USRP2 noise floor peculiarities

GNU Radio
#1

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

#2

I believe the filtering used does change with the decimation - per an
earlier posting by Matt E.: if the decimation is a factor of four
there are two half-band filters + CIC filter, if it is a multiple of
two (i.e. even, but not a factor of four) then there is only one
half-band filter + CIC, and if it is an odd number, just a CIC filter.
Doug

On Mon, Oct 26, 2009 at 4:20 PM, Jordan J Riggs [email protected]
wrote:

I’ve attached screenshots at decim 100 and 128.

Thanks,
Jordan

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Doug G.
[email protected]

#3

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:

than

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