Re: [Discuss-gnuradio] Radio Astronomy Spectrum Averaging interface; Osmosdr issue?

On 05/07/2018 05:20 PM, Glen I Langston wrote:
>
> Hello
>
> A group of us working with Gnuradio to build home radio telescopes has
> been working for
> a few years with gnu radio. The code for our observations was in a
> form that was
> "orphaned" with changes to gnu radio.
>
> The observing interface has been re-released (in an improved form) via
> git hub. It can
> be obtained with
>
> git clone http://www.github.com/glangsto/gr-nsf
>
> Despite lots of work, the result is only a few new blocks in a python
> sub-directory
> and .xml files in the "grc" sub-directory for use with with
> gnuradio-companion (GRC).
>
> The examples directory contains the .grc files to be run to make
> observations.
>
> The examples README.md has instructions.
> The examples directory also has two .grc files that test the blocks and
> show now problems on either Ubuntu or Mac OS.
>
> We find an issue for this code an Ubuntu, that is not present for Mac
> operations.
> Using exactly the same code on Ubuntu shows the problem, while the mac
> code does not.
> We guess the problem is in some way associated with our use of the
> Osmosdr block
> and an AIRSPY mini.
>
> The problem is that the spectrum has a weak alias relative to the
> center of the band pass.
> We're hoping someone might have a suggestion on how to mitigate this
> problem on Ubuntu.
>
> An example spectrum showing the interface and the problem is attached.
> For Radio astronomy the galactic hydrogen signal is near 1420.4 MHz ==
> 0 velocity. The alias signal is "folded"
> to the lower half of the band. The green curve show the problem on
> Ubuntu. The baby blue and red
> signals were taken with the Mac and have no alias. The hot load (red)
> was also taken with the Mac
Is the Ubuntu trace from an ARM system?

I wonder about numerical issues on the ARM side for the real-to-complex
converter that AirSpy uses on the host-side. Or perhaps something slightly
askew in the VOLK implementation on ARCH?

OR, maybe it's not an alias. Maybe your Ubuntu system is actually
emitting those artifacts? Is the alias *precisely* reflected about DC?




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