>>> The LFRX has a 50 ohm termination on its input, so when you connect >>> it to a signal generator with a 50 ohm output impedance, the >>> voltage goes down by half vs. open circuit. This is the >>> appropriate behavior. >>> >>> >> Yes, I would expect that as well. However, what I see is that the >> voltage on the scope, does not go to zero when the function generator >> output is off unless I also disconnect the USRP. This tells me that >> the USRP is somehow loading the function generator. >> > > The LFRX uses a single-supply differential opamp as the input. Because of > this, it presents a DC bias on its input. If you feed it with a 50 ohm > source, the DC voltage will not be zero, but the measured voltage will be > correct. This will make more sense if you look at the schematic and analyze > the opamp circuit. Thanks, that explains the bias
> In all likelihood, it doesn't matter, and won't affect your source. If your > source does not like the DC bias then you will need to use a DC-blocking > capacitor. 10 uF will have less than 0.2 ohms impedance at 100kHz. However, > if you need DC response, AND the source really doesn't like the DC bias then > you'll need to use a buffer with an opamp and have both a positive and a > negative supply for it. > The DC bias won't really affect our experiment, as long as the measured signal is true, which looks to be the case. Thanks again for your help. Arvind _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
