Re: [Discuss-gnuradio] USRP / DBSRX calibration

[Matt Ettus]

Replying a bit late here...

Jens Elsner wrote:
> As Eric Matlis a while ago (see link to thread at the bottom), I would
> like to calibrate my USRP/DBSRX combination for measurements. As noted,
> the USRP is not a measurement device, but nevertheless I'd like to give
> it a try. My application are GPS based measurements in the GSM downlink
> band.
>
> 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?

> Offset (to convert from dB to dBm): -177.7
> Noise floor: -112.5 dBm, sensitivity -165 dBm/Hz (NF: 8.7 dB at 20 degrees)
> Linear measurement range: -100 dBm to -50 dBm
>
> 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.

> 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.

Matt