Computing Power Spectra

Hello,

I wrote a simple grc graph to compute power spectra of the output of the
usrp. My test setup is a signal generator outputting a tone at the same
frequency that I set as the usrp center frequency, directly connected to
the usrp. But when I examine the output in Matlab, the first vector is
exactly what I expect, a peak at the center and then the noise floor.
The following vectors show the peak at lower and lower power levels
until, after 10 or so, it disappears into the noise. My code is
attached. Anybody know what could be causing this?

Any help would be appreciated.

Scott

On Tue, 2011-03-15 at 17:48 -0400, Scott Johnston wrote:

Hello,

I wrote a simple grc graph to compute power spectra of the output of the
usrp. My test setup is a signal generator outputting a tone at the same
frequency that I set as the usrp center frequency, directly connected to
the usrp. But when I examine the output in Matlab, the first vector is
exactly what I expect, a peak at the center and then the noise floor.
The following vectors show the peak at lower and lower power levels
until, after 10 or so, it disappears into the noise. My code is
attached. Anybody know what could be causing this?

My guess is DC offset correction. Receiving an unmodulated baseband
signal will just look like DC, which will get offset corrected in the
FPGA. Try generating a signal very slightly offset from the USRP receive
freq.

–n

On Tue, Mar 15, 2011 at 4:48 PM, Scott Johnston
[email protected] wrote:

Hello,

I wrote a simple grc graph to compute power spectra of the output of the
usrp. My test setup is a signal generator outputting a tone at the same
frequency that I set as the usrp center frequency, directly connected to the
usrp. But when I examine the output in Matlab, the first vector is exactly
what I expect, a peak at the center and then the noise floor. The following
vectors show the peak at lower and lower power levels until, after 10 or so,
it disappears into the noise. My code is attached. Anybody know what could
be causing this?

My guess is that you are seeing a DC offset correction loop closing.
This would occur if the RF tone of interest is downconverted to DC
(which sounds like what you described). I don’t recall how DC offset
correction is actually implemented in USRP land, but it may very well
take a few tens/hundreds of samples before it is completely corrected,
which lines up with what you’re seeing.

Try changing your sig gen to output a tone at, say 100 Khz from your
RF tuning frequency, and it should not attenuate over time.


John O.
CEO/System Architect
Epiq Solutions
http://www.epiq-solutions.com

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