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Re: [Discuss-gnuradio] offset at input of LF_RX

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] offset at input of LF_RX
Date: Thu, 21 Jun 2007 14:44:30 -0700
User-agent: Thunderbird (X11/20070326)

address@hidden wrote:
> Hi all-
> can anybody explain why the USRP/ LF_RX would seem to introduce a DC
> bias or offset to a signal generated by a function generator?  I have
> a function generator configured to produce a 1 kHz .1 V P-P signal
> into a 50 Ohm load.  I have a Lecroy digital oscilloscope configured
> with the input at high-impedance to monitor the signal.  When I
> connect the signal to the USRP, I see the trace on the Lecroy gets
> shifted UP by about .06 volts, ie I have to add a -0.06 V offset at
> the function generator to zero the bias.

Are you sure everything is on the same ground?  Disconnect everything
but leave it powered up, and measure the voltage on the ground of the
scope, lfrx, usrp power supply, and sig gen.  Then reconnect things and
do the same.

> Oddly enough, usrp_oscope.py doesn't show any DC bias whatsoever,
> regardless of what offset I put into the function generator.  Maybe
> it's removed inside the flow-graph?

DC offset is automatically removed in hardware, but you can turn that
feature off.


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