PXIe-5668 Self-Calibration

NI RF Vector Signal Analyzers (NI-RFSA 18.1) Help

Edition Date: June 2018

Part Number: 372058U-01

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You can configure NI-RFSA to perform a self-calibration on the PXIe-5668 system, which includes the RF downconverter, the PXIe-5653, and the PXIe-5624. You can also configure NI-RFSA to perform a module-level self-calibration on the PXIe-5624 or PXIe-5653 modules.

The niRFSA Self Cal VI and niRFSA_SelfCalibrate function perform the following self-calibration operations for the PXIe-5668 system:

  • Preselector alignment
  • IF flatness
  • Gain reference
  • LO self-calibration

If your application requires module-level self-calibration, you can configure the niRFSA Self Cal VI or the niRFSA_SelfCalibrate function to perform only the module-level self-calibration steps, namely, LO self-calibration and digitizer self-calibration. You can also run the module-level calibration steps in MAX.

Note Note  After you perform a self-calibration at the module level for either the PXIe-5624 or the PXIe-5653 modules, you may need to perform a self-calibration on the PXIe-5668 system. To determine whether you need to perform any of the system self-calibration steps after module-level calibration, use the niRFSA Is Self Cal Valid VI or the niRFSA_IsSelfCalValid function.

During a system self-calibration, the downconverter measures the reference source and compares the resulting measurements to a value stored in the PXIe-5606 EEPROM. Self-calibration is designed to work seamlessly with the PXIe-5624. If you use the PXIe-5606 downconverter only, without the PXIe-5624 digitizer, use the onboard, high-precision tone signal to correct for losses to your receiver by comparing the tone value stored in the PXIe-5606 EEPROM with a recent measured value. To retrieve the tone value that is stored in the PXIe-5606 EEPROM, use the niRFSA Get Gain Reference Cal Baseline VI or the niRFSA_GetGainReferenceCalBaseline function.

Note Note  The PXIe-5606 RF downconverter module provides a bandpass preselector that can be included in the RF input signal path when downconverting frequencies greater than 3.6 GHz. The preselector can reject signals that might otherwise interfere with or degrade the accuracy of many measurements. The center frequency of the preselector is controlled by the magnetic field generated by an electromagnet. Ideally, the preselector center frequency varies linearly with current supplied to the electromagnet. In reality, nonlinearities and thermal characteristics cause the preselector center frequency to vary with temperature and exhibit some nonlinearity versus drive current. Each individual preselector has a unique tuning curve, which is a function of temperature, and it can vary slightly over time. The PXIe-5606 must account for the preselector tuning curve when controlling the preselector drive current to ensure that the filter is properly centered at the desired tuned frequency.

When the PXIe-5606 is associated with the PXIe-5624 digitizer and the PXIe-5653 synthesizer/LO source, NI-RFSA characterizes the preselector tuning curve as part of the device self-calibration process using an internally generated calibration signal and algorithm. When using only the PXIe-5606, the preselector tuning curve must be periodically determined with an external algorithm. The preselector tuning curve can be measured by controlling an LO and digitizer (or other IF receiver) appropriately. Then, you can calculate the tuning coefficients to correct for the preselector nonlinearity at the current operating temperature. Finally, you can program the tuning coefficients into the PXIe-5606 using the niRFSA External Alignment Adjust Preselector VI or the niRFSA_ExternalAlignmentAdjustPreselector function. Visit ni.com/info and enter exk8hs as the Info Code to download a LabVIEW example demonstrating how to perform self-calibration for an NI VSA with an external digitizer and preselector alignment.

Self-Calibration Guidelines

The PXIe-5668 modules are independently calibrated at the factory; however, you should perform a self-calibration in any of the following situations:

  • After first installing and interconnecting your PXIe-5668 system
  • When there is a physical change to any of the system components
  • When there is a change to system cabling
  • When the system is in an environment where external variables, such as temperature, can affect measurements

You should also perform a self-calibration if your device has exceeded any of the following temperature or time limits:

  • Preselector alignment (PXIe-5606 only): ±5 °C and/or 7 days
  • IF flatness: ±5 °C and/or 7 days
  • Gain reference: ±5 °C and/or 7 days
  • LO self-calibration: ±10 °C and/or 30 days

To determine whether your device has exceeded any of these limits, use the niRFSA Is Self Cal Valid VI or the niRFSA_IsSelfCalValid function.

It is important to perform self-calibration after first connecting your system because of cabling and mismatch effects and some degree of residual error, which result from module interconnections. These undesired effects can be determined and reduced through self-calibration. Additionally, you should periodically run a self-calibration to adjust for performance drifts that occur with product aging.

Performing a Device Self-Calibration using the NI-RFSA SFP

NI recommends you perform the self-calibration from the NI-RFSA SFP. You can also run a self-calibration programmatically using the NI-RFSA API by calling the niRFSA Self Cal VI or the niRFSA_SelfCalibrate function.

  1. Launch the NI-RFSA SFP by navigating to Start»All Programs»National Instruments»NI-RFSA»NI-RFSA Soft Front Panel.
  2. Click Device/System»Calibration»Self Calibration.
  3. Select the self-calibration steps you want to perform from the Self-Calibration dialog box.
  4. Click Next to run the self-calibration.
Note Note  NI recommends that you perform a self-calibration if the temperature has drifted more than ±5 °C. For stable operating environments, NI recommends that you run a self-calibration on a weekly basis to help minimize residual errors, drifts, and effects from aging.

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