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[Discuss-gnuradio] noise problem with USRP

From: ematlis
Subject: [Discuss-gnuradio] noise problem with USRP
Date: Mon, 18 Jun 2007 19:20:08 -0400 (EDT)

Hello all-

I'm seeing a strange noise issue with my USRP. I have a simple antenna fashioned out of a spool of wire connected to a LF-DC daughterboard with no anti-aliasing filters. The USRP is connected to a laptop and powered off it's standard brick power supply. When the laptop is plugged into the mains, my am-demod gui shows a lot of noise in the spectrum and time series; when I disconnect the laptop from the power socket to operate on the battery this noise goes away. I have two images demonstrating the difference at:

www.nd.edu/~ematlis/z.gnuradio/usrp_on_mains.jpg www.nd.edu/~ematlis/z.gnuradio/usrp_on_battery.jpg

I don't believe I was seeing this in the recent past, however I recently upgraded to Fedora 7, although I don't see how this could be a factor.

You can see in the pic that the demodulated time series has a strong content at roughly 120 HZ; this could be a power-supply induced issue (60 Hz supply harmonic). The odd thing is there is no noise at all when I disconnect the antenna, so the noise does not seem to be entirely a function of the power supply alone. I also do not see the noise at all when using a desktop PC.

My supposition is there is some kind of interaction between the power supply ripple coming from the laptop power supply and the circuitry of the USRP, but I have no direct evidence.

Any thoughts?  Has anybody seen this also?


Eric H. Matlis, Ph.D.
Aerospace & Mechanical Engineering Dept.
120 Hessert Center for Aerospace Research
University of Notre Dame
Notre Dame, IN 46556-5684
Phone: (574) 631-6054
Fax:   (574) 631-8355

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