[Discuss-gnuradio] noise problem with USRP

[ematlis]

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?

thanks,
eric

************************************
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