Re: [Discuss-gnuradio] What is USRP oscope showing me?

[Eric Blossom]

On Mon, Dec 04, 2006 at 10:03:25PM -0800, Chris Stankevitz wrote:
> 
> Hi,
> 
> I'm new to radio and I have a lot to learn.  I have a question about 
> oscilloscopes and the usrp_oscope.py.
> 
> I believe the following.  Please correct me where I am out to lunch. 
> There is a strong 103.3Mhz FM station in my area.  Connect an antenna to 
> a band pass filter that passes 103.3Mhz +- a little bit.  Plug the 
> output of the filter to an oscilloscope (a real one) and you will see a 
> 103.3Mhz wave.  Note: you do not tune the oscilloscope.
> 
> My questions about usrp_oscope.py:
> How do I see the carrier wave at 103.3Mhz?

Assuming you've got a TV_RX daughterboard on side A:

  $ usrp_oscope.py -R a -f 103.3M -d 256

This will plot the signal that is received in a 250kHz wide window
centered at 103.3 MHz.  (64e6 / 256 = 250e3)

> What are the units of the Y axis?

They are the output of the digital down converter.
They are not calibrated to any particular voltage level.
The levels depend on the gain setting too.

> Why does it want to tune?

Because it wants to know what portion of the RF spectrum you are
interested in.

> What is usrp_oscope.py showing me?

The real and imaginary parts of the complex baseband signal coming
from the output of the digital downconverter in the USRP.


Since it sounds like you're not too familiar with what's going on, you
may want to play with usrp_fft.py first.  It'll show you the same
data, but in the frequency domain instead of the time domain.

If this isn't making much sense, try spending some time with Richard
Lyon's "Understanding Digital Signal Processing".

Eric