FFT Clutter Depending On LO Offset

[Meelis Nõmm]

Hello everyone,

We have been using the USRP N210 with the WBX 50-2200 (40 MHz) daughter board to generate FFT plots at various frequencies. Note that the experiments were done done without the antenna being connected. Our objective is to get as clean of an FFT view as possible.

In order to get rid of the large DC spike in the middle of the FFT, we started offsetting the LO frequency with the use of tune requests. However, this produces unexpected results. In theory, we should be able to move the DC spike of the LO "out of view" by applying an offset that's greater than half the sample rate of the FFT. However, depending on the sample rate of the USRP, the center frequency, and the exact offset given to the LO, smaller secondary spikes will appear at unexpected locations on the FFT. See attachments for illustration: they're all taken at the same sample rate, with the same centre frequency, and only the LO offset is being modified.

The research that we've done into the matter indicates that it might be impossible to wholly remove the DC spike in the middle of the FFT, however, with the LO moved off of it, the remaining spike isn't that large of an issue. What is an issue, however, is the fact that we have secondary spikes that appear depending on the LO offset frequency, and sometimes on the center frequency of the FFT. We are also aware of the fact that we should not be moving the LO out of the bandwidth of the daughter board (40 MHz total).

What might be the reasons for such spikes, and what other factors should we look at when trying to get a clean FFT picture?

Regards,

Meelis

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