Re: [Discuss-gnuradio] Phase Lag in USRP Output Signal

[Balijepalli, Arvind]

>>> 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