Improving NI-TClk Synchronization of Oscilloscopes with Manual Adjustment

Manual adjustment significantly reduces skew between instruments and enhances synchronization repeatability, which results in greater coherence of multi-instrument measurements.

For background on manual adjustment, refer to Manual Adjustment of NI-TClk Synchronization .

Before beginning this procedure, complete the following:

• Self-calibrate all instruments in your test system.
• Obtain a reference signal with as low skew and jitter as possible.
  

  Tip

  The lower the skew and jitter of your reference signal, the better the synchronization you can achieve between instruments using manual adjustment.

This procedure applies to:

• Manual adjustment of NI oscilloscopes that support NI-TClk synchronization
• Synchronizing multiple NI oscilloscopes of a single bus type and model number (for example, all PXI-5922 modules, but not PXI-5922 and PCI-5922 modules)

Complete the following steps to perform manual adjustment for your oscilloscopes.

Set Up

1. Connect the reference signal to all instruments you are synchronizing.
   

   Tip

   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. (LabVIEW, LabVIEW NXG only) Call NI-SCOPE Get Session Reference to generate a reference from NI-SCOPE that NI-TClk can use.
   2. Call NI-TClk Configure for Homogeneous Triggers.
   3. Call NI-TClk Synchronize.
   4. Call NI-TClk Initiate.

Calculate Adjustment

4. Fetch data from the instruments and measure the skew and jitter between the waveforms each instrument is acquiring, for example, by performing a point-by-point average.
5. Use any of the adjustment techniques applicable to your instrument to reduce the skew and jitter you measured:

   Instrument	Adjustment Technique	Description		When to Use
   PXIe/PXI/PCI-5105 PXIe/PXI/PCI-5114 PXIe/PXI/PCI-5122 PXI/PCI-5124 PXI/PCI-5142 PXI/PCI-5152 PXI/PCI-5153 PXI/PCI-5154 PXIe-5622 PXI/PCI-5922	NI-SCOPE Adjust Sample Clock Relative Delay function	Offsets the sample clock from the reference clock by the specified amount. Reduces skew between instruments.	Adjustment range: ±1 Sample Clock Period Adjustment unit: seconds Tip You can call this function multiple times to implement cumulative delay.	Primary technique for reducing skew and jitter for supported instruments. [1]
   PXIe-5110 PXIe-5111 PXIe-5113 PXIe-5163 PXIe-5164 PXIe-5170 PXIe-5171 PXIe-5172	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	Primary technique for reducing skew and jitter for supported instruments.
   All oscilloscopes that support NI-TClk synchronization except the following: PXIe-5185 [2] PXIe-5186 [2]	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		To apply offset when adjustment beyond the range of other techniques is needed. Sole technique for reducing skew for the PXIe-5160 and PXIe-5162.

6. Repeat steps 4 and 5 until the measured skew and/or jitter are as low as possible.

Apply Adjustment

7. Depending on your adjustment technique(s) from step 5 , save the adjustment you used to minimize skew and/or jitter.

   Adjustment Technique	Description
   NI-SCOPE Adjust Sample Clock Relative Delay	The NI-SCOPE Oscillator Phase DAC Value property converts the time offset you applied with NI-SCOPE Adjust Sample Clock Relative Delay into a value that can apply the manual adjustment across sessions and improve synchronization repeatability. Add the NI-SCOPE Oscillator Phase DAC Value property to your program and read its value. Save the Oscillator Phase DAC Value you read. Note You do not need to include the NI-SCOPE Adjust Sample Clock Relative Delay function in your program after you have applied the adjustment with the NI-SCOPE Oscillator Phase DAC Value property.
   All other adjustment techniques	Save the value of each property.

8. Write the saved value(s) of the property or properties you used prior to calling the NI-TClk Synchronize function in other 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 for all supported instruments in the following situations:

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

Reperform manual adjustment for specific instruments in these additional situations:

• PXIe-5185 and PXIe-5186 only:
  • If any instrument is reset
  • If any instrument is self-calibrated
  • If the host computer or a chassis within the test system is rebooted
• PXI-5922 and PCI-5922 only: If any channel-based properties are changed