[Discuss-gnuradio] USRP / DBSRX calibration

[Jens Elsner]

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

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.
Furthermore, -165 dBm/Hz is a better sensitivity than my spectrum analyzer 
offers, which I find rather odd. Do these figures make sense?

Best regards,
Jens Elsner

Link to relevant thread:
http://lists.gnu.org/archive/html/discuss-gnuradio/2007-03/msg00414.html