On 2025-05-22 21:31, Nikos Balkanas wrote:
The spike is very clean to come from outside.
Must be from my X310. My tuner must be adding a signal to the
center frequency. The small artifact at 2 Ghz is probably the tuner not
equilibrating fully.
I recently updated my FPGA image. Is that where the tuner lives?
You haven't mentioned in this thread which daughtercard you're using.
RF front-ends that use complex-baseband
downconversion suffer from something called "DC-offset", which
produces a spike at 0Hz in the complex spectrum.
The radio block in the standard FPGAs has methods for reducing this,
unless you turn it off. This is a very very
*normal* thing for complex-baseband receiver chains.
If the algorithms are engaged and working, then there'll still be a
central spike, but *considerably* reduced, and I find that
said spike is usually swamped by external signals, even in radio
astronomy.
The other method that people use is to use "offset tuning". Where the
tuner is tuned to a different RF frequency, and the
DDC brings your signal of interest down to 0Hz.
https://files.ettus.com/manual/page_general.html#general_tuning
The "tuner" is an analog collection of components, including an LO
generator, and mixers. While it is *controlled* through
the FPGA, it is an analog subsystem.
On Fri, May 23, 2025 at 3:19 AM Nikos Balkanas <nbalka...@gmail.com>
wrote:
Hello,
Whenever I look at my spectrum I always see an energy spike at the
center frequency.
In the first image you can see a spike at 2, but not at 2.001 Ghz.
In the next image,
at 2.001 Ghz you can see the energy spike at the center
frequency, but also a small
spike at 2 Ghz.
I have verified these results by both fosphor (OpenCL fft) and
fftw3f. Besides, if it were
an fft artifact, why is the spike at 2 Ghz still visible after a
few mins? These spikes
seem to be transient, but real. In that part of the spectrum, you
there is no traffic. Could it be harmonics from my power supply?
Problems with my X-310? My transmitter
doing funny things (I have 2 boards and not enabling my
transmitter anywhere)?
Naming of images is freq_sr.jpg. All are in Mhz.
TIA
Nikos
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