Improving PXIe-5113 Synchronization with Manual Adjustment

Perform manual adjustment to obtain the best possible NI-TClk synchronization between supported instruments.

Manual adjustment requires a reference signal with as low skew and jitter as possible. The lower the skew and jitter of your reference signal, the better the synchronization you can achieve between instruments using manual adjustment.

Self-calibrate all instruments in your test system before beginning this procedure.

This procedure is valid under the following conditions:

  • All instruments to be synchronized have identical bus types and model numbers
  • Application includes only homogenous triggers

You can use the NI-TClk API to synchronize the action of multiple instruments. Manual adjustment is the process of tuning the sample clocks or triggering of instruments that have been synchronized using NI-TClk to obtain better alignment than you can achieve using NI-TClk synchronization alone.

The following definitions apply to this procedure:

  • Skew—Misalignment between module timing across slot of a chassis or between multiple chassis that is caused by clock and analog path delay differences
  • Jitter—Variation in skew over time
  • Repeatability—Consistency in the effect of NI-TClk synchronization across instruments (reduction in jitter)

    • Manual adjustment significantly reduces skew between instruments and enhances synchronization repeatability, which results in greater coherence of multi-instrument measurements. Refer to instrument specifications for the degree of NI-TClk synchronization you can obtain without and with manual adjustment.

    Note Manual adjustment can minimize skew only between multiple instruments, not between multiple channels of the same physical instrument.

    To perform manual adjustment for the PXIe-5113, complete the following procedure.

    1. Connect the reference signal to all instruments you are synchronizing.
      Note For best results, use cables of the same model and of identical length for each connection.
    2. Configure an acquisition as appropriate for your reference signal.
    3. Synchronize the instruments in your session using the NI-TClk API.
      1. Call NI-SCOPEGet Session Reference to generate a reference from NI-SCOPE that NI-TClk can use.
      2. Call NI-TClkConfigure for Homogeneous Triggers.
      3. Call NI-TClkSynchronize.
      4. Call NI-TClkInitiate.
    4. Fetch data from the instruments and measure the skew and jitter between the waveforms each instrument is acquiring by performing a point-by-point average.
    5. Use any combination of the following techniques to reduce the skew and jitter you measured:
      Adjustment TechniqueDescription
      NI-SCOPE Absolute Sample Clock Offset property

      Offsets the sample clock from the reference clock by the specified amount.

      Reduces skew between instruments and improves synchronization repeatability.

      • Adjustment range: ±0.5 Sample Clock Period
      • Adjustment unit: seconds
      NI-TClk Sample Clock Delay property

      Offsets the sample clock of an NI-TClk session relative to the other synchronized NI-TClk sessions.

      Reduces skew between instruments.

      • Adjustment range: ±∞
      • Adjustment unit: seconds
    6. Repeat steps 4 and 5 until the measured skew and jitter are as close to zero as possible.
    7. Save the value of NI-SCOPEOscillator Phase DAC Value you user to minimize skew and jitter.
    8. Write the value of the property or properties prior to calling NI-TClkSynchronize in future sessions when using the test system in an identical physical configuration.
      Writing the manual adjustment property or properties to a session applies the improved synchronization to the session, regardless of the signals the test system is acquiring.

    Reperform manual adjustment in any of the following situations:

    • If any chassis, instruments, cables, or other physical components of your test system are moved, replaced, or removed
    • If the ambient temperature[1] changes by ±1 °C since the last self-calibration
    • If the trigger type, filtering characteristics, or any clocking parameters of your application are changed

    1 The ambient temperature of a PXI system is defined as the temperature at the chassis fan inlet (air intake).