RE: [Discuss-gnuradio] USRP receiver filter response

[Kell, Scott]

> 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".  I then did the same at 100 kHz, 150 kHz, 200 kHz, ... 32
MHz.  I then plotted this "response". [3] It just doesn't seem to be as
good as what I thought it would be.  [4]

Any ideas?    

Thanks!
[3]
http://picasaweb.google.com/sctkell/USRPResponse?authkey=Z3gyRLgXOZc#524
9597153645337122
[4]
http://www.nabble.com/file/8500/%280%20-%2032%29MHz%29%20CIC%20Frequency
%20Response%20%28CIC%20decimation%20%3D32%29.JPG