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

From: ematlis
Subject: Re: [Discuss-gnuradio] offset at input of LF_RX
Date: Thu, 21 Jun 2007 20:34:26 -0400 (EDT)

On Thu, 21 Jun 2007, Matt Ettus wrote:

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.

Using the ground of the USRP power supply as a reference, I saw a maximum of 0.005 volts difference between the grounds of the scope, the function generator, and the inputs to the LF-RX boards. When I check continuity, all the grounds save that of the sig gen are connected (the sig gen has a battery isolation). Obviously, when I connect everything back up those grounds are all on the same plane.

So to sum up. With the sig gen configured for .1 V p-p with 50 Ohm impedance, when I connect only the Lecroy (configured for 50 Ohm impedance) the signal is .1 V p-p centered about zero as expected. When I then switch the Lecroy to high impedance and connect the USRP, the signal now is roughly .1 V p-p but centered about roughly 0.06 V, ie shifted up by slightly more than half scale.

Just to confirm that the problem is not my function generator, instead of the USRP I connected a second Lecroy configured also with 50 Ohm impedance (the other Lecroy at high impedance) and verified that the signal is as expected, ie .1 V p-p centered about zero.

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.

Can you explain that in more detail, please? Do you have a capacitor is series with the signal flow to remove any dc?



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