[USRP-users] Improving Sensitivity of the Radio.

Marcus D. Leech mleech at ripnet.com
Tue Nov 14 22:37:00 EST 2017

On 11/14/2017 10:31 PM, Kyeong Su Shin wrote:
> Dear Marcus D. Leech:
> Thank you for the reply.
> Is the only advantage of the TwinRX the pre-selector filter bank? Or, 
> can the superheterodyne design of the radio also affect the data 
> quality in some ways?
A Low-IF superhet design has some advantages, but the UBX has the same 
overall architecture.  With a Zero-iF design, mixer balance is extremely
   important, and not always easy to achieve "perfectly".  In the end, 
it's a matter of where your mixer images end up.
> We do turn down the gain to get optimal results. (The 'noise figure' 
> that I mentioned was the noise figure that is observed by us, after 
> such adjustments). We will definitely try adding filters.
Keep in mind that filters "out front" necessarily add to the noise 
figure of your receiver, since they aren't loss-free.  RF system design 
is all about

> Regards,
> Kyeong Su Shin
> On Tue, Nov 14, 2017 at 7:12 PM, Marcus D. Leech via USRP-users 
> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
>     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
>>     (UBX/SBX/CBX/WBX).
>>     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
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>     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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