Dark Level Calibration
The Dark Calibration calibrates Eye/Mask mode's NRZ Extinction Ratio measurement (:MEASure:EYE:ERATio command) and PAM/NRZ Output Extinction Ratio measurement (:MEASure:EYE:OER command). These measurements are performed on optical channels.
Calibration commands correspond to the features in the Calibration dialog. You can click on the tabs in this picture to learn about the different calibrations.
The calibration identifies internally generated offset (dark level) signals generated by external optical-to-electrical converters and removes them during the extinction ratio measurement calculations. The dark calibration applies to the currently selected filter rate for a channel. If a new filter rate is selected, you must perform another dark calibration. Prior to performing a dark calibration, adjust the vertical scale and offset so that the eye diagram uses the full display. Also, the dark level (the signal level when there is no input to the measurement) must be on the screen to be correctly measured.
To perform a dark calibration, use the :CALibrate:DARK:CHANnel:STARt.
To query the status of a dark calibration, use the :CALibrate:DARK:CHANnel:STATus? command.
The dark level is a function of the vertical scale parameter. Perform the dark calibration at the vertical scale level to be used when making the extinction ratio measurement. If you change the vertical scale after you perform the dark calibration, the calibration accuracy can be degraded. To test several devices with a broad range of output powers, set the vertical scale so that the devices with the largest output levels are completely on the screen. Perform the dark calibration at this vertical scale. The extinction ratios for the smaller signals can still be accurately measured if the eye diagram is at least 2 graticule divisions in height. Performing the dark calibration in this manner can prevent the need for multiple extinction ratio calibrations.
A dark calibration is recommended if either the vertical scale or vertical offset (or both) are changed after the last calibration. However, if either of these values is changed back to the calibrated value, the calibration is then considered valid. For example, a channel calibrated at a scale of 500 μW/div will be invalid if the scale is changed to 50 μW/div, but it is valid if it is changed back to 500 μW/div.