With the Timing and Synchronization upgrade, the chassis has an internal precision 10 MHz Oven-Controlled Crystal Oscillator (OCXO) that serves as the default reference for the backplane PLL. The user can still provide a 10 MHz reference via any of the supported input ports if a different reference signal is needed.

The main source of frequency error in reference oscillators is temperature variation. An OCXO minimizes this error by housing the crystal oscillator circuit inside a sealed oven, which is maintained at a constant temperature higher than the ambient temperature external to the OCXO. This results in a reference oscillator that is several orders of magnitude more stable and accurate than regular crystal oscillators.

Because the OCXO must warm up to a higher temperature than the ambient temperature around it, there is a warm up time required to achieve the specified frequency accuracy. For this reason, to achieve the most stable operation of the OCXO, avoid powering off the OCXO.

The OCXO that the PXIe-1095 uses features electronic frequency control. This allows the OCXO to be fine-tuned by varying the control voltage to the OCXO. The chassis uses a 16-bit digital-to-analog converter (DAC) to give precise control of the tuning voltage. The PXIe-1095 is calibrated during the manufacturing process and should be recalibrated annually to remove frequency error that accumulates over time (such as crystal aging). Refer to the PXIe-1095 Calibration Procedure at ni.com/calibration for more details.

You also can route the OCXO as the 10 MHz output reference to support systems with tight synchronization requirements.