Tektronix WFM 601i, User Manual

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Measurement Applications
WFM 601i Serial Component Monitor
3–13
Note that there is a STATUS field just below each ERR SEC readout. These
fields will indicate either “Valid,” “Invalid,” or “Missing.” “Valid” means that a
transmitted CRC is available to check for errors. “Invalid” means that some EDH
information is present but that the CRC used to check for errors is not available.
“Missing” means that no EDH information is present. In this case the EDH DET
LED will also be off. To check error rate, the EDH CRC being used – either FF
(full field) or AP (active picture) – must be “Valid.” Note that if a CRC goes
“Invalid” while error rate is being determined, there is no change to the ERR
SEC readout; it will hold the current value. But any errors that happen while the
CRC is “Invalid” will not be detected.
This measurement method is possible because the destination receiver’s
regenerated serial output is typically derived after the receiver has equalized the
input signal, recovered clock, and latched the data. Any detection errors affecting
the receiver will also appear in the regenerated serial output, and will be detected
by the WFM 601i.
This ability to measure a receiver’s error rate by monitoring its regenerated
output has tremendous practical significance. Although most equipment does not
yet include EDH error detection, almost all has regenerated (or reclocked) serial
outputs. Thus an EDH-equipped monitor can bring error rate measurement
capability to virtually any point in a facility. Strategic placement of these
monitors can help keep track of a plant’s performance. Similarly, a single EDH
error monitoring instrument can proof an entire digital chain, by moving from
one regenerated output to the next.
The WFM 601i calculates CRC check-words for the active picture area of each
video field. The two CRC values are displayed on the SERIAL STATUS screen:
one for field 1 (F1) and one for field 2 (F2). With a fixed pattern test signal,
these CRCs will have constant values. Any change in the CRC indicates an error.
This forms a “single ended” error detection system that does not require EDH.
Connect the WFM 601i as shown in Figure 3-7. The source does not need EDH,
but it must be a fixed pattern signal (repeating every two fields). Display the
SERIAL STATUS screen. Note that there are two hexadecimal CRC values
given, one for field 1 (F1 AP CRC VALUE) and one for field 2 (F2 AP CRC
VALUE). Directly below the CRC values is a flag (CHANGE SINCE RESET)
which indicates if either CRC value has changed since the RESET soft key was
pushed. Press the RESET key and check that the CHANGE SINCE RESET flag
reads “No.” If an error occurs, a CRC value will momentarily change, and the
CHANGE SINCE RESET flag will toggle to “Yes.” The elapsed time counter
indicates the length of time since the flag was last reset to “No.” Note that the
flag can only indicate that one or more errors has occurred, so an “error rate”
cannot be calculated. But it does differentiate between zero errors and at least
one error.
Error Rate Measurement
with AP CRC Values