On Thu, 21 Jun 2007, Don Ward wrote:
----- Original Message ----- From: <[EMAIL PROTECTED]>
To: <discuss-gnuradio@gnu.org>
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
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