Eye Diagrams (I-Eye or Q-Eye) (Trace Format)
Eye diagrams are commonly used in digital communication systems. Eye diagrams help identify problems such as ISI Inter-Symbol Interference: An interference effect where energy from prior symbols in a bit stream is present in later symbols. ISI is normally caused by filtering of the data streams. (Inter-symbol interference) and jitter Variation in the amount of time that it takes packets to traverse the network and arrive at a destination.. The following Eye diagram information is provided:
- General Information
- Positioning of Eye Diagram
- Eye Diagrams and Frequency-Domain Data
- Width of Eye Diagrams
- Eye Diagrams and Incorrect Symbol Timing
and trace data use chips and not symbols. So, for these trace data types, the following information still applies except that the symbols with chips need to be replaced.
General Information
An eye diagram is simply a display of the I (real) or Q (imaginary) signal versus time that is triggered by the symbol clock.
The eye diagram is a superimposed display. For example, the VSA draws the first trace, then overlays the second trace, the third trace, and so on until the number of symbols specified by Result Length (Digital Demod) (or by Meas Interval (W-CDMA/cdma2000) is displayed.
The second trace is a continuation of the first trace; the third trace is a continuation of the second trace, and so forth. In other words, the VSA draws one trace to the end of the display and then wraps it back to the beginning of the display to start the next trace.
To create a complete eye diagram, the I or Q signal must alternate between all states. The number of eyes (vertically) in the eye diagram indicates the number of states.
For example, if there are two states, one at 1 volt, and another at -1 volt, the eye diagram would have 1 eye: the top of the eye corresponds to the state at 1 volt, the bottom of the eye corresponds to the state at -1 volt. The degree to which the two signal levels are discernible at the clock instant determines the clarity of the data in the eye diagram.
As is evident, there are m-1 eyes in an m-level system; where m is the number of states. In other words, if a vector modulation system has 4 states, the complete eye diagram would have 3 eyes¾one above the other.
Positioning of Eye Diagram
The VSA centers the eye diagram on the display such that the eye is in the center of the display. For example, if the length of the eye diagram is one symbol, the eye diagram is positioned on the x-axis between -1/2 symbol and +1/2 symbol.
In the eye diagram, the VSA displays vertical lines at the symbol positions. Symbol positions are at the location where the maximum eye opening should occur and correspond to the symbol clock.
For closer examination, set the eye length less than
one (1).
The eye diagram for some modulation formats, such as , look better when
the eye length is odd (1, 3, etc.).
For on signals, the symbol positions are offset from the display's center by 0.5 symbol. This looks best when the eye-length is an even number of symbols.
Eye Diagrams and Frequency-Domain Data
Eye diagrams are relevant only for time-domain data. However, if the eye diagram is seleced for frequency-domain data, the VSA will display the I or Q signal corresponding to the selected eye format versus time but will not superimpose the trace segments.
Width of Eye Diagrams
The width of the eye diagram can be varied by varying the parameter (click ). The width ( in time) of the eye diagram is calculated as:
Replace symbol rate with chip rate for and demodulation composite time trace data.
Eye Diagrams and Incorrect Symbol Timing
If the eye diagram shows incorrect symbol timing, see Troubleshooting for details.
