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

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Phase Lag in USRP Output Signal
Date: Thu, 03 Jun 2010 16:11:16 -0700
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100430 Fedora/3.0.4-2.fc12 Thunderbird/3.0.4

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.


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