Tektronix WFM 601i, Руководство пользователя

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Measurement Applications
3–10
WFM 601i Serial Component Monitor
Monitoring Transmission Channels
The high data rate of the serial system (270 Mb/s) places severe demands on a
coaxial channel. Impedance discontinuities in the cable path will create
reflections, causing pulse distortion and eye pattern closure. Further, the coax
loss, expressed in dB, is assumed to increase with the square root of frequency.
Variations in this loss characteristic can cause problems for receiver equalizers.
The WFM 601i can help locate channel problems. Its passive loopthrough allows
monitoring an operating system at a variety of points. By observing the system at
several locations, from the signal source to the destination receiver, channel
problems can be discovered and pinpointed. This is especially useful when
“proofing” a newly installed system.
The WFM 601i, with its passive loopthrough serial inputs, can either be used as
a bridging or terminating input device. Depending on the measurement
application, one connection may be more effective than the other. Please see
“Installing the WFM 601i in a Serial Video System,” in the Installation section
of this manual, for recommendations.
The WFM 601i, in conjunction with a serial signal source, can inspect serial
interface points for return loss problems. This time domain return loss
measurement uses the serial signal as the step generator, and the WFM 601i as
the bridging sampler.
Connect a serial source having low aberrations to one of the WFM 601i serial
inputs. Place a high return loss (>25 dB to 270 MHz) 75 Ohm terminator on the
other side of the loopthrough. The terminators supplied with the WFM 601i (p/n
011–0163–00) are recommended. Select EYE mode and observe the eye pattern.
Check for a minimum of overshoot, ringing, or other aberrations. These artifacts
will limit the accuracy of the measurement; under 5% is recommended. Choose a
better signal source if aberrations are excessive; both the Tektronix TSG-422 and
the TSG-601 handheld generator are suggested.
Using Variable Gain, adjust the eye pattern for a 10 division display. Now
remove the terminator and connect the open side of the loop-through to the serial
interface point being examined, using a short length (<2 meters) of high quality
coax (if a source is being measured, the output signal must be removed). Any
reflections from the serial interface point under test will distort the eye pattern.
These amplitude distortions, expressed as a percentage of the original 10 division
display, are the reflection coefficient.
Most serial equipment return loss is specified in the frequency domain, for
example 15 dB to 270 MHz. How this maps to a time domain reflection
coefficient measurement depends on the magnitude and phase of the individual
frequency components. While there are no standards for the time domain
Recommended Monitoring
Connections
Checking Return Loss