Tektronix WFM 601i, User Manual

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Measurement Applications
WFM 601i Serial Component Monitor
3–11
measurement, properly designed serial interfaces should achieve reflection
coefficients of under 0.1 (10% or 1 div of a 10 division display).
When the output of a video processing chain is a black picture instead of the
expected program material, the video engineer must start searching for where the
signal was lost. For baseband analog video this is reasonably straightforward to
do with the aid of a waveform monitor. In serial video, however, the video
information is modulated onto a digital carrier. Loss of either the modulating
input (video applied to the serializer) or the digital carrier (the serial video
signal) can lead to the black picture. Determining which is the case can be a
trial-and-error affair unless both the digital carrier and the video coded onto it
can be viewed.
Use the WFM 601i in PICTURE, WAVEFORM, or PARADE mode to check for
signal presence and content. If no signal is observed, switch to the EYE or EQ
EYE display to view the digital carrier. If no eye pattern is seen, then a serial
channel problem is the likely culprit. Continue moving toward the source until
the fault is found. If a proper eye pattern is seen, then look for a problem where
the video is serialized.
The eye pattern can be viewed with or without equalization. The choice of
display mode depends primarily on where in the coax channel the signal is being
inspected. For measurements directly on the output of sources, or with only short
lengths of cable, EYE mode is recommended. In this mode the serial signal, as it
appears at the rear-panel loopthrough connector, is displayed. Accurate
assessments of amplitude and pulse shape can be made in EYE mode (see
“Measuring Serial Sources,” page 3–5).
In situations where the signal is being inspected near the end of a long cable run,
the EYE mode will show what appears as a band of noise. The serial signal is
still there, but has been heavily attenuated by the coaxial cable loss. Serial
receivers compensate for this by employing equalization. Ideally, this restores the
wave shape to what it was at the source. In EQ EYE the output of the WFM 601i
serial receiver equalizer is displayed. This allows the eye pattern to be viewed
after even several hundred meters of coax.
The EQ EYE mode is most useful for establishing serial signal continuity in a
long cable. Interpretations on the EQ Eye pattern shape can be misleading,
however, since the WFM 601i conditions the receiver input signal to improve
performance with specific video patterns. This causes peaking of the EQ Eye
signal even with normal amplitude serial inputs. While an experienced operator
can learn to “read” this display and infer information on the link performance,
EQ EYE should be regarded as an uncalibrated, qualitative measurement mode.
A good rule of thumb is: use EYE mode whenever possible. Use EQ EYE when
EYE mode fails to provide a useful signal.
Checking Signal
Continuity
EYE and EQ EYE Displays