NI PXIe-5667 Vector Signal Analyser Attenuating the Input Signal at Low Frequencies

Updated Jun 12, 2023

Reported In

Hardware

  • PXIe-5667

Issue Details

My project requires the PXIe-5667 vector signal analyzer (VSA) to fetch the complete HF band (2 - 30MHz) in IQ mode. The device specifications allow this instantaneous bandwidth. However, my input signal is experiencing attenuation at frequencies < 6MHz. I have verified that for the preselector module (PXIe-5693), the low-frequency bypass path is enabled.
 

Solution

The PXIe-5667 VSA has both AC and DC channel coupling modes. When using in ‘Spectrum’ mode with low-frequency configuration, the drivers will automatically generate an error in case the acquired bandwidth lies inside the roll-off range of the AC filter. However, the same is not the case in the IQ mode. The device will not report any error, rather the user will see attenuation because of the AC coupling filter. To avoid this, set the ‘Channel Coupling’ property to ‘DC Coupled’.
The following are the two critical configurations to be performed in any RFSA IQ fetch example available in NI Example Finder for any such use case:
  1. On your block diagram, place the NI RFSA property node before NI RFSA Commit VI or NI RFSA Initiate VI (right click>>Measurement I/O>>NI RFSA>>Property Node)
  2. Search for the properties “Low Frequency ByPass Enabled” and “Channel Coupling”. Set their values to “Enabled” and “AC Coupled” as shown below.
image.png
 

Additional Information

The PXIe-5667 has no internal DC block, which allows the device to make measurements at frequencies as low as 20 Hz. High-frequency components in the RF downconverter can be damaged when DC signals are applied directly to the RF IN connector. NI is not responsible for damage resulting from improper signal connections. If the intention is to measure a frequency higher than ~10KHz, please keep the DC block filter applied at RF IN connector to save any potential damage.
 

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