Noise problem with USRP

GNU Radio
1

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

2

— [email protected] wrote:

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

So if (1) antenna is connected and (2) you are using notebook
connected to mains then you see 120Hz noise. So I think the
noise comes from the power brick and enters via the antenna.

Sounds like the notebook power supply or the cables it
connects to radiates RF. Not surprizing at all for a
switching power supply

Try installing some ferrite toroids on both of the cables on
the power brick. Use as many turns as you can through the
toroid.

Another option is to use an antenna that is farther away from the
power brick and bring the signal back using a long coax feed line.
Use the inverse square law to your advantage.

Chris Albertson
Home: 310-376-1029 [email protected]
Office: 310-336-5189 [email protected]

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3

Chris-

thanks for the suggestions. I may try to add the ferrite toroid. It’s
just curious that I didn’t notice this before. Maybe my power brick is
functioning differently, although it is brand new.

thanks,
eric