[USRP-users] Improving Sensitivity of the Radio.

Marcus D. Leech mleech at ripnet.com
Tue Nov 14 22:12:18 EST 2017

On 11/14/2017 10:00 PM, Kyeong Su Shin via USRP-users wrote:
> Hello Everyone:
> We are using USRPs for spectrum sensing (on pretty much any 
> frequencies). Before asking my faculty to order a TwinRX 
> daughterboard, however, I would like to see if there are any ways to 
> improve the sensitivity of the hardware that we currently own 
> Our USRP + UBX configuration works well, except that we cannot really 
> increase the gain level much without putting the RF frontend into the 
> nonlinear region. Because of this, the noise figure of our data is 
> about ~30dB in the worst case. I believe that the main problem is that 
> we are using a wideband outdoor discone antenna for this - dumping 
> quite a lot of RF energy to the RF frontend.
> In this case, what could I try to improve the sensitivity of the 
> radio? I think one option could be adding additional filters to the 
> chain (when we know that we are only looking at a certain 
> frequencies), but I wonder if there are anything else that I can try.
> Also, I wonder what differences we can expect if we switch our 
> daughterboard to a TwinRX. Would it be worth it? What noise figures 
> did people experience when a wideband outdoor antenna was connected to 
> the board?
> Regards,
> Kyeong Su Shin
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
In the case of overload, the noise-figure is NOT the dominating factor 
in determining system sensitivity.

If you are "sensing" across a limited band, then a filter that covers 
that band, and that band only, will definitely help keep the very first 
gain stages,
   which are "wide open" from going into overload.

The TwinRX has some significant advantage here, since it has 
pre-selector filters, but those filters may or may not have band edges 
that correspond
   adequately to the band that your are "sensing".

Basically, there's a "tension" in small-signal RF amplifiers.   They can 
either have very high dynamic range, or they can have very-low noise figure.
   That general axiom still applies, although things are getting 
somewhat better in this regard.    But taking the output of an outdoor 
disc-cone antenna
   in a normal urban environment is pretty-much begging for 
non-linearity.    You might try turning *down* the gain.

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