[USRP-users] Connecting a test signal source to the SBX's RX2

mleech at ripnet.com mleech at ripnet.com
Tue Jun 12 11:32:56 EDT 2012


 

On 12 Jun 2012 11:26, Ryan Connelly wrote 

> Oh yes of course what
was I thinking.. The FFT is not even plotting
> w.r.t. power. But the
zero must be some reference value? Are you just
> saying that the
reference is not accurate and varies with frequency?

The effective
analog gain at any given frequency will vary by up to a couple of dB,
depending on the daughtercard. 

Furthermore, the voltage samples coming
off the ADC are processed by the decimator chain in the FPGA, and by the
time they get to Gnu Radio flow-graph, they've been scaled into
{-1.0,+1.0}. So, those samples are linearly-proportional to the incoming
voltage, but aren't a direct measurement. 

Which is why you have to
calibrate over several frequencies, at your operating bandwidth.
Otherwise, the values shown in dB in the FFT graph are just
relative--they don't indicate any particular power level. 

-Marcus 

 
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