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

From: Balijepalli, Arvind
Subject: Re: [Discuss-gnuradio] Phase Lag in USRP Output Signal
Date: Fri, 4 Jun 2010 12:23:20 -0400

>>> 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.


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