Tektronix WFM 601i, Руководство пользователя

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Measurement Applications
3–6
WFM 601i Serial Component Monitor
Select the desired input channel (SERIAL A or B) and select EYE mode. Either
the graticule or the voltage cursors can be used to make the measurement. For
quick, approximate measurements, use the graticule. Select the desired vertical
gain, making sure that variable gain is off. For most measurements X1 gain,
giving 100 mV/div, will be used. Note that the volts/div is indicated in the upper
left screen corner, and that a “>” symbol is added if the variable gain is turned
on.
For more accurate measurements, use the voltage cursors. Any gain setting,
including variable gain, can be used since both the waveform and the cursors are
affected by the gain. Higher gain settings, such as X5, can often facilitate
matching the cursor to the waveform. Use the on-screen readout to determine the
voltage difference between the two cursors.
Either the OVERLAY or 10-EYE display mode can be used. Since time jitter can
obscure the measurement, particularly if waveform aberrations are present, the
“JITTER HPF” should be set to 1 kHz.
Signal sources should measure 800 mV
p-p
10%. Signal amplitudes outside this
range can degrade receiver performance. Experience has shown that proper
amplitude is important.
Serial sources should produce good signal transitions, with a minimum of
overshoot and ringing. Otherwise, automatic equalizer circuits in receivers may
not perform properly. While there is presently no specification for this, Tektronix
recommends that aberrations be held to 10%. At this level they are not likely to
degrade system performance.
To measure aberrations with the WFM 601i, use the Variable Gain to obtain a 10
division display. Aberrations should not exceed
1 division.
Since the Eye display bandwidth is 450 MHz, it will not faithfully display
aberrations that are faster than this. Most receiver circuits are band limited and
usually ignore very fast transients. Accordingly, the WFM 601i is a good
indicator of problem sources in systems.
Adjust the variable vertical gain for an Eye display of exactly 10 divisions.
Position the bottom of the display to the “–.3” graticule line and check that the
top is aligned with the “.7” line. Activate the timing cursors and align the first
cursor to the intersection of the waveform rising edge and “–.1” line. Align the
second cursor to the intersection of the waveform rising edge and the “.5” line.
The indicated
t time value is the risetime from 20 – 80%. Use the horizontal
MAG for greater accuracy when adjusting the cursors. Falltime is measured in a
similar manner. See Figure 3-6.
Measuring Amplitude
Measuring Aberrations
Measuring Risetime