Dark Level Calibration
Use the Dark Level tab on the Calibrations dialog to optimize Eye/Mask mode's Extinction Ratio measurements (NZR waveforms) and Outer Extinction Ratio measurements (PAM waveforms).
The Dark Level tab lists dark level calibrations for both optical and electrical channels. 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. Prior to performing a dark level 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.
A Dark Level calibration is very fast. However, if the Module calibration is out of date the Module calibration will automatically be started. When the Module calibration completes, perform the Dark Level calibration.
A Dark Level calibration is very sensitive to environmental changes and as a result should be frequently performed. A Dark Level calibration is recommended when one of the following changes occurs:
- vertical scale is modified
- vertical offset is modified
- channel filter rate is changed
- module is installed/removed
- ±5°C temperature change from the latest module calibration temperature
- approximately 8 of hours of continuous operation is exceeded
- power is cycled
| Icon | Description |
|---|---|
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Calibration is selected |
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Calibration is unselected |
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Calibrations are partially selected |
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Calibration is current |
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Calibration is recommended |
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Calibration has never been performed |
Disable all input signals to the channel that will undergo calibration.
The dialog shows the status of all available calibrations for each module that is installed in the DCA-X or DCA-M module.
Do not remove a module from the DCA-X during a calibration.
The Dark Level calibration 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 Level 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.
The instrument identifies the offsets by measuring displayed signal levels when no light is present at the instrument input. Any offset that is detected is measured and included in the extinction ratio measurement algorithm. The instrument removes the offsets by subtracting the dark levels from the histogram means of both the one level and zero level.