Re: [Discuss-gnuradio] High noise level reading GSM spectrum.

[Matt Ettus]

José Carlos Reyes wrote:
> Hi all,
>
>  We were reading GSM channels with Tvoid software 
> (http://wiki.thc.org/gsm/tvoid) and at the beginning it worked fine. 
> Suddenly,  we do not how, it seems that noise level has increased in 30 
> dB approx. and only a few GSM channels can be now identified (but they 
> are all only 10/15dB above noise power, when at the beginning there was 
> at least 40dB) but this channels can be read so we think the front end 
> is not broken, it seems the RF front end is not working properly or its 
> configuration is wrong, but we do not know how we have made it. We think 
> that the LP filter in our dbsrx daughterboard may be working poorly.  I 
> attach you a screenshot of the best received GSM channel (it is only 
> 15dB above noise power).

There is no low pass filter.  Assuming that you are setting the gain 
properly, then most likely problem is that you blew out the LNA in the 
front, U4.  Static or strong signals can do that.  You could try 
replacing U4, which is an MGA82563.

Matt