On 11/04/2019 08:25 AM, Meelis Nõmm wrote:
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
Try terminating the antenna input with a terminator--does that make a
difference?
At lower frequencies you can fully-expect there to be spurs picked up
from the environment, even with the antenna disconnected--terminating
the input will give a better picture.
But spurs are inevitable with any widely-tunable receiver--whether SDR
or a more-traditional architecture--particularly if there are digitial
circuits within the receiver architecture. One of the reasons that
high-end laboratory RF instrumentation is so costly (10s of $K) is that each
subsystem is EM-isolated from other subsystem with individual
sub-enclosures. Even THAT doesn't eliminate all spurs.
