Re: [Discuss-gnuradio] bandwidths< 400Khz no longer work with USRP2

[Matt Ettus]

On 11/16/2010 11:13 PM, Marcus D. Leech wrote:
> On 11/17/2010 02:00 AM, Tom Rondeau wrote:
> > WBX with latest firmware downloaded from Ettus Monday.
> Hmmm, I'm using Basic_Rx.  That should make *zero* difference.
>
> I discovered that there's a "magic" break at any sample rate that
> requires a decimation>256.
>    So *somewhere* is having a hard time with greater-than-eight-bit integers.


Decimation is filtering.  When you decimate by 512 you are reducing 
noise by a factor of 512 (27dB).  Since you are using a BasicRX, there 
will be very little noise, and 27dB less after decimation.  In fact, 
there is so little noise that the output of the filters is a constant 0 
once it is rounded to 16 bit ints.  That is why the FFT results 
essentially show negative infinity.

The solution is to decimate less in hardware and more in software, or to 
use more amplification.

Matt