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Re: [Discuss-gnuradio] USRP / DBSRX calibration

From: Jens Elsner
Subject: Re: [Discuss-gnuradio] USRP / DBSRX calibration
Date: Wed, 26 Dec 2007 23:44:00 +0100
User-agent: Mutt/1.5.13 (2006-08-11)


thanks for your reply. 

> >Using a noise generator, I came up with the following figures:
> >
> >USRP settings: DBSRX, gain 52, decimation factor 32 
> >Calibration bandwidth: 190 kHz around center frequency, 3dBm cable loss
> >accounted for
> >  
> That's a lot of cable loss.  Are you sure its that much?

The figure I posted was wrong, it was actually 1.3 dB. 

> >I noticed that (with gain 52) the USRP goes into saturation above -50
> >dBm and does not quite show linear behaviour below -100 dBm.
> >  
> Not sure what you mean above.  With gain of 52, the system gain is 
> pretty high.  For a strong signal like -50dBm you would want to have a 
> lower gain.

I was trying to measure signals starting at roughly -100 dBm (400kHz).
That's why I chose a high gain. The "saturation effect" I'm seeing
around -50 dBm might well be due to numerical limitations of the
algorithm (integral over PSD estimate) used to calculate the power in
the band. I have to check on that again. 

Should I be able to measure linearly over more that 50 dB dynamic range
with the USRP/DBSRX combination?

> >Furthermore, -165 dBm/Hz is a better sensitivity than my spectrum analyzer 
> >offers, which I find rather odd. Do these figures make sense?
> Spectrum analyzers rarely have good noise figures.  -165dBm/Hz 
> translates to a roughly 9dB noise figure, which is a little high for the 
> DBSRX, but not unreasonable.  It is better than my spectrum analyzer.

Thank you very much for that insight. I was rather suprised, given the
20dB$ price difference. 


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