Re: [Discuss-gnuradio] Re: Spurious Spikes Seen with usrp_fft.py

[Thomas]

John Clark <address@hidden> wrote:
>  Thomas schrieb:
> > Thanks for you reply, Robert.
> >
> > I am in the U.S.
> >
> > I have used usrp_fft.py to tune to 921.6 MHz, and the spike is still 
> > there at 921.6 MHz.
> >
> > I tried powering the USRP from batteries, but the spike was still 
> > there. It was also still there using a different computer. I tried 
> > using a USB cable with a ferrite bead, but it didn't change anything.
> >
> > I positioned the USRP very close to the antenna connected to a real 
> > spectrum analyzer, and I was able to observe the spike on the spectrum 
> > analyzer. I guess the USRP is actually radiating this frequency. It 
> > might go away if I place the USRP's antenna farther away from the USRP 
> > itself, which I haven't been able
>  to try yet.
>
> If you can adjust the band so that you can look at 64 Mhz up from the 
> 'current spike' to see if there is another, you may be seeing the
> digitization clock of the USRP.
>
> In my activities, with an additional LNA in front of the USRP, I see 
> spikes at say, 512 Mhz, (the start of my region of interest...), and 
> then depending
> on weather, cosmic rays, whatever, I will see spurs on 64 Mhz intervals 
> upward from there. The top of my range is about 700 Mhz.
>
> Since I'm using the TVRX as well, I may be also seeing some spurs from 
> that as well.
>
> However, the spurs I see are very narrow, on the order of 2 or 3 bins, 
> at 4k per bin, and again depending, sometimes the
> spurs will 'move' a couple of bins one way or the other.
>
> This of course is one of the reasons why I did not go into RF... and 
> chose 'software'...
>
> On discussion this with 'real RF engineers', I was informed that some 
> spectrum analyzer
>  companies actually inject
> 'noise' into their displays to cover any 'spurs' generated by the 
> equipment itself. This giving the user the impression
> that all that's there is background noise...
>
> John Clark.
There are other spikes at 64-MHz intervals. I guess that pretty much answers my question.

Thanks!

Thomas

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