Trellis-Eye Diagrams (Trace Format)

Menu Path: Trace > Format > Format box

Trellis-Eye diagrams show time on the x axis and unwrapped phase change on the y-axis. The following information is provided:

• General Information
• Trellis Diagrams and MSK
• Tips When Using Trellis Diagrams
• Width of Trellis Diagrams

General Information

The Trellis-Eye diagram is similar to an eye diagram in that it consists of superimposed lines. For example, the VSA draws the first trace line, then overlaps the second line, and the third line, and so on until the number of symbols specified are shown on the trace. (number of symbols is determined by the parameter that determines result length (name varies by demod: Result Length (as in Digital Demod) , Meas Interval (as in W-CDMA/cdma2000), etc.).

Here's some additional information:

• Most trellis diagrams seen in text books start midway between symbols. In this VSA, trellis diagrams start at the first symbol. Furthermore, the VSA wraps traces such that the trellis diagram begins within +/- 90 degrees.
• For best results, set the Eye Length greater than or equal to four (4).
• The vertical trace grid lines are located at the detection decision points (at the symbol positions):
  

Trellis Diagrams and MSK

The trellis diagram is especially useful for analyzing the Minimum Shift Keyed (MSK) modulation format.

With MSK, the symbols occur in 90 degree intervals, at 45 degrees, 135 degrees, 225 degrees, and 315 degrees. Each symbol represents one bit¾a one (1) or zero (0). A positive, 90-degree phase change represents a one (1); a negative, 90-degree phase change represents a zero (0).

Therefore, with MSK, the trellis diagram shows a rising phase ramp if the bit is a one; it shows a negative phase ramp if the bit is a zero.

Tips When Using Trellis Diagrams

Here are some additional tips to help use and interpret trellis diagrams:

• Use Time Units to set the x-axis units in seconds or in symbols (W-CDMA and cdma2000 Composite Time trace data use chips).
• Use the arrow keys on the keyboard or the mouse wheel to examine transitions between symbols. Pressing the right-arrow key moves the marker forward in time (from one symbol to the next symbol). Pressing the left-arrow key moves it backward in time (from one symbol to the previous symbol).
• Use the offset marker to easily place the marker on any part of the trellis diagram. First turn on the offset marker and position it on the point of interest. Then select Move Offset to Mkr to quickly move the marker to the point nearest the offset marker.

Width of Trellis Diagrams

As with eye diagrams, the width of the trellis diagram can be varied. Vary the width of the trellis diagram the same way as an eye diagram¾with Eye Length (Trace > Digital Demod).

The Eye Length determines the width, in symbols, of the trellis diagram. If the value of Eye Length is 5, the trellis diagram is 5 symbols in width, and the width, in time, is calculated as: