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Re: [Discuss-gnuradio] offset at input of LF_RX

From: ematlis
Subject: Re: [Discuss-gnuradio] offset at input of LF_RX
Date: Fri, 22 Jun 2007 13:24:57 -0400 (EDT)

On Fri, 22 Jun 2007, Matt Ettus wrote:

address@hidden wrote:
On Thu, 21 Jun 2007, Matt Ettus wrote:

If there is no dc-removing capacitor in the circuitry , then should I
not expect that for a board using a single (positive) supply, that the
signal is always above zero?

No, you can put a negative voltage in, as long as it doesn't go below
-3.33V.   You need to look at the schematics -- the 8132 is a
differential opamp with a common mode output set at 3.3V/2.

I meant that if you operate single supply, a negative input would be
mapped to a voltage referenced to the common mode output, not to zero.
So -.1 V would become 3.3/2 -.1 = 1.55 V.  That's what the data sheet
seems to indicate if I read it correctly.  Am I reading it correctly?

If you put 0V in, both outputs of the amp should be at 3.3/2.

If you put in a positive voltage V, the + output should go up by V/2 and
the negative output should go down by V/2.  The reverse happens for
negative voltages.

Basically, the differential amps will clip if you go outside the range
of -3.3V to +3.3V.  You will also damage the differential amp if you go
below -3.3V.

The ADC will clip if you go outside the range -2V to +2V when set for
minimum gain.

Back to the subject of  what your signal generator is doing, I don't
know.  If you measure the voltage on the sma connector with nothing
connected, you will see that it is 0.  If you connect a 1 V source
through a 50 ohm resistor, you will see that there is 0.5V at the
connector.  If you connect -1 V through a 50 ohm resistor, you will see
-0.5V at the connector.

I do not.  In fact, with no voltage applied to a powered up USRP, I
actually see +.109 V!  So when I apply a -.1 (through a 50 ohm
resistor) I see just above zero.

I see the behavior you describe only if I unpower the USRP/LF-RX.
What gives?

Sounds broken.  The LFRX has 2 inputs.  Does the other one behave the
same way?

I am measuring at the output connector of the LF-RX, which is of course the input to the 8132. I wasn't measuring at the outputs of that chip on the circuit board. Is that what you intended me to do?

And I have 2 LFRX's and all 4 inputs behave the same way.


I'm actually giving a demo with the USRP in a couple of minutes to some visiting students, so I'll have to wait to open up my case and make those measurements... I'll get back to you asap though.


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