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

[Matt Ettus]

On 06/03/2010 11:19 AM, Balijepalli, Arvind wrote:

> > 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.

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.

Matt