NI 5651 Signal Generator RF OUT and REF OUT Not Synchronized

Updated May 15, 2018

Reported In


  • PXI-5651
  • PXIe-5651



Issue Details

I have a National Instruments RF Analog Signal Generator, such as a PXIe-5651, and I've noticed that the reference clock output and the RF output of my device drift from each other, even when I'm generating at 10 MHz for both.


This is expected behavior. The REF OUT terminal can be used to export the reference clock of a PXIe-5651 in order to synchronize multiple cards together. This reference clock acts as a stable source to which the cards can look to in order to discipline their outputs and prevent them from drifting.

Additional Information

The REF OUT is specified at 10 MHz for the PXIe-5651, but in reality the frequency isn't exact because synchronization by sharing a reference clock (using Ref out and Ref in on two modules) doesn't rely on the phase or the accuracy of the Reference clock, only that its frequency remains precise and constant.

Therefore, if you have an analog signal that's being generated at 10 MHz, it will not line up exactly with the Ref out frequency.


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