Tektronix OI1125 Instruction Manual page 60

Performance Verification
48
Compensate the Dark Level on the Oscilloscope. The optical sampling heads are
susceptible to temperature fluctuations. Therefore, do the following prior to
taking measurements:
NOTE. Repeat the dark level compensation routines between every table in these
tests. The extinction ratio measurements rely on accurate oscilloscope com-
pensations. Perform the compensations more frequently if the ER measurements
are failing.
1. Disconnect the optical cable from the input to the oscilloscope.
2. Execute a dark level compensation. The Vertical menu of the TDS8000 lets
you access the optical module Dark Level and User Wavelength Gain
Compensation procedures.
First select the channel you want to calibrate in the Waveform section of the
menu. Select the Setup Optical, Dark Level or User Wavelength Gain
Compensation boxes to start the compensation. Follow the displayed
instructions to complete the compensation. For more information, consult
the instrument online help.
3. Connect the optical cable to the oscilloscope.
Take Measurements. The equipment is now set up for recording measurements in
Table 18 in the Test Record:
1. Press the Clear Data button and then the Run/Stop button on the oscilloscope
to begin a measurement. You may need to adjust the horizontal position
slightly to center the eye pattern around the center mask boundary.
2. Verify that the eye pattern does not violate the mask boundaries.
3. Record the measurement data from the oscilloscope display (see Figure 17),
in the first row of Table 18 (2.488 Gb/s, 1.5 V). Check that the data meets
the limits in Table 17.
NOTE. You must connect an attenuator on the BERT output to take the 0.25 V
measurements in Table 18. Therefore, it is easier to take the 1.5 V measurements
first, and then connect the attenuator and complete the 0.25 V rows in Table 18.
4. Recall the 9.953 Gb/s data rate that you stored in the BERT.
5. Set the filter field on the oscilloscope to OC-192. See steps 1 through 3 in
the Oscilloscope Setup on page 45.
6. Repeat steps 1 through 3 for the 9.953 Gb/s, 1.5 V row in Table 18.
7. Recall the 12.500 Gb/s data rate that you stored in the BERT.
OI1125 E/O Transmitter Instruction Manual