Reference

Eye / Jitter Mode (WFM700M Only)

Press the EYE button to access the Eye / Jitter menu and measurements.

Making Eye 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-correlated 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.

WFM700 Series Waveform Monitors User Manual

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