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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.
Arvind