[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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 20:36:49 -0400 (EDT)

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.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]