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

[ematlis]

On Thu, 21 Jun 2007, Don Ward wrote:

> ----- Original Message ----- From: <address@hidden>
> To: <address@hidden>
> Sent: Thursday, June 21, 2007 5:22 PM
> Subject: [Discuss-gnuradio] offset at input of LF_RX
>
>
> > 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.
> >
> > 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?
> >
> > Any thoughts on either of these questions?
>
> From looking at the schematic, it appears that the LFRX requires a 50 ohm 
> input resistance (at DC) to bias it for zero output.  Does your function 
> generator do this?

Yes, the function generator has two modes; high impedance output and 50 
ohm load output.  It's a switch that seems to basically just multiply the 
setting on the amplitude by a factor of 2 to account for the voltage 
divider created by the 50 ohm load.

> -- Don W.
thanks,
eric