PXIe-5668 Timing Configurations

The timebases of the PXIe-5624 IF digitizer and the PXIe-5653 synthesizer/LO source module must be frequency-locked to a common reference clock. The following clock sources are available:

  • 100 MHz PXIe-5653 LO synthesizer/LO source onboard Reference Clock—The PXIe-5653 supplies this 100 MHz source through the REF OUT (100 MHz) connector.
  • 10 MHz external Reference Clock—Connect the external clock signal, from your stable frequency reference, to the REF IN connector on the PXIe-5653.
  • 10 MHz PXI backplane clock—This 10 MHz Reference Clock signal is supplied on the PXI backplane.
    • Note Although using the 10 MHz PXI backplane clock as a reference is possible, NI does not recommend doing so.The frequency accuracy and stability of the 10 MHz PXI backplane reference results in increased phase noise.
    Note Do not use the PXIe-5653 REF OUT (10 MHz) connector as a reference source for timing configurations.

    Configuring for Onboard Reference Clock Timing

    The default configuration of the PXIe-5668 allows the PXIe-5653 to export a 4 GHz LO2 signal to the PXIe-5624 so that the PXIe-5624 and the PXIe-5653 devices are frequency-locked.

    Complete the following steps to configure the PXIe-5668 to use the PXIe-5653 internal clock.

    1. Connect the LO2 OUT connector on the PXIe-5606 front panel to the CLK IN connector on the PXIe-5624 IF digitizer front panel.
    2. Set the Ref Clock Source property to OnboardClock or the NIRFSA_ATTR_REF_CLOCK_SOURCE attribute to NIRFSA_VAL_ONBOARD_CLOCK_STR.
      Figure 2. PXIe-5668 Default Cable Configuration

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    Configuring for External Reference Clock Timing

    Complete the following steps to lock to an external reference source.

    1. Connect the external signal to the PXIe-5653 REF IN connector.
    2. Connect the LO2 OUT terminal on the PXIe-5606 to the CLK IN connector on the PXIe-5624.
    3. Specify RefIn as the Reference Clock source using the niRFSA Configure Ref Clock VI or the niRFSA_ConfigureRefClock function.
      NI-RFSA uses a default Reference Clock rate of 10 MHz. Use the niRFSA Configure Ref Clock VI or the niRFSA_ConfigureRefClock function to specify a different Reference Clock rate. You can specify any frequency from 5 MHz to 100 MHz, in increments of 1 MHz.

    Configuring for PXI 10 MHz Backplane Clock Timing

    NI recommends that you configure your system so the PXIe-5624 IF digitizer uses the source provided by the PXIe-5606 LO2 OUT front panel connector. You can also configure the PXIe-5668 to lock to the PXI 10 MHz backplane clock, but doing so results in worse phase noise.

    Complete the following steps to configure the PXIe-5668 to use the PXI 10 MHz backplane clock.

    1. Lock the PXIe-5653 to the PXI backplane clock.
    2. Connect the PXIe-5606 LO2 OUT to the LO2 IN connector on the PXIe-5624.
    3. Set the Ref Clock Source property to PXI_Clk or the NIRFSA_ATTR_REF_CLOCK_SOURCE attribute to NIRFSA_VAL_PXI_CLK_STR.
      The PXIe-5653 can lock to the PXI Express backplane Reference Clocks if the backplane Reference Clocks meet the frequency accuracy requirements of the PXIe-5653.
      Note The PXI Express backplane Reference Clock, when left free running, may not meet the accuracy requirements of the PXIe-5653. The PXIe-5653 requirements are more stringent than PXI Express backplane accuracy requirements because of the high-performance OCXO on the PXIe-5653. Locking to the PXI Express backplane Reference Clock requires that the PXI Express backplane Reference Clock be locked to another, more accurate external reference.