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

[ematlis]

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?

thanks,
eric

> Matt