Tektronix WFM 601i, User Manual

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WFM 601i Serial Component Monitor
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Measurement Theory
Eye Pattern Display
Eye pattern displays are commonly used to view binary coded digital signals.
The display shows the voltage levels used to represent the two logic states, and
the transitions between them. Proper “distance” between the logic levels, as well
as proper position of the transitions, is important for error-free data transmission.
Eye patterns are typically formed by triggering an oscilloscope at some fraction
of the serial clock frequency while applying the serial bit stream to the scope’s
vertical channel. Sometimes the serial signal is holding at a “low” or “high”
logic level, and sometimes it is transitioning between levels. These combinations
end up producing an overlaid display showing both logic levels and all the
different ways to transition between them. In between these transitions, the
separation or “distance” of the two logic levels must be sufficient for the receiver
to clearly detect whether a “high” or “low” occurred. This separation is the so
called “eye opening” of the signal.
Eye patterns are often used in conjunction with a template or “mask” to estimate
system error rate. Serial video systems operate at such low error rates however,
that eye closure masks are not practical. Instead the eye pattern is used to check
signal parameters such as amplitude, risetime, and jitter, as well as to identify
signal distortions such as those caused by transmission channel reflections.
The WFM 601i uses an equivalent time sampler to create the eye pattern display.
Equivalent time samplers are used where waveforms are repetitive. The
waveform voltage is sampled at many different points at a rate much slower than
the basic repetition rate of the signal. Over many cycles of the signal, a
representation of the waveform is created. Although the serial bit stream may not
be repetitive, the eye display formed by overlaying the bit patterns is repetitive.
In the case of the WFM 601i, the eye pattern sampler samples at 7 MHz, with an
equivalent time bandwidth of at least 450 MHz.
Vector Display
The vectorscope and its XY Cartesian display has long been a staple of the
television industry. In its more familiar configuration it presents a display of the
two color difference signals (R–Y & B–Y in NTSC, or V & U in PAL) that are
decoded from the composite video signals. In the composite world a sample of
subcarrier (burst) is supplied with each line of video to synchronize the decoding
of the coloring information contained in the color subcarrier. This provides a
display where it is possible to measure color phase errors (angular displacement)
and color signal amplitude errors (radial displacement). The system is based on
the accuracy of the color burst(s), which are placed at the correct phase and then
any variance in the color bar vectors is measured by the mislocation of the dots