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

[John Clark]

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.