[USRP-users] N210 RX has fixed 900MHz carrier

Marcus Müller marcus.mueller at ettus.com
Tue Sep 9 09:08:22 EDT 2014


Hi 1158053788,

that might be a harmonic of the master clock. SBX is pretty sensitive
and has a low noise figure, so I'd expect noise floor with fully pumped
up LNA to be equivalent to something like -166dBm/Hz or so; without
knowing your spectral resolution when looking at the background noise,
it's hard to estimate the power of your 900MHz peak.
Let's just juggle some numbers around, anyway: the GSM-specified
sensitivity is -102dBm for a GSM channel (which has, for this aspect,
IIRC, a 200kHz = 53dBHz equiv. noise bw.). Johnson-Nyquist noise
including a noise figure of 8dB would then lead to a whopping -113dBm of
noise energy in that channel[1].
Now, your carrier most likely really looks like a single line of
seemingly infinitesimal bandwidth. Now, you say its 20dB= 100times
higher than the noise floor. As long as it's substantially narrower than
-20dB =1/100th of the channel bw., we're fine.

Greetings,
Marcus

[1]This illustrates one typical aspects of Wireless communication
standardization: You usually push the limits of what can be done at the
time of standardization just a bit to keep things possible, but get most
out of the state of the art. -102dBm sensitivity is but 10dB over this
value, which we can achieve 20 years after with comparatively complex
hardware. It was certainly not easy back in the times GSM got
standardized to achieve this value, but if the standardization agencies
hadn't pushed the engineers, you wouldn't get receive GSM in a train
speeding through european rural areas today, and handset technology
wouldn't have evolved fast enough to make 3G and 4G possible.

On 07.09.2014 11:35, 1158053788--- via USRP-users wrote:
> Hi everyone.I was using the N210 with daughter board SBX to run the OpenBTS.
> When I check the receive data from the usrp, I find  there is a fixed 900MHz carrier, athough the data is noise. The carrier is 20dB higher than the noise level. 
> First, I thought maybe it is the LO influence, so I used the advanced tuning,setting the LO_OFFSET.But it turned out not the LO problem. I can see another carrier, 10dB higher than the noise level. The position will change as I change the value of LO_OFFSET.But the 900MHz carrier will never change.
> Then I thought may be there is interference in the air, so I remove the antenna of the usrp, but the 900MHz still exist.
> I don't know why.Can anyone tell me? The usrp is N210, and the daughter board is SBX
>
>
> 1158053788 at qq.com
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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