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Reference
WFM700 Series Waveform Monitors User Manual
3-17
Eye / Jitter Mode (WFM700M Only)
Press the EYE button to access the Eye / Jitter menu and measurements.
The Eye pattern display presents a voltage-versus-time waveform composed of
multiple sweeps of the SD or HD serial bit stream.
You can set the sampled eye pattern to display either a full word of correlated
data (10 Eye display in SD; 20 Eye display in HD) or a three-data-bit overlay
showing jitter uncorrelated to the parallel word (3 Eye display). In the word-cor-
related mode, the sweep is also synchronized to the video, so that you can see
any shift in the data stream timing correlated to horizontal or vertical video
information. This mode is most useful for detecting jitter patterns or modulation
related to parallel-to-serial conversions.
Jitter is seen in the eye diagram as a horizontal thickening of the trace. As jitter
increases, the opening in the eye shrinks until the receiver can no longer reliably
decode the serial stream.
The usefulness of the eye measurement decreases with long cable lengths. Even a
short cable will slow the rise time and make evaluation of a source difficult.
Slightly longer cables will make one eye run into the next. This is called
intersymbol interference. It makes the eye appear to have multiple levels and
closes the eye opening vertically long before the receiver fails to decode the
signal. The only information the eye measurement gives on longer cables is
some idea of the cable length.
Different clock recovery bandwidths can be selected to help isolate the frequency
of any jitter that is observed. When a bandwidth setting is chosen, only jitter at
frequencies above that bandwidth are observed.
You can turn on or off a readout that displays automatic measurements and a
histogram of the eye waveform. Refer to Automatic Eye Measurements on
page 3--18 for more information.
Setup for Eye Measurements. Perform the following steps to prepare the
instrument for Eye measurements:
1. Connect a 100% Color Bar, serial video-signal to the instrument using a
75 Ω cable of two meters or less in length. Use high quality, low loss coaxial
cable, such as Belden 8281.
2. Press the GAIN button and select 1x vertical gain.
3. Press the EYE button to select the Eye Pattern display.
Making Eye
Measurements