Re: [Discuss-gnuradio] USRP receiver filter response

[Eric Blossom]

On Wed, Sep 24, 2008 at 10:53:36AM -0400, Kell, Scott wrote:
> > I then saw the post on the USRP filter response by Firas [1] where he
> > showed that for an overall decimation rate of 64 the CIC filter
> response
> > had better than a 55 dB anti-alias rejection. [2]  But I'm seeing only
> 42
> > dB (overall response).
> > 
> > Does anyone have any ideas that explain this 13 dB discrepancy in the
> > filter response?  Or am I missing something obvious...
> > 
> > [1] http://lists.gnu.org/archive/html/discuss-gnuradio/2007-
> > 05/msg00191.html
> > [2] http://www.nabble.com/file/8500/%280%20-
> >
> %2032%29MHz%29%20CIC%20Frequency%20Response%20%28CIC%20decimation%20%3D3
> 2%
> > 29.JPG
> 
> [Kell, Scott] 
> To illustrate what I'm talking about, I set the USRP as follows:
> Data rate: 500 kSamples/sec, 
> Center frequency: 1250 kHz
> 
> I put a 9dBm tone in at 50 kHz and measured the largest alias in my
> "passband".

What daughterboard are you using?

If it's the Basic Rx, it's spec'd from 1 MHz to 250 MHz.
There's a transformer on the board that limits the bottom end.
See the datasheet http://www.ettus.com/downloads/er_ds_txrx_dbrds_v4.pdf

Eric