A “System selftest failed” warning typically indicates an abnormal low-level system state. If this warning appears, do not disassemble the instrument or repeatedly force-restart it. Please contact the Lianxun FAE team directly. We will arrange for a qualified engineer to perform an on-site firmware upgrade to restore normal operation.

An abnormally high TDECQ value can be caused by multiple factors across the test link. Troubleshoot the issue step by step to isolate the source:

1、Check for Contamination and CDR Status

Check the CDR fiber connector and optical port for contamination. Replace the CDR with another known-good unit and check whether the issue persists. If the TDECQ value returns to normal, the original CDR can be identified as the source of the issue. If both CDRs operate normally but the TDECQ value remains high, check whether the source signal itself is normal.

2、Check Clock Quality

Check whether the correct CDR data rate is selected. An incorrect data rate setting may degrade the quality of the clock signal supplied to the DCA, which may result in an abnormal TDECQ measurement.

3、Check Physical Connections

Check the TRIG port for physical damage and make sure the RF cable between the DCA and CDR is securely connected.

4、Cross-Check with Another CDR

Cross-check with another CDR and verify whether both units can lock normally to determine whether the issue is caused by the CDR itself. If the issue is confirmed to be instrument-related, please contact the FAE team for further assistance.

Failure to recover the clock is typically related to the input signal conditions or an incorrect data rate setting.

1、Check the Optical Input Power

Check whether the CDR is receiving sufficient optical input power.

2、Verify the Data Rate Setting

Check whether the correct CDR data rate is selected. You can confirm the current setting in the SymbolRate field on the interface. An incorrect data rate setting will prevent the instrument from locking properly.

3、Cross-Check with Another CDR

Perform a cross-check using another CDR to determine whether it can lock normally. This helps identify whether the issue is caused by the CDR itself. If the issue is confirmed to be instrument-related, please contact the FAE team for further assistance.

Abnormal power readings are typically caused by optical path contamination, incorrect settings, or physical hardware damage. Troubleshoot the issue in the following sequence:

1、Check for Optical Port Contamination

Use a fiber inspection scope with an FC/PC adapter to inspect the optical port. If contamination is found in the center of the port (the optical transmission path), it must be removed using a fiber cleaning pen. Failure to remove the contamination may interfere with signal transmission and cause significant measurement errors.

2、Verify the Wavelength Setting

Check whether the wavelength setting on the LCD screen matches the source signal. For example, when testing a 1310 nm signal, incorrectly setting the wavelength to 850 nm will result in an abnormal power reading.

Note: This wavelength check applies to the CR6256 only.

3、Check for Fiber Adapter Damage

Visually inspect the fiber adapter in a well-lit environment. If the ceramic sleeve inside the adapter is cracked, replace the adapter.

If all the above checks are completed and no issues are found, but the problem persists after restarting the instrument, please contact Lianxun FAE for confirmation before returning the instrument for repair.

Strictly keep the optical input power within the safe range during testing. The two models have different damage thresholds, so particular attention is required:

Recommended Linear Optical Input Power Range

For single-mode (SM) input, an optical power level above -12 dBm is recommended for linear operation. For multimode (MM) input, a level above -10 dBm is recommended. For routine testing, > -8 dBm is recommended to ensure signal quality.

CR6256 Optical Power Damage Threshold

The CR6256 damage threshold is +7 dBm.

CR3302 Optical Power Damage Threshold

The peak incident optical power of the CR3302 must never exceed +5 dBm under any circumstances.

⚠ Do Not Overload

Any signal exceeding the applicable threshold will cause immediate, irreversible, and permanent physical damage to the internal photodetector assembly.

To prevent damage to these precision instruments, the following four steps must not be skipped:

1、Grounding

Before powering on the instrument, make sure it is properly grounded.

2、End-Face Inspection

Before inserting the optical fiber, use a fiber inspection scope to inspect both the fiber end face and the CDR optical input end face. Ensure that both end faces are completely clean and free of any contamination.

3、ESD Protection

When changing the cable connected to the TRIG port, power off the instrument and wear an ESD wrist strap to prevent electrostatic discharge from damaging the TRIG port.

4、Connector Inspection and Cleaning

When connecting the TRIG cable, check the TRIG port for any damage. Before use, it is recommended to clean the RF connector with an alcohol-moistened cleaning swab.

Troubleshoot the issue based on the following two scenarios:

Scenario 1: No Eye Diagram Is Displayed

① Check the test settings in the host software, including SIRC, Wavelength, and Pattern Length, and make sure they correctly match the current test data rate and pattern length.

② Confirm that a clock signal is being supplied to the DCA.

Scenario 2: Only One Channel Displays an Eye Diagram

① Check whether the other channels are correctly configured under Channel Setup, and make sure that only CH1 has not been configured by mistake.

② Check whether the required channel tabs (1A / 1B / 1C / 1D) in the lower-left corner of the host software interface are selected.

Troubleshooting TDECQ measurement deviations requires checking the following four areas:

1、Dark Current

As with ER measurement deviations, check the temperature difference and perform the dark current calibration again if necessary.

2、Clock Signal Issue

Check whether the clock signal cable is loose. Try replacing it with a new cable, or perform a cross-check to confirm that the CDR is operating properly.

3、Optical Port Contamination

Use an FC fiber inspection scope to check the DCA optical port and fiber connector end face for contamination.

4、Incorrect Configuration

Check whether the corresponding data rate and wavelength settings are correctly configured in the host software.

If the TDECQ measurement still shows a significant deviation after completing the above checks, perform a cross-check with another DCA under the same test conditions. After ruling out external environmental factors, contact FAE for further assistance.

Large extinction ratio (ER) measurement deviations are typically caused by environmental conditions or incorrect configuration:

1、Excessive Temperature Difference Since Dark Current Calibration

If the current instrument temperature differs significantly from the temperature at which the previous dark current calibration was performed, the ER measurement may deviate. Under Setup → Display Setup, enable the temperature display; the temperature will then appear above the eye diagram. If a temperature warning appears below the eye diagram, perform the dark current calibration again. The temperature-difference threshold can be configured under Calibration, with 2–3°C recommended.

2、Incorrect ERCF Setting

In the host software, go to Measure → Config Base Measurments to configure the ERCF for each channel and data rate. The ERCF can be set to either a positive or negative value, but values beyond ±5% are not recommended. Select Apply for the setting to take effect.

3、Incorrect Single-Mode/Multimode Wavelength Selection

The 850 nm and 1310 nm wavelength settings use different parameters. Selecting the wrong wavelength can cause significant errors in the calculated results.

If the ER measurement still shows a significant deviation after completing the above checks, perform a cross-check with another DCA under the same test conditions. After ruling out external environmental factors, contact FAE for further assistance.