NI-TClk Manual Calibration on NI-SCOPE Devices

Overview

This article discusses how to improve the out-of-the-box synchronization accuracy of NI-TClk synchronized devices through Manual Calibration.

NI-TClk Manual Calibration on NI-SCOPE Devices

Using NI-TClk synchronization, you can have multiple instruments simultaneously respond to triggers, align sample clocks, and simultaneously start multiple instruments.  Skew between devices can vary depending on the devices selected, for the PXIe-5114 skew between devices is typically around 500ps.  This skew may not be satisfactory for some applications where the phase accuracy between devices requires a higher level of synchronization.  Manual calibration, the topic of this article, can lower skew between devices. For example, the PXIe-5114 can have an average skew of less than 20ps after manual adjustment.

 

For a deeper dive into NI-TClk, its capabilities and use cases, see National Instruments NI-TClk Technology for Timing and Synchronization of Modular Instruments.

 

Manual calibration involves the adjustment of the sample clock on each device with respect to each other using the phase adjustment DACs in the Reference Clock PLL circuitry.  Through Manual Calibration, skew between devices is eliminated, and jitter is minimized.  For more information about NI-TClk and how it works, see the section below titled “Overview of NI-TClk Technology”.

 

                Note:  Manual calibration cannot adjust skew between channels on the same device.

 

Procedure for Manual Calibration

These procedures are valid for NI-SCOPE devices, EXCEPT the PXIe-5160/5162 and PXIe-5185/5186.  These devices do not support manual calibration, but have mechanisms for adjusting for skew between devices. 

 

To perform manual adjustment, an external signal is needed that has low jitter and skew

  1. Connect the external signal to all digitizers.  For best performance, minimize unnecessary skew by use matched length cables.
  2. Configure the digitizer for acquisition, using NI-TClk.  If using LabVIEW, use the following example from the NI Example Finder: “niScope EX Multi-Device Configured Acquisition (TClk).vi”
  3. Measure the skew between devices, also note the amount of jitter. Remember you can only adjust skew between devices, not between channels of the same device. (See Attachment: average_phase_8.6.vi)
  4. Use “niScope Adjust Sample Clock Relative Delay.vi” to apply skew correction.
  5. Repeat Step 4 until you are satisfied with the synchronization results.
  6. Read the value from the property “Oscillator Phase DAC Value”, store for future synchronization sessions.

To apply the synchronization attained from Manual Calibration, write the value saved to the “Oscillator Phase DAC Value” property before calling the “niTClk Initiate” VI.

 

Adjustment Function Details

This section provides more information regarding NI-TClk Manual Calibration and adjustment functions on NI Digitizers/Oscilloscopes.  Skew adjustment is only part of Manual Calibration.

 

niScope Adjust Sample Clock Relative Delay.vi

This VI delays to the sample clock relative to the Reference clock.  This is the VI that should be used to manually calibrate a NI-TClk session for all supported NI digitizers.

This VI is intended to only be used for NI-TClk Manual Calibration, and behavior outside of this use case in unspecified.  This function can only adjust up to +/- 1 sample clock period per call, but the effect of multiple calls is cumulative.

 

“niScope Trigger Delay” on the niScope Trigger VI

Trigger delay is the amount of time that elapses between when a trigger occurs and when the digitizer actually triggers. 

See High-Speed Digitizers Help >> Fundamentals >> Triggering >> Trigger Parameters >> Trigger Delay 

 

When using trigger delay, no trigger or synchronization signals are physically altered or delayed, therefore it is not possible to use this property to create a trigger delay generator.

 

NI-TClk Sample Clock Delay Property

This property is for setting a delay to a device's sample clock, additional to the NI-TClk synchronization.  This property needs to be set before calling the niTClk Syncronize VI.  This property is useful if you want to have the sample clocks synchronized, but have a constant delay/offset/phase relationship.  This property can only adjust sample clocks within +/- 1 sample clock; NI-TClk should have adjusted the sample clocks to within this window.

 

For example:  To implement TIS (Time Interleaved Sampling) between two digitizers, use this property to configure the two devices have a 180 deg phase relationship (1/2 of period).  They would be synchronized using NI-TClk, and then an 180deg phase offset would be applied.

 

Note: This property can also be used for Manual Skew Adjustment for PXIe-5160/5162 devices.

 

Property: niScope Oscillator Phase DAC Value

This property is adjusted indirectly through the use of "niScope Adjust Sample Clock Relative Delay.vi", and is the raw value used to adjust the Sample clock relative to the Reference clock. 

 

The intended use case for this property is to allow for repeatable synchronization, when used as outlined in the NI-TClk Manual Calibration steps above.  For NI digitizers when this property is set, the synchronization process initiated by the “niTClk Synchronize” VI, will rely on this property for the fine clock adjustment, which leads to reduced time to perform the NI-TClk synchronization.

 

Note: This property can also be used for Manual Skew Adjustment for PXIe-5185/5186 devices.  The value written to the devices will remain valid until Self-Calibration is initiated, any changes to the Reference Clock or External Clock are made, the device is reset, or after reboot.