Why Do I Get a Spectral Hump Using USRP-2945?

Updated Mar 12, 2018

Reported In

Hardware

  • USRP-2955
  • USRP-2945

Driver

  • NI-USRP

Other

UHD

Issue Details

Using certain IQ rates there is roll-off on the edges of the spectrum using a USRP 2945/55. See the attached images for comparison. Figure 1. shows the difference between a flat spectrum and spectrum with roll-off.

Figure 1. The spectrum on the left is flat. The spectrum on the right shows the roll-off behavior.

How do I get around this behavior?

Solution

There are three ways to mitigate this issue:
  1. The issue only occurs when using an odd decimation/interpolation factor. Use an even decimation/interpolation factor instead. An even (divisible by 2) factor enables a half-band filter before the CIC filter and that significantly flattens out the CIC filter response.
  2. Equalize the response in software.
  3. Utilize the Instrument Design Libraries, which implement a different filtering process.

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