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Re: [Discuss-gnuradio] USRP / DBSRX calibration

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] USRP / DBSRX calibration
Date: Wed, 26 Dec 2007 14:25:04 -0800
User-agent: Thunderbird (X11/20071115)

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

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.


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