Tektronix WFM 601i, User Manual

Page: 93 / 135

Measurement Applications
Rev Aug 1994
WFM 601i Serial Component Monitor
3–15
The WFM 601i has two trigger modes for the Eye pattern displays: 10-EYE and
OVERLAY. In 10-EYE the Eye pattern display is triggered on parallel
word/tv-line boundaries, with 10 eyes shown per sweep. Parallel word and
tv-line correlated behavior can be seen in this mode. If a serial system is
experiencing a disturbance that appears related to video patterns, either word or
tv-line, use the 10-EYE mode in either EYE or EQ EYE to try and identify the
problem. Use line select (see below) to window the Eye pattern display to the
area of interest in the video field.
In OVERLAY trigger mode, the Eye pattern display is purposely decorrelated
with the parallel word boundaries. That is, each of the 10 eyes that occur
between parallel word clocks are all overlaid, with equal probability of
occurrence, at a fixed location on the screen. This causes each of the three
displayed eyes to appear identical, with peak jitter shown at each eye crossing.
The choice of the 10-EYE and OVERLAY trigger modes is made in the EYE
PATTERN CONFIGure menu. The chosen trigger mode is indicated with a CRT
readout in the upper left corner of the screen.
The Eye pattern displays can be swept at field rate. While individual eyes cannot
be seen in this mode, low frequency changes in the envelope of the serial signal
can be observed. Some video patterns, such as the Equalizer SDI Check Field
Signal defined in SMPTE RP-178 (also known as the “pathological” signal) can
contain a large low frequency component. Serial equipment with inadequate
coupling time constants may not properly pass this signal without errors.
Sweeping the eye pattern at field rate can help locate these sources. Other
disturbances, such as mains hum, can be seen in field sweep.
Use the SWEEP button to toggle the eye pattern between the normal 3 ns/1 ns
displays and field sweep. When in field sweep, the time/div display in the upper
right screen corner will say FIELD. Normal one field magnification (X25) is
available with the MAG button. Also, the line selector (see below) can be used in
this mode, with the selected lines identified with a z-axis bright-up.
The unique eye pattern triggering used in the WFM 601i allows the familiar
video line selector to be used just as it is with standard video waveforms. One
line and 15 line displays, on either or both fields, are available. The line selector
can be used to correlate events seen on the picture monitor, the analog waveform
display, and the Eye pattern display.
For example, suppose a disturbance is observed on the picture monitor. Activate
the line selector, in 15 LINE mode, and adjust the line selector knob until the
bright-up strobe overlays the picture defect. Select an Eye pattern display, and
choose FIELD sweep mode. Observe that 15 lines of the eye pattern envelope are
intensified. Check for any disturbances of the envelope in the intensified region.
Push the SWEEP button to return to a normal Eye pattern display. Only the eye
pattern corresponding to the 15 selected video lines is displayed. Similarly, if
Observing Field Rate
Behavior
Using the Line Selector in
Eye Pattern Modes