Re: [Discuss-gnuradio] USRP2 noise floor peculiarities

[RH200]

Hi we to have been getting interesting stuff at low power levels. Some one is
trying to implement and RFI scanning system using a small dish to scan for
RFI between 800 MHz and 1800 MHz. Using the USRP2 and DBRX and its a bit
hard to tell the difference at times between the birdies in the system and
RFI. 

Also does anyone have a reliable estimate of the noise tempreture for the
USRP2 DBRX combination?

Attached is three images each image has two plots one is a linear plot the
other is a logarithmic scale. One plot is the dish and LNA coupled with the
log perodic antenna. The other plot is the LNA terminated, and the last one
is the USRP2 terminated. The plots are just lots of little data captures
stiched together.
http://old.nabble.com/file/p26159950/rfi_logper_20091020b.png
rfi_logper_20091020b.png 
http://old.nabble.com/file/p26159950/rfi_termamp_20091020.png
rfi_termamp_20091020.png 
http://old.nabble.com/file/p26159950/rfi_termcoax_20091020.png
rfi_termcoax_20091020.png 

Jordan J Riggs wrote:
> 
> Folks,
> 
> I have been trying to get a better understanding the noise characteristics
> of the USRP2/RFX2400 platform. To get an idea of the noise floor at
> various
> bandwidths, I connected the SMA's on the RFX2400 to a 50ohm load and
> averaged out the FFT at various bandwidths. Naturally, I would expect the
> floor to drop at lower bandwidths (higher decimation), but this is not
> always the case. For example, the floor at decim 100 is slightly lower
> than
> it is at 128. This is counterintuitive. The only explanation I can come up
> with is some filtering quirk in the FPGA. Does anyone have any ideas?
> 
> I've attached screenshots at decim 100 and 128.
> 
> Thanks,
> Jordan
> 
>  
>  
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