What to do when the measurement result doesn't match expectations?

What to do when the measurement result doesn't match expectations?

It is possible that signals with excessive jitter, noise, overshoot or other signal aberrations may cause the measurement system to report incorrect results. Care should be exercised when doing automated measurements on such signals. The TDS8000 and CSA8000 provide measurement annotations that can assist in checking the rationality of automated measurements.

Troubleshooting Automatic Measurements is a document with examples of \troubleshooting of automatic measurements; it also has comments on compensation ("self-calibration") and Dark Level compensation.

For Extinction Ratio measurements it is recommended to follow these guidelines:

• Always perform a Dark Level compensation before taking this measurement to ensure accurate results. Access the Dark Level compensation from the Optical page of the Vertical Setup dialog box. For lower values of ER, (e.g. 5dB), it is sufficient to perform the Dark Level compensation every few hours. For high level of ER (e.g. 12dB) it's recommended either to perform the Dark Level compensation often, or at least verify the dark level setting right after the measurement (disconnect any signal, verify that the trace is close to dark level).
• Optimize the vertical resolution before taking this measurement.