you with data assessment. Press the VIEW softkey. Use the arrow keys to choose the menu item you prefer and press ENTER to make your selection.

- Select Pause to suspend measurement updates (Keystroke shortcut: SHIFT 8).

- Select (or deselect) Reset to manually reset the Constellation function.

- Select Single Channel to view the level measurement screen for a single channel.