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). FlexDCA identifies the offsets by measuring displayed signal levels when no light is present at the instrument's 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.
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.
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 However, if the Module calibration is out of date the Module calibration will automatically be started.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.
Disable all input signals to the channel that will undergo calibration.
Do not remove a module from the DCA-X during a calibration.
Dark Calibration Applies to Channel/Reference Filter Combination
The dark calibration is performed for the current Channel/Reference Filter combination. If the filter is changed, the channel's dark calibration will no longer be marked as valid. For the most accurate measurements, perform the calibration with the reference filter that you intend to use for your measurements.
For example, consider the situation where no dark calibrations have been performed for Channel 2A and you perform these steps:
- Select a 27.7393 GBd reference filter for Channel 2A and perform a dark calibration. The dark calibration is marked as valid.
- Select a 28.0500 GBd reference filter for Channel 2A. The dark calibration is now marked as uncalibrated.
- Select the original 27.7393 GBd reference filter for Channel 2A. The dark calibration is again marked as valid.
When to Perform a Dark Cal
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:
- Channel filter rate has changed.
- Details message: This optional calibration has not been performed.
- Module is moved to a different slot or PC.
- Details message: This optional calibration has not been performed.
- Temperature changes more than ±5°C.
- Details message: The temperature has changed by more than 5.00 deg C.
- Elapsed time exceeds 10 of hours.
- Details message: Elapsed time exceeds 10 hours.
- External hardware added and enabled in the Extended Module Configuration dialog.
- Details message: The external input device configuration has changed since calibration.
- Vertical Calibration is expired or hasn't been performed. Vertical calibration is required before Dark Level Calibration can be performed.
- Details message: Channel xx Optical Dark Level Calibration for this channel has not been performed since the last Channel xx Vertical Calibration.
- Power is cycled.
If a message is available, the Details button is displayed in the dialog. Click the Details button to view the message.
| Icon | Description |
|---|---|
|
Calibration is selected |
|
Calibration is unselected |
|
Calibrations are partially selected |
|
Calibration is current |
|
Calibration is recommended |
|
Calibration has never been performed |