Why Does My Constellation for My Communications Algorithm Appear to Have a High Amount of Phase Noise During Operation in LabVIEW Communications 1.1?

Updated Nov 12, 2018

Reported In

Software

  • LabVIEW Communications System Design Software

Driver

  • NI-USRP

Issue Details

I have a communications algorithm and my constellation appears to have phase noise for short bursts during operation. Why am I seeing this behavior and how can I reduce the phase noise?

Solution

There are many ways in which a constellation can appear noisy. Certain schemes are highly sensitive to phase noise. If you see this behavior and are using LabVIEW Communications 1.1, one method you can try is to adjust a setting on the PLL which locks your USRP to a 10 MHz reference (Internal or External Ref In). The VI attached has this change already applied. You will need to:

1. Download the attached gVI.
2. Open your LabVIEW Communications 1.1 vi.lib directory:
<LV Comms installation directory>\Resources\vi.lib\Native\
3. Rename the gVI below or copy it to another directory for back-up purposes:
niUsrpRio Config v1 Host.lvlib__Construct LMK04816 Initialization.gvi
4. Copy the downloaded gVI to the directory from step 2.
5. Re-open and run your LabVIEW Communications 1.1 application to see if the new gVI improved your constellation.

There is a change in the gVI which changes the Charge Pump current on PLL1. This change affects how fast the PLL locks which, depending on the quality of your reference clock, can result in better phase noise performance.
 

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